Definitions for jack kerouac
This page provides all possible meanings and translations of the word jack kerouac
Kerouac, Jack Kerouac, Jean-Louis Lebris de Kerouac(noun)
United States writer who was a leading figure of the beat generation (1922-1969)
Jean-Louis "Jack" Kérouac was an American novelist and poet. He is considered a literary iconoclast and, alongside William S. Burroughs and Allen Ginsberg, a pioneer of the Beat Generation. Kerouac is recognized for his spontaneous method of writing, covering topics such as Catholic spirituality, jazz, promiscuity, Buddhism, drugs, poverty, and travel. Kerouac became an underground celebrity and, with other beats, a progenitor of the hippie movement, although he remained antagonistic toward some of its politically radical elements. In 1969, at age 47, Kerouac died from internal bleeding due to long term abuse of alcohol. Since his death Kerouac's literary prestige has grown and several previously unseen works have been published. All of his books are in print today, among them: On the Road, Doctor Sax, The Dharma Bums, Mexico City Blues, The Subterraneans, Desolation Angels, Visions of Cody, The Sea is My Brother, and Big Sur.
The numerical value of jack kerouac in Chaldean Numerology is: 6
The numerical value of jack kerouac in Pythagorean Numerology is: 9
Images & Illustrations of jack kerouac
Find a translation for the jack kerouac definition in other languages:
Select another language:
Discuss these jack kerouac definitions with the community:
Word of the Day
Would you like us to send you a FREE new word definition delivered to your inbox daily?
Use the citation below to add this definition to your bibliography:
"jack kerouac." Definitions.net. STANDS4 LLC, 2017. Web. 26 Feb. 2017. <http://www.definitions.net/definition/jack kerouac>.


Brunelleschi's Dome - Ross King
The Building Centre,
Free to attend
Share this talk:
Author and historian Ross King will talk about the construction of Santa Maria del Fiore Florence and how the architect and engineer Filippo Brunelleschi completed the cathedral in 1436 with the largest masonry dome ever built.
Brunelleschi is considered the main initiator of stylistic changes in Renaissance architecture being able to build an octagonal self-supporting dome engineering the perfect placement of brick and stone on the model of the Roman Pantheon.
Ross King’s book Brunelleschi's Dome: How a Renaissance Genius Reinvented Architecture (2000) is considered to be the definitive account of the project.


BY DANE WENZEL
Software Quality Assurance (SQA) should be considered an integral part of any software project. Adopting SQA efforts early in product development will involve meeting specified requirements, fixing bugs early, and preventing bugs. SQA should not be an afterthought or project phase that the product must pass through in order to ship. Nor should SQA be viewed solely as a testing effort, though testing is a big piece of it. When SQA is properly implemented throughout the Software Development Life Cycle (SDLC), the results are better products at a lower cost due to well defined requirements, improved software quality, and bug prevention.
There are differences between Software Quality Assurance and Software Quality.
Software Quality Assurance can be defined in a couple of ways:
- “A planned and systematic pattern of all actions necessary to provide adequate confidence that an item or product conforms to established technical requirements.” (IEEE, 1991).
- “A systematic, planned set of actions necessary to provide adequate confidence that the software development process or the maintenance process of a software system product conforms to established functional technical requirements as well as with the managerial requirements of keeping the schedule and operating within the budgetary confines” (Galin, 2004).
Software Quality is defined as:
- “The degree to which a system, component, or process meets specified requirements. The degree to which a system, component, or process meets customer or user needs or expectations” (IEEE, 1991).
- “Quality consists of those product features which meet the needs of customers and thereby provide product satisfaction. Quality consists of freedom from deficiencies” (Juran, 1988).
The SDLC is comprised of many different phases that historically follow the order of: Requirements, Design, Development, Testing, Deployment, and Maintenance. Notice, testing falls in the middle of these phases, which is an important element of SQA. The benefits to initiating SQA efforts further upstream in the SDLC include meeting specified requirements, fixing bugs early, and preventing bugs.
Clearly not all software projects and teams are the same. Software projects come in a variety of complexities and so do teams. However, some form of SQA can be adopted to any project which will address the specific needs of the project and the unique capabilities of the team. SIGMADESIGN strives to provide SQA to all software projects in order to meet the client’s specified requirements. Here’s how we do it:
SQA can be especially helpful during the early stages of the SDLC, specifically the Plan/Design phases as it focuses the design around the client’s requirements. Also, it can ensure the requirements provide clear product testability measures. A good development team will assess if the product requirements contain evaluation criteria, if the requirements are consistent, and if the requirements are measurable. During the Design phase, the SQA will also help identify any missed requirements or lost product functionality.
SQA brings a unique perspective to requirement reviews as developers check evaluation criteria, measurability, and consistency involved in meeting those requirements. During the requirement phase, SQA addresses any concerns regarding incomplete or missing requirements that will be needed to both plan and execute testing.
SQA focus areas during the Requirement/Design phases include:
- Requirement reviews for completeness/consistency
- Specification reviews for completeness/consistency
- Design reviews for testability
- Risk study/mitigations
- Bug prevention measures
- Test Plan (Master Test Plan)
- Establishing bug tracking process/procedures
Identifying and fixing bugs early in the SDLC will, in most cases, will be less expensive. According to data gathered by the System Sciences Institute (SSI) at IBM, “the cost to fix a bug found during implementation was around 6 times costlier than one identified during design, bugs found during the testing phase could be 15 times more costly than during design.” Using these estimates, a bug found and fixed during the Plan/Design phases would cost $100 to fix. In these early phases, the bug would be considered a requirement/specification bug, not a coding bug. If, however, that same bug escaped detection in an earlier phase and was not identified until the Testing phase began, that bug would now cost $1,500 to fix.
Additionally, other costs attributed to bugs can be found after a product has shipped. These costs can be direct or indirect. Direct costs may include lost revenue for the current product or future products. Indirect costs may include reduced company or brand reputation and customer product loyalty.
Interestingly, there are a few examples of real-world cases where companies learned the hard way that identifying and fixing bugs early in the SDLC will cost less. Here are a couple of examples documented by SmartBear:
NASA – The Mars Climate Orbiter was a robotic space probe that NASA Launched in 1998 to study the climate and atmosphere of Mars. All was well until communication got cut-off a year into the mission when the Orbiter got lost in space. When dealing with spacecraft engineering, there’s a lot that could go wrong — it’s literally rocket science. However, the error could have been an easy fix if caught on time. The $125 million spacecraft presumably orbited too close to Mars’ surface and disintegrated because the engineering team failed to convert their measurements from U.S. units to metric.
AT&T – In January of 1990, AT&T failed to meet the basic needs of their customers as thousands attempted to call their friends, family, and airports. Long distance calling was down for nine hours, and customers sparked outrage after experiencing a total of 75 million missed phone calls and 200,000 lost airline reservations. Initially, AT&T thought they were being hacked, but the issue was due to a software bug. AT&T had just recently “updated” code, but the change of speed ended up being detrimental to a system that couldn’t keep up. AT&T lost an estimated $60 million from long distances charges that didn’t go through after being down less than a full day in an error that was the result of one line of buggy code.
Bug prevention does not mean that every bug is prevented from occurring. Complete bug prevention is not feasible. The goal in bug prevention is to minimize the number of bugs introduced during the life of a project, starting at an early phase. Finding bugs in the later phases of the project, such as the testing phase, or worst case after shipping, will be far more detrimental to the project budget and potentially that company’s reputation than if found earlier in the SDLC or in the best scenario prevented in the first place.
There are multiple areas where SQA plays an active role in bug prevention early on in a project, including the following:
- Architecture risk analysis
- Setting quality goals
- Setting quality measurements
- Requirement standards
- Requirement reviews
- Design reviews
- Coding standards
- Code reviews
- Unit tests
- Continuous building and integration
- Test automation
Moving SQA upstream on your next software project might seem like an insurmountable undertaking. However, it may be easier than you think. The goal is to identify and fix bugs earlier by including SQA in the planning/design reviews and minimizing the introduction of bugs during earlier phases of the project. Ensuring SQA is an early foundational cornerstone on your next software project will help to lower costs in both the short and long term.
SQA should not be considered a single person or a constrained ideology. SQA should be considered a quality mindset that each project team member can and should embrace for the overall success of any software project. When stakeholders see and appreciate the value and benefits SQA brings to a project when it is adopted earlier in SDLC, they will more likely become advocates for early adoption on future projects.


Like humans, animals have extraordinary abilities, too, abilities that are used and utilized to adapt to the environment. There are a plethora of animals with the best sense of smell, species that have a keener and more remarkable ability than others.
If you want to know which animals are this, keep on reading because we have prepared a great list of animals that dominates fin the ability to have the best sense of smell.
Humans have 396 genes for olfactory receptors; however, animals have more OR genes. It is smaller than the 768 olfactory receptors found in rabbits. However, more animals have a high olfactory receptor count, a gene that gives an excellent ability to smell.
You must be looking forward to this, so let’s get started on the list of species with the best sense of smell right away, shall we?
Table of Contents
List of Animals with the Best Sense of Smell
Let’s get into our list of animals with some amazing smell sensors.
Among the great animals that can smell the best are bears. Its sense of smell is 300 times better than humans. Bloodhound, a dog species also have 2,100 times better sense than a human’s.
