Mounting VHF antennas
 

I am preparing to mount my vhf antennas on the belly right behind the firewall on an RV 7, have seem some right in front of the spar but I would like as much ground clearance as possible. Any know problems with this ?
TIA, Kirk S

The problem with this is that you loose ground plane forward of the antennas. Will make them somewhat directional.

I have dual bent whips mounted aft of the spar right under pilot and copilot seats. No issues.

The noisy (electrically) engine area is not ideal for antenna, along with SS firewall, engine, nose gear (as installed). More distance from these items would be better. You don't have a nice 360 degree ground plane around the base of antenna mounting near the firewall (as Mel said above). Also it's a dirty oily area in general.... With that said, if you want max clearance, mount it near firewall. Many have and it works... it's a compromise but up to you. All locations are a compromise one way or another...

Bent whip has plenty of clearance as said (for landing/TO/taxi on smooth surfaces). I think even a straight angled back VHF will work Tail dragger or Trike, but a bent whip will work. There are different bent whips. Get one that has the most vertical politicization. Bent whips where element that is parallel to fuselage is not ideal and will reduce your RX and TX range significantly. All locations and antennas are a compromise. You have to decide what is important.

Top of fuselage? You want clearance. With slider canopy you have to mount antenna a little further aft on fuselage.Longer coaxial so adds a little weight and minor loss in signal, but it will be more than be made up using a straight angled antenna with more vertical polarization (not bent) and an unobstructed location as you can get on an airplane with good ground plane. Up to you.

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!).

Just me
 

My general rule was to mount com 2 on top and com1 on bottom. Then use com2 for all ground communications because some people have had trouble with a bottom mounted antenna communicating with the tower while on the ground.

For me, I have a slider with the Supertracks. so my ELT antenna is going on top. This leaves little room for a top mounted comm antenna. So I am trying to figure out where to mount com2 besides next to comm 1 on the bottom.

JMHO

Two bent whips on the bottom, aft of the spar, outboard bay of seat ribs. No issues whatsoever. Very rarely, like once a year or so, some Ground control at an airport will report weak and scratchy. Turning the airplane a couple of degrees (while taxiing or whatever) usually fixes it. I think it's the gear leg interfering.

Don't overthink it.

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!

My RV7A



Combination GPS/Com:

Top mounted antenna
 

Note that some airports, such as Tucson International (KTUS) require that you use a top mounted antenna for ground use.

As printed on the FAA airport chart here -

http://aeronav.faa.gov/d-tpp/2005/00430AD.PDF

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.


(click to enlarge)

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.

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.

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

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) ??