Comm Antenna Question

RFazio · #1 06-22-2010, 05:23 PM

I am going to install a second antenna in my RV. We were talking about it at the airport and someone said that the wire to the antenna should be multiples of 22". I can't see how the length of coax cable could effect the radio but I figured I'd ask here. Does it matter? Should I measure the length of cable to the antenna and make it 22" multiples or is that an old wifes tale?

kentb · #2 06-22-2010, 05:38 PM

The length of the antenna is important, but I have never heard of making sure the coax is tuned. It seams possible that it could have and effect, but if it does I think that it would be hard to measure. If it is true than the number of inch to multiply by will be dependent on the frequency of the radio being used, not 22 inches for all.

All my coax cable is how ever long that was need to get from point A to point B, plus a little extra for future service. Mine work fine.

Kent

Glovebox · #3 06-23-2010, 12:07 AM

Coax length in your application is not important. All VHF airband antennas work with any reasonable length of coax.

22" is ? of a wavelength at approx. 134 MHz. Unless you were making your own antenna, it's a useless number... a little shorter than what I would use for an aviation antenna, and a little long for the two-meter ham band. Might be a reasonable compromise for both, though.

Coax cables cut to multiples of ? wavelength, however, can be useful for measurement purposes, but must take into account the 'velocity factor' of the particular coax cable used. The velocity factor can physically shorten the coax length by more than 30%. Without further calculation, such a cable is only useful in a very narrow frequency range.

Use as much cable as you need. I'd do the same as Kent and leave some extra for servicing. No harm there.

RFazio · #4 06-23-2010, 09:01 AM

I thought that was the case but figured I'd ask anyway. Thanks again.

Richard Fazio

elippse · #5 06-23-2010, 10:39 AM

Quote:

Originally Posted by RFazio

I am going to install a second antenna in my RV. We were talking about it at the airport and someone said that the wire to the antenna should be multiples of 22". I can't see how the length of coax cable could effect the radio but I figured I'd ask here. Does it matter? Should I measure the length of cable to the antenna and make it 22" multiples or is that an old wifes tale?

As others have replied, having a resonant transmission line means that you have a very narrow band, resonant antenna, which these aren't. The best thing for good DX is to use low-loss, low leakage coax such as Andrew FSJ1-50A. And if I might recommend a better mounting location for VHF Comm, put one antenna on the bottom of each wing about 1/3 of the way toward the tip, and about the chord mid-point. These 1/4 wave antennas have very poor radiation in the horizontal direction due to limited counterpoise(groundplane) and instead radiate better upward when mounted on the top of the plane. This isn't bad if you're talking to high-flying airliners or satellites, but not in flight when you're talking toward the ground. Having them on opposite wings gives lots of spacing to minimize crosstalk and interference.