Since the olfactory bulb area in the bear’s brain is five times larger than the human brain, bears have the best sense of smell of any animal in the world. This kind of ability makes bears scary and fascinating at the same time.
Bears have enormous noses, each of which contains thousands of smell receptors. As a result, the species is ranked first among the species with the best sense of smell in the world.
Bears are capable of detecting a deceased animal from up to 20 miles away due to their exceptional ability and sharp sense of smell.
When camping or in the woods, especially when bringing food that is appealing to bears, the dominant smell receptors, which are 100 times more sensitive than those of the human nose, will detect the scent of the food being brought.
If you do not store your food in an odor-proof plastic bag, it will be quite challenging to keep a bear away from your food in this situation.
See Related: Animals that Start with X
When thinking about what animal has the best sense of smell, the sharks are among the list. This predatory fish relies mostly on its ability to smell. Smelling is so important to this creature that two-thirds of its brain is dedicated to the olfactory bulb.
Since of this, sharks are frightening predators because they can detect the presence of a wounded marine animal or even an injured human victim from a mile away.
In addition to using its keen sense of hearing to seek food, the shark makes use of its powerful sense of smell. The nostrils of this species can be found on the bottom of its snout, on either side of its mouth.
They are aware of their surroundings by hearing and their excellent sense of smell, which is made possible by the shark’s amazing ability and body structure, allowing water to flow continually over its nostrils.
With the help of its incredible ability to smell, a shark can recognize a drop of blood in almost 100 liters of water!
See Related: Endangered Species in Oklahoma
When talking about animals with the best sense of smell, Elephant is part of the list. According to research, this animal possesses the best sense of smell, and it is possibly the strongest ever discovered in a single species. Elephants have almost 2,000 unique genes that enable them to identify scents in their environment.
Elephants have a high sense of smell and can discover water sources from up to 12 miles away, thanks to their keen sense of smell and long-range. According to previous research, African elephants are able to discern between odor molecules with exceedingly minute structural changes that humans and other primates are utterly unable to detect.
Their nostril is placed located at the tip of the trunk and is responsible for breathing, smelling, and obtaining water from the environment. Elephants’ sense of smell is constantly used, making this animal among animals that smell the best. Its trunk is always used that helps to detect new smells and information.
See Related: International Elephant Foundation
Wondering which animals have the strongest sense of smell? Snake is definitely among these animals. Its remarkable ability of keen sense of smell makes it a fascinating animal.
Snakes do have nostrils despite they don’t have a conventional and can smell with their tongues. The tongue of the anaconda, ground-dwelling snakes, and other aquatic snakes is used to detect scent.
Snake sniff through the mouth; it relies on tongue-flicking gestures to gather airborne particles where they are able to smell through the mouth. It facilitates the collection of samples from the air particles, soil, and water and the analysis of the various compounds.
These animals use their tongues to detect all kinds of scents in the air, which allows them to identify a smell and locate prey or to know where danger is lurking. Like the sharks, snakes use their keen sense of smell to be aware of their surroundings and survive.
See Related: Best Wildlife Conservation Job
Dogs have a powerful nose that makes them among the animals with an amazing sense of smell. Generally speaking, dogs are considered to be good smellers, as evidenced by the fact that police and military service dogs are trained to detect explosives.
However, despite the fact they do not have a particularly big number of olfactory receptor genes, their sense of smell is 100 times better than humans. Bloodhounds are a dog species that have an exceptionally keen sense of scent and have 300 times better smelling abilities than humans.
The portion of a dog’s brain that is dedicated to evaluating odors is approximately 40 times larger than the portion of a human brain. These animals are drawn to odors that are new, novel, and intriguing to them.
Dogs have an extremely delicate sense of smell and a very powerful one because of their keen sense of smell that dogs are trained and employed to detect bombs and drugs. They can also track down individuals based on the smell of their clothing and discover bodies.
See Related: Best Book on Endangered Species
Small but with an incredible sense of smell. The olfactory bulb of the kiwi is indeed the second largest among all birds in terms of its relation to its forebrain size. It is the reason why it is among the animals with the best sense of smell.
For the simple reason that they are unable to fly and that their food is located on the ground, Kiwis have developed an acute sense of smell.
It is exclusively by the use of their sense of smell that they can locate earthworms. Kiwi’s nostrils are also positioned near the tip of its bill, which is a unique adaptation that facilitates detecting and locating food much easier for the bird.
See Related: Amur Leopard
Animals with the best sense of smell include huge tamed cloven-hooved herbivores like cows. This animal has a keen sense of smell, as evidenced by the presence of 1,186 olfactory receptor genes compared to humans.
Its strong nose was capable of detecting odors from as far away as five to six miles away. This skill of the cows was valuable in situations where they could detect impending danger.
Aside from possessing a keen sense of smell, cattle are also equipped with outstanding hearing capabilities. Even though the cows’ primary senses were sight and hearing, their excellent sense of smell also benefited them in understanding what was going on in their environment.
See Related: Environmental Organizations in Africa
You might be wondering why there is a moth on the list, and the answer is simple. The moth, namely the male silkmoth, is the world’s best among all of the insects. A male moth can sense the scent of a female moth from more than seven kilometers away. This demonstrates how intense and acute the animal’s sense of smell is.
In order to extract smell molecules from the air, male moths use their feathery antennae to do so.
Their antennae detect scent components as they fly, rather than utilizing their nostrils to smell them as most would normally do. A single scent is detected by their highly developed sense of smell, which has evolved to detect only the scent of female moths.
See Related: Best Posters on Saving Earth
If you’re looking for answers of which is the best, rat or mice, in terms of sense of smell, the Rats beat the mice. Rat is among the animals with the best sense of smell.
They have strong sniffing abilities, which allows them to have a more varied and versatile diet than cats, which can smell out but are extremely picky eaters. They are not very well-versed in detecting odors that what rats can do.
The presence of 1,207 olfactory receptor genes in rats contributes to the animals’ excellent sense of smell. A piece of skin on the back of the rat’s neck comprises smell receptors, allowing air to pass past their nose.
When a rat smells, the fragrance particles travel to their vomeronasal organ, where they process it. Rats also use this organ to detect and distinguish between different individuals of the same rat species by detecting pheromones.
See Related: Best Conservation Posters
Horses are among the animals that have a high sense of smell, and they are also among the most intelligent animal. Even though their sense of smell is not as good as a dog’s when it comes to scent detection, they have a far better sense of smell than humans do.
Through this skill, horses are able to distinguish between predators and other horses, and the anatomy of their nasal cavity has provided horses with a relatively big nasal cavity to use. Furthermore, due to the large size of the horse’s head, there are a disproportionately large number of receptors.
When they inhale a scent that they are intrigued by, their flexible nostrils flare. It is similar to how dogs learn about one other by sniffing one another’s backsides, although horses are less straightforward in their communication when meeting for the first time.
See Related: Environmental Organizations in Europe
Opossums are marsupials, which means they are mammals with a keen sense of smell. It is similar to the development of kangaroos and koalas, where part of their development takes place in the mother’s womb, and the rest takes place in her pouch.
Because of its limited vision and hearing, its sense of smell and its excellent sense of touch take precedence over these senses.
These genes are found in more significant numbers in this species than in humans, who only have 396 olfactory receptor genes in total. Because of their vast number of OR, opossums are remarkable animals who have a keen sense of smell.
There are notorious foragers who are frequently observed sniffing in gardens and grounds in search of a food source.
A high sense of smell helps opossums adapt to their surroundings, allowing them to survive in urban environments.
Aside from having incredibly keen olfactory capabilities, this animal is also recognized for having an exceptionally powerful immune system, which allows it to defend itself against illnesses and toxicity from the venom of the snakes on which it preys.
See Related: Types of Lions Around the World
12. Chinese softshell turtle
The Chinese softshell turtle, also referred to as the Asiatic soft-shelled turtle, is a reptile and among animals with the best sense of smell. In addition to China and Hawaii, this species can be found in other places of the world as well.
The fact that this reptile made it onto this list was attributed to its 1,137 olfactory receptor genes that are almost close to the number of the Cows OR.
The Chinese softshell turtle is a type of softshell turtle that is also considered to be an invasive species in the United States. It has a maximum height of 13 feet, depending on gender.
These turtles are olive in color, and they may have dark markings on their bodies as well as a long-looking snout. This animal uses its exceptional sense of smell to capture prey and determine whether there is a predator or incoming harm in its surroundings.


What is the best way to display a variety of surfaces in such a way as to encourage many people to interact with them? Stage an exhibit. In March of 1996, the Providence Art Club, one of the oldest such clubs in the country, hosted the show ``Surfaces Beyond the Third Dimension'' in their Dodge House Gallery. The first incarnation of that exhibition was for two weeks, and the gallery book includes the signatures of dozens of visitors, including artists, students, and mathematicians. The physical exhibit has long since been dismantled, yet the show lives on as a virtual experience and we still receive comments in the on-line guest book.
