A 1/4 wave monopole antenna above a counterpoise, the so-called ground plane, needs to have a lot of very conductive metal in the direction of transmission in order to get good TX/RX in that direction. When you mount one of this type of antenna on the top of an airplane, the majority of the radiation pattern is upward; not too bad when you're on the ground speaking to the tower or another plane in flight, but a bummer when you're in the air trying to reach a station below you. For most purposes when you're trying to contact someone within 10 to 20 miles it probably doesn't make much difference where you mount an antenna. But if you want superb long distance TX/RX, mount your antennae on the bottom. Here's a location that will probably make you choke, but if you put a comm antenna on the bottom of each wing just outboard of the fuel tank you will get a good pattern and much isolation between the two. But a big part of TX/RX problems lies in using lossy coax. The best coax for the job, especially for the higher DME and transponder frequencies, is Andrew FSJ1-50. It is about the same diameter and weight as RG58U, has a total solid outer metal jacket so its EMI-RFI is as low as you can get, has half of the loss of RG58U, and can be bent in a 1" radius. The last time I checked you could get it for about $1.29/ft. So why spend big buck$ on radios and then scrimp on coax? 'Doesn't make sense, does it?
Last edited by elippse : 04-04-2010 at 08:26 PM.