Mounting VHF antennas

[N941WR]

Originally Posted by BobTurner

I would quibble a bit about the word ‘significantly’. A bent whip which is bent in the middle (half vertical, half horizontal) will theoretically radiate 76% of its energy vertically polarized. In practice I’d say that’s not significant. Other factors are more important. On the ground, rebar in the concrete, landing gear, etc., can screw up the underside antenna’s ground plane (plus the tower is above the airplane) so a top antenna is better. But if flying high the bottom antenna may be better (a ‘perfect’ top mounted whip cannot communicate to the ground at all!).

I have had conversations with other planes that were over 100 miles away, now much degradation could there be?

As for placement, I put the antenna under the pilot's seat, just forward of the spar. In fact, the aft two machine screws for the antenna go through the spar flange and the front two are secured to platenuts on doubler. I am thinking of adding a second radio and will put it on the other side and mount it the same way. That way you don't have to run those thick cables through the spar, which as you know has limited space to pass wires through.


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[FinnFlyer]

Originally Posted by BobTurner

One option is an Archer wingtip antenna. Properly installed, it?s still not as good as an external antenna, but not as bad as some seem to think (an unscientific test - because I used two different radios - I seem to get a useable atis on the wingtip at about 75% of the range for the same signal on the belly whip.). I use it for atis and backup, and works fine in that role. And one less external antenna, probably gains me 0.05 knots!

As far as I understand, the Archer wingtip antenna is for Nav (VOR) -- horizontal polarization.

Or is there also a vertical Archer? I can imagine that you could get partially vertical going from bottom of rib and up and out towards tip before you have to bend it horizontal.

Finn

[BobTurner]

Originally Posted by FinnFlyer

As far as I understand, the Archer wingtip antenna is for Nav (VOR) -- horizontal polarization.

Or is there also a vertical Archer? I can imagine that you could get partially vertical going from bottom of rib and up and out towards tip before you have to bend it horizontal.

Finn

Yes, there is an Archer cut for the com band. And yes, you want to run the leading edge diagonally up/down as much as possible (due to the asymmetric tip shape I ran mine from the top of the rib down to the fiberglass wing tip.) And yes, as previously noted, it is not as good (but not horrible) as a ?real? vertically polarized antenna. Since nav signals are horizontally polarized, the Archer nav antenna works quite well.

[gmcjetpilot]

Originally Posted by BobTurner

I would quibble a bit about the word ‘significantly’. A bent whip which is bent in the middle (half vertical, half horizontal) will theoretically radiate 76% of its energy vertically polarized. In practice I’d say that’s not significant. Other factors are more important. On the ground, rebar in the concrete, landing gear, etc., can screw up the underside antenna’s ground plane (plus the tower is above the airplane) so a top antenna is better. But if flying high the bottom antenna may be better (a ‘perfect’ top mounted whip cannot communicate to the ground at all!).

Originally Posted by N941WR

I have had conversations with other planes that were over 100 miles away, now much degradation could there be?

As for placement, I put the antenna under the pilot's seat, just forward of the spar. In fact, the aft two machine screws for the antenna go through the spar flange and the front two are secured to platenuts on doubler. I am thinking of adding a second radio and will put it on the other side and mount it the same way. That way you don't have to run those thick cables through the spar, which as you know has limited space to pass wires through.

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I ham radio guy. I agree. It is not practically significant. It is however something, polarization does matter. Not to drive into the weeds, RF has an electrical field and magnetic field (electromagnetic field). Polarization matches the electrical field. The orientation of element of the antenna is the polarization. Matching the polarization of the RF antenna of incoming signal gets maximum signal. If the RF antenna polarization does not match there is a loss by the factor of cosine of the angle between the polarization of the RF antenna and the signal. Theoretically it could be ZERO! However there is no pure polarization but antennas can have deep nulls and be directional. Often RF signals bounce around and polarization is mixed. Usually a VHF line of sight signal does not change much unless it bounces off objects. We are rocking the unbalanced dipole in a metal airplane. At 12,000 feet you might get 100 miles with 10 watts and a coat hanger in your mouth. Ha ha. (don't try it)

I did an experiment with an FM radio broadcast transmitter (FCC Part 15, power 100 mw) whip antenna vertical. I set up a receiver and determined line of sight range, a few blocks with receiver antenna vertical. I laid the receiving antenna down horizontal. I still got the signal but lots of static, signal became weak. Put receiver antenna back vertical and signal was again solid. If you noticed bent whips give you at least 10" to 13" vertical polarization. The electrical length is still 1/4 wave or about 20". Folding antennas is a bit of an art. It works to be sure but it is not without some losses. So when you say you got 100 miles with a bent whip, a pure vertical dipole may give you 130 or 150 miles range at 12,000 feet.

Originally Posted by N941WR

Bill R.

I did a similar thing in my old RV-4. Great location.

[emsvitil]

If you're using 2 comm antennas, would it make any sense to use the wing as a ground plane by mounting 1 whip antenna each on the underside middle of each wing to maximize distance between comm antennas and VOR antennas (assuming tip mounted VOR) ??