The Dodge House Gallery has a square base and one interior partition, providing well-lighted wall space for twelve large photographic reproductions of computer graphics images as well as an alcove for display of a continuous videotape featuring two three-minute videos. There was a guidebook that gave information about the nature of the objects, including technical descriptions of the software and hardware used in the design of the objects and the Ilfochrome process used in their reproduction. Additional pages gave mathematical descriptions of the various pieces, as well as references to places where they had appeared either in research articles or as illustrations in books and journals. There was also a well-attended afternoon gallery talk, describing the origins of the project and including a guided tour of the exhibit. All of these aspects of the physical exhibit are enhanced in the virtual counterpart. In a certain sense, the on-line version contains much more than the original. To what extent does it capture and augment the experience of those who visited the actual gallery opening, and came back to see and respond to the images on the walls? There are many questions raised by this means of portraying mathematical art and design, and we will address some of them now.
What is it that we are showing? Most of the images included were originally studied as abstract geometric constructions given by parametric surfaces in three- and four-dimensional space. In some cases, there is an elaborate theory behind the illustration, whereas in other cases, the phenomena are not yet well understood. In several instances, the display itself represents an innovation not only in the method of displaying a surface but in representing its mathematical properties in a way that suggests new results. Visitors to the virtual exhibit can read about the mathematics behind any of the images via a link from the main page for the image.
It is clear that this electronic gallery faithfully reproduces a great many of the aspects of the actual exhibit, while altering the experience in other ways. In some cases, the electronic version loses information, while in others it provides the opportunity for significant enhancement, particularly in satisfying the viewer's curiosity about the different parts of the mathematics and computer science that made it possible for us to produce these works. We are grateful for the opportunity to present our work in a way that will continue long after the physical exhibit has given way to the work of other artists, and we look forward to further responses from visitors to our virtual gallery.


Descriptive definition is - presenting observations about the characteristics of someone or something : serving to describe how to use descriptive in a sentence. Painting essay example- sample essay on how to describe a work of art or a painting. I'm writing a descriptive essay on running i need an attention getter and any other ideas i can use i don't know how to do that with describing this. Essay about describing person place, teacher, sensory imagery enhances descriptive essay describing someone close to you can your life story is to ask that.
Do you know how to write a descriptive essay describe something or someone you’re very of your essay in which you’ll be doing the most describing. What is a descriptive essay answers, writing tips, and 100 examples of topics. Writing about hair: the thick and thin of descriptions descriptive hair phrases the thick and thin of descriptions erin.
How to describe people - appearances how to describe someone in english learn english basics. How to write fanfiction how to write a funeral scene share this site with your fellow the pastor or someone will say final words and maybe even a. Check descriptive essay descriptive essay writing: person, event celebration touch and taste will show the readers that the writer is describing descriptive.
Descriptive essay outline developing a descriptive essay outline helps in writing a good descriptive essay this will facilitate better exposition of the topic. Descriptive essay detailed writing guide including essay structure patterns, introduction and conclusion techniques, useful examples, tips and best practices. What are some good ways to describe a smile especially if you need to write an essay on how about describing someone's smile — not by what it looks. Here's how to make a valedictorian-quality descriptive essay, how to write a descriptive essay on a person: write about someone who genuinely intrigues you. “show, don’t tell” – this is the cardinal rule in writing a descriptive essay but even with elaborate illustrations showing the specifics of its topic, this type of essay can easily seem like a senseless rambling if a proper descriptive essay outline is not followed.
How to write a descriptive essay if you are describing an have someone else ready your essay and then ask them if anything needs to be clarified or if. How to write a descriptive essay search the describing descriptions i would bounce all the way up the porch and rattle the door until someone finally let me. List of descriptive essay topics when describing a person, someone who means a lot to you a person that frightens you.
Instead of a story simply write a short piece of purely descriptive writers never describe for the sake of describing but someone still was yelling out. Descriptive essay: definition a short and a long conversation that you've had with someone about a descriptive writing: definition, techniques & examples. Adjectives to describe a person some words are better suited to describing the physical appearance of someone, has its own set of suitable descriptive words.


The Student Experience
WiSE Beyond Their Years:
Women in Science and Engineering
By Deanna Duff
Elise Groves is proud to continue her family's engineering legacy – with a twist. Upon graduating from the UW in 2014, she will become a third-generation engineer. Following in her father's and grandfather's footsteps is rewarding, but it has also been a journey to find equal footing. As the first woman in her family to pursue engineering, she couldn't always look to them for role models.
"Engineering is a male-dominated field and sometimes it's a struggle to attend class and realize you're the only female. In the back of your mind, you wonder if choosing another field would be easier," says the 21-year-old. "It's programs like WiSE that made me realize female engineers really do exist."
Women in Science and Engineering (WiSE) is a nationwide program that endeavors to recruit and retain female engineering and science students. UW WiSE is one of many successful programs in the College of Engineering Student Academic Services (SAS ) focused on increasing the number of female engineering students. And WiSE is working.
Creating equal gender opportunities benefits the sciences and engineering. "We bring our own, unique perspectives and when you combine women and men, it creates a fuller, whole picture," says Groves.
Anne Goodchild, assistant professor of civil and environmental engineering, understands firsthand the impact of WiSE. For her, the seed to pursue engineering was partially planted when a WiSE group presented at her California high school. "In hindsight, it was really powerful. Even for upper-middle class students like me, the exposure made an impact," says Goodchild. "There are so many messages sent to girls that they should do other things and it undermines their confidence. WiSE makes a difference in helping counteract that."
UW Leading the Nation in Women in Engineering
The UW's efforts with a variety of programs to increase the number of female students have yielded solid results. According to the National Center for Education, in 1988 only 15.4% of engineering graduates nationally were women. Over 20 years later, the American Society for Engineering Education (AS EE) reported that the UW's 24% undergraduate rate in 2011 was ahead of the 18% national average. While parity has yet to be reached across the board, some engineering disciplines demonstrate particular progress. According to AS EE, for the 2010-2011 academic year, 48% in UW industrial and systems engineering graduates were women, 41% in bioengineering and 37% in chemical engineering.
"Every one of those fields is known to have strong, positive social impacts. Women tend to choose engineering areas where they see more social benefit," says Suzanne Brainard, current executive director for the Center for Workforce Development. Brainard, who designed and implemented the Women in Engineering Initiative and formerly oversaw WiSE, believes that reframing the discussion was essential to making progress. "This isn't an issue of 'fixing' women. Research showed that women don't decide against engineering or drop out of programs because of personal failure. Their grades are often better than the men," says Brainard. "We had to work towards institutional change. How can we (the education system) be more supportive and emphasize these fields as available options to young women?"
Impressive progress has been made in the last 20 years, but there is also much that remains to be accomplished. "It's labor-intensive to run these programs and takes a lot of people power and financial support," says Brainard. To safeguard the progress already made and to continue moving forward, ongoing and vigilant advocacy is essential.
Engaging Students Early
A major way that WiSE helps the UW achieve that mission is through early outreach. "The transition from high school to college and the first two years at university are crucial in encouraging young women to pursue disciplines such as engineering," says Cathryne Jordan, WiSE program manager.
Many female students are not overtly or intentionally discouraged. Rather, they are deterred by feelings of isolation and the lack of role models. "Some people don't mind being in the minority, but it can be overwhelming to come from a typical high school where the classes are about (gender) even one of only a few girls in a class of 100. No one really has to say anything (negative) for it to feel intimidating," says Alexandra Gramling. Gramling served as a WiSE board member and graduated this year with a mechanical engineering degree. She will pursue her engineering master's degree at the UW this fall.
In addition to providing students opportunities to connect with their peers, having female, academic role models is equally important. Anne Goodchild specifically chose to come to the UW as a faculty member because of its welcoming environment. "Our department has hired a lot of really talented young women. I think in the long-term, we (female professors) support the younger generation by obtaining positions of leadership and influence and that filters through," By Deanna Duff says Goodchild.
A Community of Support
WiSE offers several events and partners with the Society of Women Engineers and Phi Sigma Rho, a sorority of women engineers, to provide ongoing, community support. WiSE receives some university funding, but is largely supported by industry contributions, individual gifts and volunteers. Students and alumni serve on the advisory board planning and overseeing projects. To fully connect the various communities, the annual WiSE conference is the program's flagship event hosting high school students, current and pre-engineering college students, faculty, alumni and industry leaders. Held every winter, a few hundred attendees network and attend lectures and workshops. This not only creates a sense of community, but also encourages long-term consideration of how to eventually integrate into the workforce.
For major industry leaders like Microsoft, Boeing, BP, CIS CO and Intel (among the many WiSE corporate sponsors), cultivating exceptional women engineers is mutually beneficial. "There aren't many companies today that aren't building worldwide products. Any aspect of diversity that you can bring to the team, including women, is critically important. The more collective knowledge, the better the product in the end," says Debra Dove, a 1998 UW computer engineering graduate. Dove works at Microsoft and, as an alumna, has repeatedly participated in WiSE events.
The current generation of students who have benefited from WiSE are already doing their part. They have learned the value of not only being great engineers, but good citizens who want to inspire future generations.
"When I have kids, whether they're boys or girls, I hope they'll be engineers!" says Gramling.
Interested in WiSE? Consider these opportunities:
A quarterly session where you can mentor aspiring female engineers.
WiE Speaks Lecture Series
Share your expertise with female engineering students as a presenter.
Women in Engineering Conference – March 2, 2013
Volunteer as a speaker, mentor, or participant in this annual event.
