Monday, February 22, 2010

Antenna and Radar Cross Section Measurements

Hi everyone -

Here is a video on antenna and radar cross section measurements put together by LabTV. LabTV produces educational "webisodes" for middle and high school students through the National Defense Education Program. This segment highlights the compact range facility at MIT's Lincoln Laboratory.

John Sandora




Bounce Back
Using Radar, the Answer Is in the Echo

RADAR -- which stands for radio detection and ranging -- is a technique that's used all around us in everyday life. It can determine the presence and the velocity of an object such as an airplane or even a person.

Radar works via a transmitter that shoots a pulse of electromagnetic energy. The pulse travels to a target, bounces off, and then the radar listens for the echo off that target.

Engineers at the MIT Lincoln Laboratory in Lexington, Mass., build the most advanced radar systems in the world. With the help of a special testing chamber, these scientists can test their radar antennas indoors -- before taking them out into the real world.


video

Monday, March 24, 2008

Cantenna Part I: Theory

The first antenna we’ll look at is called a Cantenna. It’s an inexpensive way to increase the range of Wi-fi networks and it’s been around for a few years now (it even has its own Wikipedia article: http://en.wikipedia.org/wiki/Cantenna).


Some basics:

In antenna terminology, this is an “open-ended waveguide”. Practically speaking that just means a hollow metal tube excited in a way that radiates electromagnetic energy. You pick the diameter of the tube according to the frequency band you want to work in. The length and diameter of the tube determine the antenna’s radiation pattern. You excite the cantenna with a coaxial cable and it will transmit your signals – it’s as simple as that!

http://en.wikipedia.org/wiki/Image:Coke-cantenna-SV1ML-0a.jpg


Uses:

As luck would have it, many can sized cylinders are good for the 2.4 GHz ISM band. This could be useful for connecting a laptop to the internet in a remote location or connecting to a free Wi-fi network from your home. Most 802.11 standards fall into this band: 802.11b and 802.11g use 2.4GHz. 802.11n can use both 2.4GHz and 5 GHz, but you can get away with only the 2.4GHz signal - it propagates better over long ranges anyway.

I have a slightly different application: The Ramsey SG7 Speedy Radar Kit operates at 2.6GHz.

(http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=SG7 ) This is a little Doppler radar kit that I’m putting together.




How it works:

Inside waveguides, electromagnetic fields can only exist in certain patterns called “modes.” The best cantenna designs excite one and only one mode. This is for several reasons: different modes actually travel at different speeds down the waveguide, known as “dispersion.” This can cause a problem for signals by spreading out pulses making them harder to receive. Also, different modes have different radiation patterns. Shown below is one transverse electric (TE) and one transverse magnetic (TM) mode – the main types for circular waveguide.

First let’s look at the TE11 mode a.k.a. the “dominant mode” for circular waveguide. Shown below is a side view of the waveguide and feeding coax. The grey rectangle is the cantenna, with a small green colored coax cable attached with the center conductor extending into the can. This should be one-quarter of a wavelength (in the guide!) from the back of the can, and it will excite the TE11 mode: electric fields are plotted in red and magnetic fields are plotted in blue.























Now suppose you fed the cantenna from the back instead of the bottom. This will excite a different mode than what was shown above. This is the TM11 mode (electric fields are plotted in red and magnetic fields are plotted in blue).





















How to pick a diameter:

As mentioned before, you want to excite only one TE mode. You do this by picking a can diameter large enough to allow the TE11 mode but small enough to block (“cutoff”) the TM01 mode. To repeat: if the 808.11b/g frequency band goes from 2.412-2.462GHz then you want the TE11 mode to work at 2.412GHz and the TM01 mode to NOT work at 2.462GHz.

Look at http://www.turnpoint.net/wireless/cantennahowto.html for an online calculator or you can run through the equations yourself:

(From Kraus’ Electromagnetics 8-5 “The Hollow Cylindrical Waveguide”)

TE11:

This mode will work if f = 2.412GHz > fcutoff = 2*c/(3.41*D)

TM01:

This mode will not work if f = 2.462GHz < fcutoff = 2*c/(2.61*D)

You want:

fcutoffTE11 < f < fcutoffTM11

2.87” < D < 3.67”

Practically speaking, while the lower bound is important – the can’s diameter MUST be larger than 2.87” to work at 2.4GHz – the upper bound is not such a hard boundary.

