radio/antenna installation

[wynkoop]

It is surprising how well a cheap radio will work if it is connected to a good antenna that is properly installed. The simple rod type whip antennas will work as good as any for about the first 100 channels, then it starts loosing ground. That's why they are under 20 bucks. For a belly mount the AV-17 from R. A. Miller and the Comant Ci 122 work pretty good for the money. These antennas are wound which I'm told broadens the channel range significantly. The antenna must be mounted to a satisfactory ground plain. Most Biplanes have a large enough alluminum belly panel to create a good ground plain. A minimum ground plain would be about 18" in diameter. Try to keep the antenna far enough back so it isn't blanked out by the landing gear. Don't scrip on the antenna coax. The RG-58U has been used for years and works pretty good but the RG-400U is the new kid on the block and is a better cable, particularly for transponders. Have someone that has the proper tools make up the coax. Many transmit/receive problems come from improper connections.

I have a King KY 97A connected to the AV-17 on the bottom of my Acro Sport and it works well. Even sitting inside a metal hangar with the door down I can talk and receive with airplanes flying overhead

Your off just a bit in your idea of a good ground plane.

First let's find the middle of the com band.

137 - 118 = 19 The comm band is 19 Mhz wide

So the middle of the comm band is 118 + 9.5 = 127.5 Mhz

The formula for a quater wave is 234 / Frequency-in-Mhz, so we have 1.8352941 Feet or 22.0235 inches.
In radio a ground plane is normally 5% larger than a quarter wave, so that means 1.1 inch more for a RADIAL length of 23.1235 inches.

Now since that is the size of one radial it means that a good ground plane would be about 46 inches across. A typical 1/4 wave antenna for
VHF has 4 radials of 1/4 wave+5% arranged at 90degree intervals.

In practice if you have a metal ship, or a metal frame, any piece of metal bigger than 46 inches with the antenna at the center of it will yeild good results. If you have a glass or wooden ship you should consider either a dipole antenna or install 4 23 inch radials.


On the cable front LMR-240 is lower loss than the suggested rg-400u, and will give better results.

I hope someone finds this useful.

-Brett

[Neil]

Just said it was a good ground plain, not optimum. A lot of the tube and fabric airplanes sometimes force you to take what you can get.

[flyrgreen]

On a tube & fabric airplane, the metal groundplane that is mentioned late in the thread is essential. If you have a metal belly plate already, there you go.

[Tom Downey]

I used one of these in a All wooden aircraft and it has been working great for about 10 years http://www.aircraftspruce.com/catalo...nnasystems.php

[Mike Switzer]

Tom, did you use that in the Fairchild? Where did you mount it?

[Tom Downey]

Tom, did you use that in the Fairchild? Where did you mount it?

Yes, it is mounted (glued) to the under surface of the turtle back fabric. 12:00 high on the fuselage. lots of choices on the 24. and the VAL-Com 2000c broadcasts like a louden boomer on it.