Contact Joseph Sherman at firstname.lastname@example.org if you are interested in any of the above volunteer opportunities.
Give to the Suzanne G. Brainard Scholarship for Diversity in STEM
This fund awards an annual scholarship to an exceptional student who impacts STEM diversity.


Brought to you from FIRST THINGS by Allan Gillis
“Most of those pushing for women deacons are interested not in restoring an aspect of early Christian history but in acquiring political power in today’s Catholic Church.”
The True History of Women Deacons
When Pope Francis announced his willingness to appoint a commission to study whether women can serve as deacons in the Catholic Church, my first thought was: Here we go!
And sure enough, FutureChurch, the liberal Catholic organization that has subtly pushed for the ordination of women to the hitherto all-male Catholic priesthood, not only praised Francis for his statement but announced its intention to set up a CatholicWomenDeacons.org website, sponsor a retreat for women feeling the “call” to become deacons, and, clearly most important of all, lobby the U.S. bishops to start pestering Rome about opening the diaconate to the female sex. The less subtle Women’s Ordination Conference (WOC) faulted Francis for not going all the way and opening the priesthood to women, but it did offer him some limited praise: “WOC advocates that a new commission on the diaconate include discussions on priestly ordination for women in the Roman Catholic Church.”
The “discussions” WOC has in mind seem to be historical in nature. In its press statement, WOC invokes “historical evidence” of the existence of “several women deacons” in the early Church and asserts that, in ordaining women deacons, the Vatican would merely be “recognizing its own history.”
But a look at that history may cause us to doubt whether the aspiring women deacons of today really are in line with the historical Christian women they claim as their forebears.
There is no question that women deacons, even ordained women deacons, performed an ecclesiastical function for many centuries in the early Church. Exactly what that function was, however, is difficult to figure out. We know of Phoebe, the “deacon of the church,” whom Paul mentions in his letter to the Romans. But the Greek noun “diakonos” that Paul uses—which in classical Greek had no specific female gender—meant “servant” during the first century. The same noun is used in John’s Gospel to denote the servants who filled up the water jars during the wedding feast at Cana, and it appears in the Acts of the Apostles with respect to the seven men who were chosen to feed to the poor while the apostles focused on prayer and preaching.
Christian documents of the third and fourth centuries provide more detail, and they seem to depict an increasingly elaborate and formally honored role for women deacons. These documents refer specifically to deaconesses (“diakonissai”), and at least one mentions a bishop’s laying his hands on these women in a kind of ordination ceremony that recognized their special office. Other early documents, mostly from the Eastern Church but also occasionally in the West, name some women as “deaconesses.” These women were almost invariably either widows or celibates who had chosen some form of the monastic life. Their duties largely consisted of charitable works and participating in the baptisms of adult women, in the days when immersion baptism was universal and it would have appeared scandalous for a male priest to immerse a naked woman. These deaconesses sometimes assisted priests at the liturgy—a not uncommon practice among nuns whose only male contact was their priest.
The nunnishness of the early Christian women who became deaconesses is striking. And it is a feature that persisted, even into fairly recent history. Deaconesses disappeared from both the Western and Eastern Orthodox Churches during the Middle Ages, when the office of “deacon,” with its specifically liturgical functions of preaching and reading the Gospel, became a formal part of Holy Orders and thus open only to men. Only the Armenian Church continued to ordain deaconesses to serve at the altar, up through the early twentieth century. And it should be noted that every one of those Armenian deaconesses was a nun, often the abbess of her convent.
I suspect that few of the women who suddenly feel the “call” to the Catholic diaconate in light of Pope Francis’s statement will likewise be willing to enter nunneries—much less to adopt the very feminine title of “deaconess.” Most of those pushing for women deacons are interested not in restoring an aspect of early Christian history but in acquiring political power in today’s Catholic Church. Their goal is the priesthood and, ultimately, the episcopate—just as it was for women deacons when the Anglican Church began recognizing their office as part of Holy Orders during the 1960s. Our aspiring women deacons view the female diaconate not as a historical revival, but as a camel’s nose in the clerical tent.
Charlotte Allen is a writer living in Washington, D.C.
First Things is a great online/print journal! Be sure to check them out!


Assessments for MBER-Bio
We have found that assessment practices are highly individual and variable from teacher to teacher. Rather than embed all assessments into the curricular sequence on each triangle we have collected a range of assessment types from teachers who have been implementing MBER Bio in their classrooms. In this way you will have some options to draw from as you design your own assessment scheme that works for you. We hope that you will use both formative and summative assessment practices. Please be certain to read the general information provided through the three buttons below about our philosophy on Summative Assessments and Formative Assessments-- as well as the Challenge Questions that are already integrated into the curricular resources.
The table below provides teacher-generated assessments for you to use and modify throughout the year. You may sort by Triangle or by any other column. When you click on the item description, you will be taken to a new page with information specific to that particular resource. You will see a link to an attached pdf, doc, spreadsheet, etc. that you may download to your local computer. We've chosen to include several items in the filename including the model code (when appropriate), category and type of assessment, a few descriptive words, and the username of the author (unless they chose to be anonymous). You'll also notice these assessments do not contain the MBER logo: they are the creation of users just like you. Whenever possible, we've asked the authors to give us an editable version of the resource. In many cases this was not possible. However, we expect that ALL of these resources will require modification for your classroom.
Tips for how to productively incorporate assessment while simultaneously tracking points that ultimately allow for numerical grading can be found through the links below and littered throughout the individual resources in the table.
Please also see MBER Essentials to learn more about assessment tools.
We hope that, over time, this page will become a repository of sample assessments from users like you implementing model-based instruction in their classrooms!


This work analyses Frida Kahlo’s creative process, particularly the one that took place as an aftermath of the tragic experiences the Mexican painter went through. Born in 1907 and deceased in 1954, Frida Kahlo was engaged in a tumultuous relationship with artist Diego Rivera. It is well known that this union had a tremendous impact on her, not only as a woman but also as an artist. Our focus will be to question the painter’s intimate world in liaison with her creative process, in light of the fatality that took place in 1932, which resulted in the production of the dramatic painting “Henry Ford Hospital”. This painting was groundbreaking in terms of how it dealt with the issue of miscarriage through art, as well as it offered a moving representation of the body.
[marron]Keywords :[/marron] Art, psychoanalysis, pregnancy


For antenna modelling see L B Cebik's pages at http://www.cebik.com/ which contains much introductory and advanced material on using the Numerical Electromagnetic Code NEC software.
This Java applet can be used to visualise wave motion in 2 dimensions close to antennas and other conducting structures. Highly recommended.
There is a complete set of notes by Prof Georgieva of McMaster University in pdf format at http://www.ece.mcmaster.ca/faculty/georgieva/antennas.htm which will take time to download, but contain reasonably complete maths and descriptions and novel insights.
For a propagation-of-microwaves tutorial see http://www.mike-willis.com/Tutorial/propagation.html by Dr Michael Willis, G0MJW (and the Rutherford-Appleton Lab).
The purpose of a transmitting antenna is to radiate electromagnetic waves into "free space" (usually, but not necessarily, air). The power for this is supplied by a "feeder" which is often a length of transmission line or waveguide having a well-defined characteristic impedance. One can regard an antenna as a kind of "transducer" to turn generated electrical energy into radiating energy. The acoustic equivalent of an antenna is a loudspeaker, (or microphone in the case of a receive antenna).
Antennas are also used in "receive mode" to collect radiation from "free space" and deliver the energy contained in the propagating wave to the feeder and receiver. Usually, antennas are reciprocal devices; their essential properties do not depend on whether they are used as transmit or receive devices. So an efficient transmit antenna can also be used as an efficient receive antenna for the same kind of signal. The directional pattern also does not depend on the transmit or receive mode usage. These properties are collected together and called reciprocity .
An "isotropic radiator" has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the antenna. It is a reference radiator with which other antennas are compared. If the power supplied to the isotropic radiator is P watts, the energy density (watts per square metre) at a distance R metres from the centre of the radiator is P/(4 pi R^2). This is because we are spreading the power P uniformly across the area (4 pi R^2) of a sphere of radius R.
Since propagating electromagnetic waves are transverse, the electric and magnetic field vector directions are at right angles to each other and also at right angles to the direction of travel of the wave.
Clearly then, we cannot realise an isotropic radiator in practice since there will be places on the unit sphere where we cannot specify a unique "polarisation direction" for the direction of the electric field. (For example, the lines of longitude on a sphere all meet at the poles, and the directions N and S are not defined at the poles).
This is sometimes called the "hairy ball" problem. Can you comb a hairy ball so that there is no parting or point of baldness anywhere on the ball? No; there must be a discontinuity in hair direction somewhere.
For this reason, it is impossible to construct, or even envisage, a perfect isotropic radiator. (If one allows more than one polarisation mode, one can approximate an isotropic radiator for some practical purposes. However, this can be argued to be a superposition of two antennas, and the polarisation properties are not isotropic even though the power summed over the two modes may be, approximately. To receive isotropic radiation from such an antenna over the sphere, one would have to construct a receive antenna whose polarisation properties depended on where it is located with respect to the source. An example of this dual-polarisation use is provided by the turnstile antenna, and tailored stacked turnstiles may be made a good approximation to an isotropic source in this sense.) It is however possible to have uniform radiation in all azimuth (see below) directions, or in all elevation directions at a particular azimuth plane.
