Re: Radar

Hmmm... all sorts of Far-Side-esque images of "science fair projects gone too far" come to mind.

I'm not sure what level of ambition or resource you wish to apply to this task. This project is not totally out of the question. So I will answer this in stream of consciousness fashion, and work my way towards outlining a simple demonstration.

First off, some standard engineering references to radar include

M. Skolnick, "The Radar Handbook"

D. Barton "Modern Radar Systems Analysis"

These are written for engineers working in radar, so reading these may be a bit of a tough slog, plus they won't give too many "science fair project" hints. Try the Library of Congress web page to search further, or the usual web bookseller pages.

You might try searching the Web for some tutorial info on radars, as well as searching for books at a more introductory level.

Building a simple science experiment is to demo radar is not a casual affair. First off, there's potential entaglements with the FCC, who, as you correctly surmise, would not look kindly on unauthorized radar transmissions ("dang rev'nooers!"). Secondly, a fair amount of sophistication will be needed to develop the necessary transmitter, antenna, and receivers needed.

However, it's not out of the question entirely, either. One could pursue an amateur radio operator's license with "technical class" priveleges, allowing this kind of experimentation in specially set-aside bands. Enterprising ham radio operators have built amateur radar rigs to bounce signals off the moon. The "Radio Amateur's Handbook" has some info on this, or you might try browsing the web site of the American Radio Relay League (ARRL), a ham radio operator's association, for more info. They should be able to point you at the published articles, or even put you in touch with a local ham radio club to provide some advice on this adventure.

And if cost is REALLY not an issue, navigation radars for pleasure boats can be bought commercially, as can weather radars for private aircraft for about $10,000 an up. Or why buy new when old will do? A Web search will actually turn up dealers for military surplus radar sets. Be the first on your block with a working air defense radar. Just see if that doesn't stop the neighbor's dog from messing in your yard :-)

But I digress.

Another thought: you could see if the local airport control tower is willing to give you a tour of the radar control room. They would take a dim view of allowing any science experiments, however.

But if it's scientific principles you're after, why not abandon the RF spectrum for the acoustic spectrum? The basics physics and math are the same for both radar and sonar. A sonar merely sends outs blasts of sound waves and measures the reflections.

It used to be that you could buy experimenter's kits with ultrasonic transducers and instructions for simple electronics to construct an ultrasonic rangefinder. Perhaps your local electronics retailer (e.g., Radio Shack) still sells these as kits, or electronics supply catalog houses such as Digi-Key sell them (hmmm... wait a sec... checking... checking... yes, Digkey has them). My guess is that they can be had at a quite reasonable price (certainly well under $100). These transducers were used in the 1980's in Polaroid's XR70 range-finding autofocus cameras, and may be still used in similar cameras today.

Or even simpler yet, a pulse generator, audio power amplifier, speaker, microphone, amplifier, and oscilloscope can be used to create a simple "radar" demonstrating what's called an "A-scan" radar set. This is what radar operators looked at in the old days.

Set up the pulse generator to emit a pulse (say, once per second with a pulse width of, say, 10 milliseconds), input it to the audio power amplifier and connect the power amp output to the speaker. (Caution, experiment carefully here as you are liable to burn out a speaker or three. I'd recommend an outdoor "bullhorn" PA-type speaker. It's not high fidelity you're after here, but ruggedness). Point the speaker at possible reflectors (say, a large building a hundred yards off, and point the microphone in the direction of the reflector, also. If you want, use a microphone at the focus of a parabolic dish. Toys of this nature are actually sold under the trademark "Big Ears" or some such. Try the Edmund Scientific catalog, or make your own!

Hook the microphone up to a microphone amplifier, and the output of the amplifier to the oscilloscope vertical (Y) input. Set the scope up with about a 1-second sweep and trigger off the initial pulse of sound that the mike picks up from the PA speaker. Fiddle with the mike amplifier gain and the oscilloscope gain, and you should be able to see a blip at a time delay corresponding to TWICE the distance to the reflector (the sound has to make a round trip, remember. The speed of sound is about 1100 feet per second, so for a bulding 100 yards away, the round trip is 600 feet, and the echo will return in about 0.55 seconds. That's halfway across the scope sweep.

To make this work, you will need a pretty strong blast of sound out of the speaker, and a reasonably sensitive mike. If you can hear an echo with your ears, you're well on your way.

All of these items can be had at your local Radio Shack, with the exception of the parabolic dish (which is helpful, but optional), and the oscilloscope. Perhaps the scope can be borrowed from a cooperative electronics technician. Adequate equipment is likely available in even a high school physics or electronics lab.

Becuase you're working with sound, you won't have to worry about the FCC. The setup is entirely composed of commercial, off the shelf items, so the engineering needed is minimal (just a bit of "systems integration"). The neighbors may be a bit annoyed.

The main differences between what you're doing and a radar set does is that radar works with RF energy and you are working with sound (acoustic) energy, and the time scales are different. Radar pulses travel at the speed of light, 186,000 miles per second, while your "radar" pulses travel at about 0.2 miles per second. Radar pulses are a few microseconds (milllonths of a second) long, while your "radar" pulses last a few hundreths of a second.

Hey, an even simpler thought: forget the pulse generator, power amp and PA speaker -- anything that produces a crisp burst of sounds waves will do: a starter pistol, a snare drum, a hinged pair of planks snapped together, etc. I like it! (KISS -- Keep It Simple, Stupid). That reduces the electronics to an audio mike, amp, and oscilloscope.

Steve Czarnecki

Next week's topic: "Turn those Old Lawnmowers into Lunar Orbiters"