What’s next:

· Cantenna Simulations

· Radar and Antenna Construction

· Cantenna Improvements

References:

· Cantenna – Wikipedia http://en.wikipedia.org/wiki/Cantenna

· How To Build A Tin Can Waveguide WiFi Antenna http://www.turnpoint.net/wireless/cantennahowto.html

· Wlan antenna Waveguide type http://www.saunalahti.fi/elepal/antenna2.html

Friday, December 28, 2007

Relevant Links

With the exception of amateur radio, there’s not really that much scientific info on the web about antennas (but lots of companies are selling poorly documented things) so here is what I can find in the way of helpful websites.  

 

Relevant Links:

·          http://en.wikipedia.org/wiki/Antenna_(radio)

o        Wikipedia is always a good place to start

·          http://www.ac6v.com/antprojects.htm

o        “AC6V's Homebrew Antenna Links Including Antenna Theory”

·          http://www.arrl.org/qexfiles/0105downs.pdf

o        “Why Antennas Radiate” by Stuart Downs – the ARRL is a GREAT resource, check out arrl.org

·          http://www.classictesla.com/download/emfields.pdf

o        “Understanding electromagnetic fields and antenna radiation takes (almost) no math”

·          http://www.wa5vjb.com/

o        Low-cost PCB antennas

·          http://www.microwaves101.com/

o        A great site that deals with microwave components - an antenna has to connect to something, doesn’t it?  Check out the downloads section for a nice Smith Chart and some handy spreadsheets

·          http://www.rfcafe.com/

 

E-books:

·          http://www.ece.rutgers.edu/~orfanidi/ewa/

o        Electromagnetic Waves and Antennas, an e-book by Sophocles Orfandis

·          http://web.mit.edu/6.013_book/www/

o        Electromagnetic Fields and Energy, by Haus and Melcher from MIT’s Hypermedia Teaching Facility

 

 

Thursday, December 27, 2007

Antenna References

This blog will of course not be able to explain everything about the theory of antennas. These are my three favorite antenna reference books:
  • Antennas: For all Applications, John D. Kraus and Ronald Marhefka
    • Kraus was a genius in the field of antennas and has written an excellent textbook. If you really want to UNDERSTAND how antennas work this is the book for you. Since he passed away it is becoming harder and harder to find, so you may need to head to the library.

  • Antenna Theory: Analysis and Design, Constantine Balanis
    • Balanis is another excellent author and his book is more math-intensive than Kraus. This is the book to use if you want design equations for antennas.

  • Antenna Theory and Design, Warren Stutzman and Gary Thiele
    • This book is great because it contains a little of everything. It walks the middle road between Kraus and Balanis, but it has everything you need to know. The chapters on computational electromagnetics are outstanding.

Of course, a knowledge of electromagnetics is essential groundwork for the study of antennas:
  • Electromagnetics with Applications, John Kraus
    • Again, Kraus' book is focuses on conceptual understanding and is not very rigorous. But if you want to see the basic principles of electromagnetics explained clearly, this book is great.

  • Introduction to Electrodynamics, David J Griffiths
    • This book is the widely acclaimed introduction to using the principles of vector calculus on electromagnetic problems. If you want to see the power and beauty of Maxwell's equations, I highly recommend this book.

What is an antenna?

Though I suppose this post will be the least useful for anyone who is already on this site, nonetheless, I feel the need to discuss what it is that an antenna actually does...

An antenna is a electromagnetic transition between a circuit and the medium around it. For instance, cell phones have an antenna that radiates energy towards the tower, allowing you to talk. Radios have antennas that absorb energy, literally grabbing the signal, and hence the music, out of thin air. A physicist might describe an antenna as a device that transitions from the electrons flowing in a circuit to photons traveling through the air. Engineers view antennas as filters in both space and frequency, since antennas are only looking in certain angular regions over certain bandwidths. No antenna is perfectly isotropic, and no antenna works over all frequencies. Thus, the proper selection of an antenna for each application is very important.

Welcome to the Antenna DIY Blog

Welcome to the Antenna DIY Blog!

With the popularization of 802.11, HDTV, GPS, Bluetooth, RFID, and of course cellphones there are a lot of signals in the air these days. This blog will apply a do-it-yourself perspective to a topic that many find confusing.

I'll post some designs and ideas that I hope are useful, as well as the best references to get more information.

John