Such an antenna, having uniform radiation azimuthally, is called "omnidirectional". This term is a misnomer, as the antenna is not isotropic and the radiation strength will decrease if we increase the size of the elevation angle. Thus an "omni" antenna does not radiate equally in all directions.
Imagine you are standing upright on the ground. If you look straight ahead you are looking "along boresight". The boresight direction of an antenna is usually taken to be the direction along which the radiation is most highly concentrated. There can be, therefore, more than one boresight direction (eg, a vertically orientated half-wave dipole has uniform radiation in the azimuth plane, and any direction in this azimuth plane may be defined as boresight.)
If you look around you horizontally, you are looking in various "azimuth" directions. The azimuth angle varies from 0 to 360 degrees, allowing you to look in every horizontal direction.
If you look up or down with respect to your local horizon, the angle of view up or down is termed the "elevation". The elevation angle varies from -90 degrees (straight down) to +90 degrees (overhead).
We shall call the elevation angle "theta" and the azimuth angle "phi". The distance from the antenna is the radius "R". These are commonly termed "spherical polar co-ordinates".
The set of angles phi(0 to 360 degrees) and theta(-90 to 90 degrees) allows us to specify any radiation direction uniquely. In the far field region (see below), the electric and magnetic fields fall off proportional to distance R; that is, they go as 1/R. The power therefore falls off as (1/R)^2 and the total radiated power over the entire sphere surrounding the antenna is independent of distance R.
Given a set of spherical polar co-ordinates (R, theta, phi) we can determine the power density in watts/(square metre) for both the antenna being investigated, and the isotropic reference antenna which is radiating the same total power. The ratio of these power densities gives us the "directivity" of the unknown antenna in the direction (theta, phi) at a distance R from the antenna. If the direction (theta, phi) is not specified, the "directivity" is taken to be the maximum directivity of any of the directions of radiation. The quoted definition is
"The directivity of an antenna is defined as the ratio of the radiation intensity in a given direction from the antenna, to the radiation intensity averaged over all directions. This average radiation intensity is equal to the total power of the antenna divided by (4 pi). If the direction is not specified, the directivity refers to the direction of maximum radiation intensity". We should caveat that this definition is for power radiated in the FAR FIELD region.
Here, the distance R from the antenna is taken out of the considerations by defining the radiation intensity as "the power radiated from an antenna per unit solid angle". For a directional antenna the radiation intensity will depend on direction from the antenna. However, in the far field it will not depend on the distance from the antenna.
In both cases the power at large distances falls off with distance as 1/(R^2). This is called the "inverse square law" and is because the area of a sphere increases with distance as (R^2). The "solid angle" subtended at the origin by an area A on a sphere is defined as A/(R^2). The units of solid angle are called "steradians". There are 4 pi steradians in a complete surrounding surface of the antenna.
The power radiated into any solid angle by an antenna is constant in the "far field region". Thus the directivity in the far field is not a function of the distance from the antenna.
The "gain" of an antenna takes account of the antenna efficiency as well as its directivity. The formal definition of gain in any direction (theta, phi) is "power density radiated in direction (theta, phi) divided by the power density which would have been radiated at (theta, phi) by a lossless (perfect) isotropic radiator having the same total accepted input power." If the direction is not specified, the value for gain is taken to mean the maximum value in any direction for that particular antenna, and the direction along which the gain is maximum is called the "antenna boresight".
Allied to the concept of gain is the concept of efficiency. The efficiency of an antenna is the gain divided by the directivity, in any direction.
The efficiency and gain are limited by resistive losses in the antenna structure, and by resistive loss in objects which may lie inside the "near field" region of the antenna. (see below.)
The IEEE standards specifically exclude reductions in total transmitted signal arising from impedance mismatch (reflection loss) or polarisation mismatch. These reduce the transmitted signal in any particular application by further amounts, and have to be considered in any link budget calculation.
As an example of loss produced by objects close to the antenna radiating structure, there is a very substantial reduction in gain of the 10 element Yagi array antennas on the roof of BB building at about 4pm in the afternoon, when all the birds come and roost on the antennas. Birds have a high dielectric loss tangent.
Both Directivity and Gain are quantities relative to a reference antenna. Most often this is taken to be an isotropic antenna, which radiates equally in all (spherical) directions, and which cannot be made in practice. If the gain is expressed as dB it is usual to suffix the "isotropic reference" with the letter i; thus the gain with respect to an isotropic antenna in dB is called
However, antenna designers often want to know how much better a given antenna (possibly a Yagi-Uda) is than a reference dipole antenna. A dipole antenna has directivity 2.2 dBi. So it is frequently the case that we see the term dBd, where the directivity is referenced (by the suffix letter d) to a perfect dipole. For the reference dipole, the gain and directivity are assumed to be the same, that is, the efficiency is assumed to be 100%. Thus the gain with respect to a perfect half-wave dipole antenna in dB is called
Immediately, we see that there is the additive relationship
Now since 10^(2.2/10) = 10^(0.22) = 1.6596 we see that the numerical gain of a reference dipole is about 1.66 over isotropic, and that in numerical terms
These figures are good to a percent, which is adequate for practical antenna design and measurement purposes.
The propagating wave has a transverse direction for the electric field called the "polarisation direction". This normally lies along the direction of electric field in the waveguide feed, or along the conducting driven rod element in a linear antenna.
It is of course possible to radiate from a conductor which is not constructed in a straight line. However, there will still be a preferred polarisation direction.
The polarisation direction is necessarily at right angles to the line of sight joining the observer to the transmitting antenna. It is also at right angles to the magnetic field direction, which is also "transverse".
It is possible for the plane of polarisation, or the polarisation direction, to change with time, and to change with distance away from the source antenna. This is called "rotation of the plane of polarisation".
If the antenna consists of a helix, or a crossed array of dipoles fed in quadrature, then the plane of polarisation can rotate one complete cycle every wavelength. The wave is then said to be "circularly polarised". It is possible to have right hand and left hand circular polarisation.
Clearly we can rotate the plane of polarisation in time and distance by spinning the antenna physically about an axis lying along the boresight direction.
The general dependence of directivity and gain on the angles (theta, phi) is called the "radiation pattern".
In the case of a linear polarised antenna having fixed direction of polarisation, one can draw polar sectional plots in the "E-plane" and in the "H-plane". The E-plane contains the direction of propagation and the electric field vector. The H-plane contains the direction of propagation and the magnetic field vector. The E-plane is at right angles to the H-plane.
E-plane and H-plane plots are normally regarded as sufficient to characterise an antenna.
The radiated power density may fall into well-defined regions called "lobes", separated by regions of low intensity called "nulls". Strictly speaking the nulls can only be precisely zero intensity for particular directions (points from a continuous set). There is the "main lobe", which is usually where the wanted power from the antenna is directed, and "side lobes" where the antenna sends radiated energy which is regarded as "wasted" or may even interfere with other transmitting systems.
It is possible for there to be more than one main lobe having a given maximum value of gain. For example, a linear array of dipoles can have main lobes spaced 180 degrees apart, and both having the same gain.
Any radiating structure which has rotational invariance around a vertical axis will radiate equally in all directions in the horizontal plane, because there is nothing to define a preferred direction of (horizontal) radiation.
Examples are a vertical whip antenna, or a vertical dipole, or a monopole over a ground plane. These antennas radiate with the electric field vertical, and the magnetic field horizontal.
In the case of a horizontal loop or coil, the radiation is also omnidirectional but the magnetic field is vertical and the electric field is horizontal.
An alternative method of producing horizontally polarised (nearly) omnidirectional radiation is to use crossed horizontal dipoles fed in phase quadrature. Such an arrangement is called a turnstile antenna. Each dipole produces a characteristic figure-of-eight radiation pattern in the horizontal plane; these are superposed in quadrature so the pattern, looked at from above, rotates about the axis once a cycle of radiation. Turnstile antennas also radiate circular polarisation vertically; the radiation may be concentrated in the horizontal plane by stacking turnstile antennas one above the other and feeding them in phase with each other. Turnstile antennas are commonly used as transmitting antennas when horizontal polarisation is required together with omnidirectional radiation.
In the early days of FM band II broadcasting, transmitters were horizontally polarised and the electric field was in the horizontal plane. This made reception on vertical whip antennas on motor vehicles unsatisfactory, as the polarisations were crossed. An ideal receiving antenna for this configuration would have been a horizontal loop above the top of the vehicle. Occasionally one can see such an arrangement on the streets. However, this has overheads of complexity and so the band II transmitters nowadays are slant polarised or elliptically polarised, so that there is a vertical component of electric field.
The horizontal polarisation was adopted because it was found that the reception on antennas spaced around 10 metres from the ground could be maintained over a slightly greater service area than was the case when using vertical polarisation. Also it was found that the interference from unsuppressed car ignition systems was less for the horizontally polarised case, and it was believed that the multi-path reflections from aircraft flying overhead were less troublesome for this configuration. If one measures the signal strength of a band II transmission somewhere near a major airport, it is constantly fluctuating due to these multi-path effects from the moving reflecting surfaces of the aircraft.
