I am interested in building a tracker and have a question about the transmit antenna. Would this antenna look similar to an elt antenna? I would like to install my tracker in a removable location in the baggage compartment of my RV-7A. The antenna sold by byonics is roughly 18" inch long whip. Is this the best antenna for a portable unit? My thought is to have it removable/portable so I can take it with me when I go backpacking, hiking, etc. I realize that a rubber-duckie antenna is not very good but would it be workable? The rest of the stuff is very portable. According to byonics, a 9V alkaline battery would run the system for about 9 hours. When in the plane, I would have a socket to plug into for airplane power.
Thanks,
Tom RV-7A N175TJ Flying
Very good question!
(Warning: layman's explanations to follow (because that's what I am); electrical engineers and radio experts please keep your snickering to yourselves...or jump in with better explanations
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Please keep in mind as you read this response that my experience with APRS is very limited. Hopefully Pete and others with APRS rigs will chime in with more input.
Yes, the antenna may indeed look very much like an ELT antenna....or it may look very different. There are many different types of antennae and they each have various physical mounting requirements and radiation patterns.
The typical ELT antenna is some version of a 1/4 wave whip. If it contains no circuitry that allows it to be shorter than 1/4, it will be approximately 23" since this is 1/4 of the radio wavelength at 121.50 Mhz.
We are using 144.390 for the trackers, and 1/4 wave is about 19.45". This is the number I used when constructing the 1/4 whip I installed on the bottom of my plane.
For a whip antenna to work most effectively, it needs a "ground plane" or artificial earth. In our case, the aircraft airframe serves as the ground plane. A whip with a good ground plane will usually be much more efficient than one without a ground plane. That is one reason why the rubber ducky on a handheld transceiver doesn't work as well as a ground plane antenna (the short element works against it as well).
So, the 18" whip that Byonic sells (why is it 18" instead of 19.45"?) will load the transmitter just fine, but the radiated signal will not be as good as it would if the antenna was working with a ground plane.
I have been very happy with the performance of the whip on the bottom of the fuse. Pete Howell will hopefully jump in to provide his input about using a J-pole antenna in the wingtip of his plane.
By the way, I see no reason why an ELT antenna that is designed to be mounted on the exterior of an aircraft shouldn't make a fine tracker antenna if it is trimmed slightly to optimize it for 144.390 Mhz. I have a nice little ELT antenna that I'm going to try in place of the whip to see how it performs. A SWR meter is used to tune the antenna for maximum performance at a particular frequency.
Don't know if I answered your question fully, but maybe this will get the discussion rolling.