Airplane Antennas

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Antenna Builder's Kit

RST-2802, $29

This kit includes a 35' roll of copper tape, a bag of 20 toroids, and the Antenna Reference Text (RST-8020) on disk. Under normal circumstances this is enough to make 6 antennas.

About the Antenna Reference Text:  Over the years, we've written some two hundred articles on all aspects of aviation electronics. Seven of them are directly concerned with the design and construction of "plastic plane antennas". We took the heart and soul of these articles and made them into a concise, attractive document. The latest edition includes ELT antennas. There is nothing that we know about plastic airplane antennas that is not in this on this disk. 2012 Edition

 

The Story of Our Airplane Antennas

Back in 1978 the homebuilt airplane world was a-changin'. Wood and metal were giving way to glass and foam. You see, fiberglass and resin are as transparent to radio waves as air, so there was no reason to put a drag-stick outside the airplane skin when the antenna elements could just as easily and as efficiently be put underneath the skin.

I won't tell you that your airplane will save 5% in drag, but it will be somewhere around this number -- depending on the cruise speed and the number of antennas your savings will be somewhere between 2% and 8% of the total drag of the airplane.

You can buy "plastic plane antennas" anywhere from the $5 or so I charge up to hundreds of dollars for antennas with "magic properties" that are "hidden in epoxy" and "tuned to the airframe". One of the people that wants to sell you hundred dollar antennas pooh-poohs my "ferrite donuts" or toroids (TOW-roids) as useless.

OK, here's the truth of it. The ferrite donuts act as nothing more or less than a very efficient low-loss "balun". A balun does nothing more than matching a balanced antenna to an unbalanced feed line, hence the name balun. In the direct-connected dipole that we use, the balun does nothing more than keeping reflected power from traveling down the outside of the coax back to your radio -- or in the case of a receiver, from the input stage of your radio back out to the antenna.

In either case, the little chunks of powdered iron prevent unwanted radiation from the outer surface of the coaxial cable braid. Think of it like the iron core noise filter you put on your alternator line. It lets the direct power through and strips off the noise. Those who say that iron machine nuts will do the same thing as toroids just don't understand much about the behavior of ferrite material at VHF. It took us the better part of three months of design and experiment to select the size, ferrite recipe, and number of toroids that optimizes match and minimizes loss.


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