The power density in watts per square metre is numerically equal to the rms E field in the wave times the rms H field in the wave. We remember the rms values are 0.707 times the peak values, or 1/sqrt(2) times the peak values.
We recall the SI unit of the electric field E is volts/metre, and the SI unit of the magnetic field H is amps/metre. Thus the product is (volts amps)/metre^2 or watts per square metre, as expected.
The characteristic impedance of free space (that is, vacuum or air) is 377 ohms or 120 pi ohms. This is the ratio of E field to H field. It is called Zo. Thus the power density in watts per square metre is (H^2)*Zo or (E^2)/Zo. The field strengths are therefore proportional to the square root of the power density, and they therefore fall off as 1/R, or linearly with distance.
The field strengths therefore fall by a factor of 2 every time the distance from the antenna is doubled; the radiated power density falls by a factor of 4 (or 6dB) every time the distance from the antenna is doubled.
Exercise for the reader. Calculate the electric field strength on boresight, 100 kilometres away from a transmit antenna which has boresight gain 20dB and accepted input power 10 kilowatts. Compare the received voltage from a 1 metre length of wire antenna (assumed short compared to a wavelength) with the thermal noise voltage produced by a resistance of 75 ohms across a bandwidth of 10 kHz at a temperature of 300K. Hint. Boltzmann's constant k = 1.38 * 10^(-23) watts per degree K. Express the signal to noise ratio in dB.
Apart from quantum electronic devices such as lasers and masers, radiation is always the result of accelerated charge. Since ions are so much heavier than electrons, in practice this means that radiation is almost entirely due to accelerated electrons. These do not have to be "free electrons" or even "conduction electrons"; it is possible for radiation to occur from the acceleration of electrons bound to positive ionic charge in a dielectric. We note especially that radiation does not occur from the displacement current term in Maxwell's equations if this term does not include dielectric effects. That is, normally the relative dielectric constant of a physical medium is greater than unity because of polarisation effects (displacement of physical charge). There can be a contribution to the radiated fields from this bound charge which is represented by the displacement current contribution (epsilon-relative - 1)(epsilon0)dE/dt. However, in a vacuum, the displacement current (epsilon0)dE/dt does not contribute to the radiated far fields. For a full proof of this, the reader is directed to the book "Classical electromagnetism via relativity" by W V G Rosser, Butterworths, 1968.
The contribution to the E field at large distances is proportional to the [amount of charge being accelerated] times [its acceleration]. This has dimensions Coulombs-metres/sec/sec, which is the same dimensions as the rate of change of the quantity (IL) for a current I in a little length L of conductor. Thus we see, in order to increase the radiation from a short length of antenna, we either
The effective area multiplied by the wave incident power density in watts per square metre gives the total power delivered to the antenna's feeder. This is for a receive antenna.
The effective area A of an antenna is related to the boresight gain G and the free space wavelength lambda of the radiation by the formula G = (4 pi A)/(lambda^2). This is a most important formula.
A half-wave dipole has effective area of 0.13 lambda^2, which is roughly an area lambda/2 by lambda/4. The directivity of a half wave dipole, in the azimuth direction or H-plane, is about 1.67 or about 2.23 dB. Within elevation angles of size about 32.6 degrees the dipole has higher directivity than an isotropic source; outside this range it has lower directivity.
Considering an antenna as a transmitter, if it is fed with power P (accepted power) then the power density on boresight is G P/(4 pi R^2) at distance R. Here, G is a straight number calculated from the directivity and the efficiency. It is also possible to give the gain G in decibels; remember G is a power gain so in dB a gain G of 10 is 10dB, a gain of 100 is 20dB, a gain of 1000 is 30dB and so on.
If we transmit between two antennas each of gain G, spaced by a distance R, the field strength at the second due to the first is G P/(4 pi R^2) watts per square metre, and the effective area of the second is A=G*(lambda^2)/(4 pi) so the total power transferred from transmitter to receiver is the product of these factors. The received power is therefore P*(G lambda)^2/(4 pi R)^2. This can be factorised into three parts as follows; the gain of the transmitting antenna times the gain of the receiving antenna times a "divergence factor" because not all of the power transmitted is picked up by the receiver. This latter factor is [lambda/(4 pi R)]^2 and the reciprocal of this, namely [(4 pi R)/lambda]^2 is often referred to as the "free space loss". We note that it is not really a "loss" as free space itself is a lossless propagating medium.
These antenna transmission formulae only apply in the far field region, so we need to know when we are in the far field.
In the near field region, the polar radiation pattern depends on distance from the antenna and there is reactive power flow in and out of the region. One can imagine that the energy, instead of propagating uniformly and steadily away from the antenna, has an oscillatory longitudinal component. Energy is transferred to and from the near field region which represents the reactive part of the antenna driving point impedance. As one moves further away, this oscillatory energy flow reduces leaving just the regular power flow in the resistive characteristic impedance (377 ohms or 120 pi ohms) of free space.
In the far field the polar radiation pattern is completely independent of distance from the radiating source.
The transition from near to far field happens at the "Rayleigh distance", sometimes called the "far field distance". An estimate for this distance may be made from the formula (2 d^2)/(lambda) where d is the maximum dimension of the radiating structure. In the case of a circular dish this is just the diameter; but in the case of a rectangular horn it is the diagonal distance across the mouth. This is only an estimate, and nothing suddenly happens at the far field distance thus estimated.
As an example, for a 10GHz antenna having dish diameter 30 cm, the wavelength is 3 cm and 2d^2/lambda = 2*30*30/3 cm or 6 metres. This is a sizeable distance compared to the dish dimensions.
If we consider the Tidbinbilla dish at 6GHz, shown elsewhere in this collection of notes, the wavelength is .05 metres and the diameter 75 metres so the Rayleigh distance is 2*75*75/0.05 = 225 kilometres. Thus when the dish is pointing upwards we need to be above the atmosphere before we arrive at the far field region.
For this reason, it is impossible to measure the far field radiation pattern of a deep space antenna on a terrestrial antenna range. One has to resort to complete measurements of the near field response, and computer calculation to turn them into a far field pattern. Alternatively one can measure the beamwidth by scanning across a small radio star. However it is often difficult to obtain reliable measurements of the sidelobe responses.
Part of the function of an antenna is to match the impedance of the feeder, or driving transmission line, to the impedance of free space.
A half wave dipole presents a resistive impedance of 73 ohms to a transmission line. It also has a small inductive reactance, of about 11 ohms. (The size of the reactive part depends on the length/diameter ratio of the rods of the antenna). The impedance close to resonance varies in a similar manner to a series tuned circuit. If the dipole is shortened from lambda/2 there is an additional series capacitative impedance and if it is cut too long there is an additional series inductive impedance. Thus to make a dipole which has entirely resistive impedance it must be cut a few percent shorter than lambda/2. The precise amount of shortening needed depends on the diameter of the rod elements. In general, the amounts of reactive impedance depend on the ratio of diameter to length of the antenna rods. A good discussion may be found in R S Elliott "Antenna Theory and Design", Prentice-Hall 1981, pp275ff, ISBN 0-13-038356-2, in section III.
Now we see why coaxial cable is often designed to have 75 ohms characteristic impedance.
As the dipole is shortened, the radiation resistance falls sharply and it becomes a very inefficient radiator. For example, Elliott (op. cit. p304) has a calculation which indicates that for a rod-radius/wavelength ratio of 0.2% a dipole has to be cut about 5% shorter than a half-wavelength long to have a vanishing radiation reactance (that is, to present an entirely resistive impedance to the feed) and that then, its radiation resistance has fallen to about 63 ohms. To put this into perspective, at a band II frequency of 100 MHz, the wavelength is 3 metres and a rod-radius of 0.2% of a wavelength is 0.6 cm so the rod diameter is about 1.2 cm, which is typical for such an antenna.
Incidentally, those of you who try to model dipoles using NEC (Numerical Electromagnetic Code) software may find that the returned real part of the driving point impedance stays close to 72 ohms, whatever the value of the diameter/length ratio of the rods. There is an argument that this is due to the stray capacitance between the closely-spaced ends of the thick rods. The combination of shunt capacitance with the radiation resistance in series with the residual inductance provides an impedance transformer, as is found in RF power amplifiers for example. This transformer steps up the actual radiation resistance to a higher driving point resistance; at the same time the shunt capacitance resonates with the residual inductance. There is an argument that the radiation resistance which "matters" is the driving point resistance; however, we then find that this is critically dependent on the gap capacitance and varies with the spacing of the rods, and whether they are made from solid metal or tubes.
Thus we see that these notions about the impedance of a half-wave dipole are only a guide to what we would measure in a practical installation. Indeed, a balanced feeder of characteristic impedance about 70 ohms is impracticable; so we have to incorporate some kind of balance-unbalance ("balun") transition between feed and antenna. The separation of the antenna rods also affects the total antenna length and the feed characteristics, and the physical feed structure and balun affect the near-field distribution of the dipole. It is thus possible to prefer cut-and-try methods for matching practical dipole antennas over the carefully calculated nostrums of antenna theorists.
By the time the dipole length has reduced to lambda/10 the radiation resistance has decreased to about 2 ohms and the reactance has increased to between 1 and 5 kilohms depending on the diameter of the rods.
An infinitesimally short dipole is called a "Hertzian" dipole and is important theoretically since in practice all its properties may be calculated analytically. However, it is never used in practice because of its vanishingly low radiation resistance. For many purposes, calculations on a Hertzian dipole give a useful guide to the behaviour of longer dipoles.
For practical reasons, particularly in mobile applications, it is necessary to cut dipoles short or to use monopoles loaded with inductance over a ground plane. The radiation resistance of a short dipole is given by the formula Rrad = 20*(pi*L/lambda)^2 and for a lambda/8 dipole is only 3 ohms. The series capacitative impedance for this length antenna may be as much as 1000 ohms; most of the transmission line voltage is lost across this capacitative reactance unless it is tuned out. One often sees short monopoles with a coil at the foot, to provide inductive tuning for this capacitative reactance. However, this is poor policy as it puts up the Q factor and reduces the bandwidth of the antenna. The tuning can be quite critical, especially in the presence of variable near-field obstacles.
ALL the above properties of a linear passive antenna are identical whether it is used in transmit or receive mode. There is only one exception to this rule called "reciprocity", and that is when the antenna contains magnetically biased magnetic materials such as ferrites with resonantly rotating electron spin systems.
The physical reason for reciprocity is that the only difference between outgoing and incoming waves lies in the arrow of time. Since the electromagnetic equations are invariant except for the signs of magnetic fields and currents, under time reversal, there can be no difference between transmit and receive mode in the physical current and field distributions. However, if we have a magnet providing a steady bias field, under time reversed conditions we would have to reverse the direction of this bias field. But for incoming and outgoing waves, the bias field direction remains the same. Thus it is possible for the system to be non-reciprocal.
Of course, antennas containing amplifiers, or diodes, or spark gaps, may well not be reciprocal for obvious reasons. Also, practical antenna installations having metal-oxide-metal contacts, "rusty bolts", dry soldered joints and other electrical contact imperfections are also likely to behave differently under transmit and receive modes of operation.
If we recall the definition of the Quality factor or Q factor as being the ratio of the stored energy to the energy dissipated per radian of oscillation, it is clear that in an antenna the part of dissipation is taken chiefly by the radiated energy. The stored energy is held in the near field region of the antenna structure. Since the fractional bandwidth (delta f)/f is just the reciprocal of the Q factor, for a given radiated energy the Q will be smaller and the bandwidth larger if we minimise the amount of energy stored in the near field region of the antenna structure.
One way of doing this is to make the antenna elements fatter in relation to their length. For a very fine wire antenna, the magnetic field for a given current rises as we approach the axis of the conductor, as 1/r, where r is the radial distance out from the conductor. Thus making dipole antennas out of thick rods rather than thin wires is a good method of broad-banding, up to a point.
The biconical antenna, and its derivatives, the broad-banded Yagi, the bow-tie antenna and the phantom conical antenna (which doesn't have a complete conical surface, but just conically disposed rod elements), is a good method of broad-banding a dipole type of antenna. There is a degenerate form of biconical antenna where the rods are arranged as an X with the upper v and lower ^ fed as opposing arms of the dipole. This was very common in the early days of TV broadcasting, and was also relatively broadband compared to a simple dipole. A variant of this kind of X antenna had the upper and lower arms of the > as a dipole, and the < as a reflector. The radiation pattern had a maximum in the direction away from the reflector, but again the antenna structure was more broadband than a simple H antenna. It was also easier to construct.
A rule of thumb is that a typical half wave dipole with sensible diameter rods has a fractional bandwidth of about 15%.
Given a geometrical current distribution on the antenna structure, it is relatively straightforward to calculate the radiation integrals (see any good em theory textbook) to determine the radiation patterns. Often this has to be done numerically.
As with all such activities nowadays, there is commercial software available for the PC platform to do this. One such site is at http://www.nittany-scientific.com/ which also has lots of useful links elsewhere around the web to these topics. I have not tried out this particular software so I cannot vouch for its ease of use or accuracy.
The difficulty with most antenna theory lies in determining the current distributions on the antenna conductors, given an arrangement of feeds, and the terminal voltages at the antenna ends of the feeds.
The Hertzian dipole is artificial in that it assumes there is a uniform current density along the arms of the dipole. There is thus an unphysical current discontinuity at the ends of the arms, which cannot be realised in practice. However, given a uniform current distribution, the properties of the antenna are reasonably straightforward to calculate. That is why this unphysical situation is so often presented in textbooks.
An equivalent problem pertains in simple loop antennas. Here it is often assumed that the current is constant around the loop. That is only a reasonable assumption if the loop perimeter is short compared to a wavelength. There is capacitance between the opposite sides of the loop which can carry displacement current, which results from a build up of charge due to the voltage drop around the loop which has inductive impedance. One can easily see that although there is only a single continuous conductor, the current does not have to be the same everywhere around the loop, as some of it goes to charge the stray capacitance.
Normally, self-consistent calculations are used to calculate together the current distributions and the radiated fields. The "method of moments" is popular. In this method the antenna structure is spilt up into a number of regions, on each of which the current distribution is assumed uniform. The integral equations for the antenna then reduce to solving (what may be a quite large) matrix equation. This is well adapted to computer solution. There are issues of accuracy, and sensitivity to the model framework assumed. This method is also used to work out radar cross sections of complicated objects such as helicopters and aircraft.
In a reflector-aperture antenna fed from the front by a sub-reflector and/or a feed, the far field radiation pattern can be calculated from the Fourier Transform of the field distribution across the aperture, accounting as well for phase variations across the illuminated area. The side lobe behaviour of a reflector antenna is particularly well-suited to this calculation method. A process known as "Apodisation" (after "apod" = "without foot") tapers the amplitude ("amplitude taper") illumination across a dish reflector antenna so that there are no sudden changes of excitation amplitude, especially at the edges of the reflector. We recall from Fourier Transform theory that sudden changes in a function give rise to the presence of high frequencies in the Fourier Transform. In this particular case the sudden change in spatial illumination gives rise to high spatial frequencies in the transform, which directs the energy well away from boresight as the "spatial frequency" translates into the deviation of the propagation direction from boresight.
There is a good discussion of the sidelobe suppression process by using apodisation techniques in volume 1 of the book "The Handbook of Antenna Design", publishers Peter Peregrinus, 1982, on behalf of the British IEE, editors A W Rudge, K Milne, A D Olver and P Knight, ISBN 0-906048-82-6, table 1-3 page 43. As always there is a trade-off, in this case the beamwidth of the main beam is increased by some tens of percent depending on the illumination profile, for the benefit of reducing the sidelobe levels. For reference purposes, this table 1-3 is reproduced here with minor modifications.
For a circular reflector antenna of diameter D, fed with radiation of wavelength lambda, r = fractional distance from the centre of the dish towards the rim. Distribution -3dB beamwidth level of 1st sidelobe Radian angle of 1st radians null (from boresight) (Full width at half maximum FWHM) Uniform 1.02 lambda/D -17.6dB 1.22 lambda/D Tapered to zero at edge 1.27 lambda/D -24.6dB 1.63 lambda/D as (1-r^2) Tapered to zero at edge as 1.47 lambda/D -30.6dB 2.03 lambda/D (1-r^2)^2 Tapered to 1/2 at edge as 1.16 lambda/D -26.5 dB 1.51 lambda/D 1/2+(1-r^2)^2 "Taylor distribution" 1.31 lambda/D <-40.0 dB no null? (Gaussian profile approximation?)
In array antennas, consisting of a number of identical or similar elements driven in synchronism, the problems are very similar to those encountered in filter design situations. Again, one needs to find the individual currents on each element. Often it is assumed that if the elements are identical, the currents must be also. However, consider the situation where there are four dipoles arranged in a straight line. Two of these will be end elements, and two will be interior elements. The coupling impedances between the adjacent elements will thus be different, and the currents necessarily different also.
Since the Fourier Transform method mentioned in the previous sections has an inverse, it is in principle possible to start from a knowledge of the required far-field radiation pattern, and also knowledge of the desired geometry of the source structure supporting the generating currents, and use inverse transform methods to derive the geometrical distribution of current amplitudes and phases on the source structure which would give rise to the desired far-field pattern.
Of course, whilst this is in principle no more difficult than is calculating the forward Fourier Transform to derive the far field pattern from the source currents, the difficulty, as always, from a practical point of view, comes in setting up the calculated amplitudes and phases of current elements on the specified source structure from an appropriate feed structure.
Here we list some of the common types of antenna. Apart from exotic applications, such as the banana tree, most antennas consist of a juxtaposition of conductor and insulator, which may be dielectric or it may be air or free space. This is not necessary; any structure which will support a current on its surface, or guide or modify the direction of propagation of an electromagnetic wave, may be pressed into service as a kind of antenna.
A dielectric lens may be used in front of a horn feed in a similar manner to a physical optical glass lens; but its action is then to modify the wave velocity, and therefore the curvature of the wavefront across the antenna.
This class of antenna contains important technology for satcoms applications.
The simplest kind of aperture antenna consists of a tapered waveguide transition in the form of a "pyramidal horn".
This kind of pyramidal horn aperture antenna is very important in the laboratory, as it is one of the few types of antenna whose boresight gain may be very accurately calculated (to within 0.1 dB). Consequently, it is used for producing reference field strengths and for calibrating the gains of other antennas.
A variant on the rectangular pyramidal horn is the circular horn feed. Such an aperture antenna is commonly used with a circularly symmetric waveguide mode (not the lowest mode in circular guide, NB) to produce uniform illumination of a Cassegrain antenna, which has a circular reflector dish of much larger diameter than the feed. The large reflector dish produces higher gain. The circular waveguide feed can also be used to produce circular polarisation.
Most ground based small broadcast satellite receiver dishes have a small horn feed of low gain placed at the focus of a dish between 0.5 and 1 metre diameter. Often the feed is offset from the boresight direction of the reflector dish; this "offset feed" arrangement directs the main beam away from the feed, and this results in less blockage and improved sidelobe performance.
If the main beam in a Cassegrain antenna hits the feed, or the sub-reflector, it will be diffracted around the obstacles and radiation will be scattered or diffracted into the sidelobe directions. The effective area of the dish is reduced, and the interference with other satellite systems from the sidelobes will be increased. This is not so important in deep space antennas. If we look at the Tidbinbilla deep space tracking antenna, we see there are two reflectors between the main feeds and the main beam. The sub-reflector at the focus of the large 75 metre dish is convex. The feeds are pointing along boresight, and are arranged to have a bean divergence angle which is just sufficient completely to illuminate the sub-reflector. The sub-reflector returns the energy to the main reflector, and again the reflections are arranged so that there is minimal spillover at the edges of the dish, although maintaining uniform illumination as far as is possible.
The feeds are also conveniently located at the centre of the main dish, which moves little as the dish is steered. This has mechanical advantages, and makes the final HPA and LNA electronics more accessible for servicing.
Aperture antennas such as this are used in "very long baseline interferometry" methods. Here, two or more high gain large antennas, having large collecting areas, are separated by many hundreds of kilometres, and used to synthesise an aperture array having the diameter of the baseline separation of the dishes. Radio astronomers use these systems to pinpoint the location of radio sources to great accuracy in elevation and azimuth.
If we want to increase the gain of a dipole antenna we can add another dipole antenna alongside it. This is the simplest form of array antenna.
Why is the gain increased, and what is the boresight gain of this "two element array"?
First, we assume the antennas are fed in phase with each other and spaced lambda/2 apart. Considering the radiation in a direction which is normal to the plane containing the dipoles, the contribution from each element arrives in phase with the other. The field strength in this direction is double that for one element, so the radiated power density, which is the square of the field strength, is four times that for one element. However, the two elements together are fed with twice the power of a single element. The increase in gain is therefore a factor 4/2 = 2.
This calculation scales with the number of elements. If we use a 10 by 10 array, the boresight power gain is increased by a factor of 100, which is the number of elements. The field strength is 100 times more along boresight than for a single element, so the power density is 10,000 times greater. But 100 times the power is being fed to the array compared with a single element, so the gain increase is a factor of 100 as stated.
This gain increase is over and above any boresight gain of the individual elements. If we start off with an array of 100 horn feeds at 10GHz, of size 14 cm by 14 cm each, their intrinsic gain is about 20dB and the array factor gives an additional power gain of 100 which is 20dB so the combined structure has a boresight gain of 40dB or so.
Now consider, are we "getting something for nothing" or does this increased gain along the boresight come at the expense of gain elsewhere in the radiation pattern? The answer is clearly that the array concentrates the total radiated power along certain directions at the expense of others.
If we go back to our 2 element dipole array, spaced lambda/2, there can be no radiation along a line joining the centres of the two dipoles as their contributions are in anti-phase in this direction, there being a lambda/2 path difference to get from one to the other.
In general then, the element pattern times the array pattern equals the total radiation pattern of the arrangement. What is the array pattern? It is the pattern you would observe for a set of isotropic radiators spaced as the array elements are actually spaced, and fed with the same amplitudes and phases of signals that the actual array elements receive.
If you want to read more about the fascinating subject of array antenna design, consult R S Elliott [op cit].
The argument presented in the section above just happens to be correct when the elements are spaced by lambda/2.
However, for other spacings of the elements, we run into difficulties. Really what we should do is to integrate the array pattern over the sphere surrounding the antenna, to find the averaged isotropic radiated intensity, and then compare that with the intensity on boresight.
This is the approach taken by Constantine Balanis in his book on Antenna Theory and Design. He finds that the gain enhancement depends not only on the number of isotropes, but also on their spacing.
Clearly, if we have N isotropes collected together in a very small region much less than a wavelength across, then the radiation pattern (given uniform and equal excitations of each element) will be a good approximation to a sphere, and the maximum directivity must be just 1, as it is for a single isotropic element, and not N as predicted by the arguments above.
What Balanis says is that a good approximation to the maximum directivity Dmax is given by
for N elements spaced d apart. We see that this reduces to N, quoted above, when d = lambda/2 and the spacing is a half-wavelength.
If we use two aperture antennas, spaced by a great many wavelengths, as an interferometer, the fringe spacing will be of the order of the angle subtended by an object of diameter one wavelength at a distance equal to the separation of the aperture antennas. For example, at 10GHz the free space wavelength is 3cm or 0.03m, so if we separate the antennas by 3000km or 1E8 wavelengths, we can resolve radio sources about 1E-8 radians across, or about 2 milliseconds of arc. By comparison, the beam width of one of the aperture antennas will be of the order of the angle subtended by a wavelength of radiation at a distance equal to the diameter of the reflector. Thus, if we considered a system where there were two 30 metre diameter antennas separated by 3000km, there would be (3E6)/30 = 100,000 interference fringes within the main beam of one of the apertures. Of course, the sensitivity of the interferometer is still governed by the total capture area of the two dishes; but the resolution is now comparable with that of a dish of diameter 3000km.
The interference fringes from these two circular dishes will form parallel straight lines across the circular beam, as can be seen in the pictures below:-
Two circular apertures spaced a distance apart
Measurements on antennas are difficult. The behaviour of an antenna is best seen by monitoring the reflection coefficient with a network analyser over a band of frequencies, and for convenience a frequency about 1 GHz is appropriate. At 1 GHz a wavelength is 30 cms; the antenna is a reasonable size and it is possible to investigate the effects of adjacent objects, and different feed lengths, without too much difficult physical manipulation. The results may safely be transferred to other frequency bands by thought and analogy.
When this is done, one rapidly appreciates that an antenna can not be considered as a closed, isolated component having well-defined properties. Nearly every electronic measurement on an antenna is grossly affected by its environment and physical mounting. One might well ask the question, "What is an antenna?", or equivalently, "Where does the antenna stop and the outside world begin?". A sensible answer to this question is to consider all objects inside the near field as contributing to the radiation.
A helpful example is a Yagi-Uda antenna. We might regard this as a simple dipole with lots of resonant rods placed in the near field. But if we just consider the properties of the driven "antenna", namely the driven dipole, we know we will be grossly in error in assessing the performance of the installation. So why should we stop considering the effects of metallic structures at the end of the boom? We should add in the scattering from the mast, guys, feed (outer coaxial shields can carry induced current) and even dielectric objects (like the chimney stack or adjacent building) in the near field.
Many people now have access to software which accurately simulates antenna behaviour. To do this it is necessary to construct a model. The process of "modelling" is critical to this enterprise as the simulation has limitations of accuracy depending on the kind of model chosen. In itself, the software is essentially accurate and useful. However, the results it returns, for simulation of real antennas, depends critically on what is built into the model. It is not usually possible, in the NEC2 and miniNEC and NEC4 software, to add in all the local effects which will affect the results. This is not just because it is too difficult; there are difficulties in principle, knowing the correct dielectric and conductivity parameters to put in for a real-world installation. Details of the feed arrangement are also difficult to get right. So it is often difficult to know if the results from the simulation of the model represent the real behaviour of the antenna it was intended to investigate. The process of running the software always returns a result, and the internal checks on validity, while possible, are subtle. Belief in the results often dissolves into a matter of opinion or faith. This can be the subject of strongly-held views, which can only be resolved by recourse to measurements.
Thus, simulation should be regarded (taking the most cautious view) as merely a rough guide to an antenna's behaviour in a real installation. Any modelling process needs careful validation by measurements. One is then presented with the choice of which to believe, if there is disagreement.
I have seen people worry about 1/10 dB in gain in a simulation. This is probably unsound, and one wonders how many hundreds of hours people spend (no doubt happily) in this kind of activity.
An anonymous committed modelling expert tempers the remarks above with the following comment.
Depending upon the conditions of use and application, NEC can be quite accurate. Under some conditions of application, accuracy deviation of dimensions has run to well under 1% in some design projects. In others, accuracy of construction guidance can be considerably off. The real question is this: what are the conditions for each kind of case? However, that question requires an application-by-application analysis, not a wholesale posture toward modeling.
Yes, but application-by-application analysis usually requires experimental measurements to be made to validate the models.
Considering this italic comment, what can be said with confidence is that an unvalidated antenna model is not necessarily a good guide to how a practical antenna may behave.
One of the thrusts of the complex-systems research reported elsewhere on these pages (see complexsim.html, for example) is that simulations are only of use if one knows they are going to be accurate before one validates them against measurements. Most people using antenna models are making extrapolations from validated simulations to applications that they believe to be similar. It is the thesis presented here that this method of proceeding is unsafe and unwise in many cases.