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Comm Antenna Location - RV-7A

KatanaPilot

KatanaPilot

Well Known Member
I know it's customary to locate both comm antennas on the bottom of the airplane. They are less visible and there is a good ground plane with the wing and fuselage.

That being said, I'm an airline guy and with my previous light aircraft (Diamond) - one comm antenna was on top and one was on the bottom. The usual protocol on them and the big iron was #2 Comm to top antenna - better Tx/Rx for ground ops and #1 Comm to bottom for in flight.

I'm less concerned about appearance and more interested in performance. We know that a single lower antenna will be offset from centerline and some structural beef up will be required for a top antenna. Any issues or concerns I'm overlooking?
 
KatanaPilot said:
I know it's customary to locate both comm antennas on the bottom of the airplane. They are less visible and there is a good ground plane with the wing and fuselage.

That being said, I'm an airline guy and with my previous light aircraft (Diamond) - one comm antenna was on top and one was on the bottom. The usual protocol on them and the big iron was #2 Comm to top antenna - better Tx/Rx for ground ops and #1 Comm to bottom for in flight.

I'm less concerned about appearance and more interested in performance. We know that a single lower antenna will be offset from centerline and some structural beef up will be required for a top antenna. Any issues or concerns I'm overlooking?
Click to expand...

That's exactly how I have mine. I used a Comant "Comdat" VHF/GPS antenna on top with the GTN650 (com 2) and a standard Comant CI-121 on bottom for com 1 (GTR200). Works beautifully, for me it's always about function first and appearance 2nd.
 
Comant

I have both antennas on the belly with no issues. Having said that, I have a new Comant antenna that Walt references I bought from Stein and never used. I'll make you a good deal on it if you want it.
 
I've had both comm antennas on the belly just forward of the main spar for over 24 years. Never had a problem on the ground or in the air.
 
Krea,

Assuming good installation practice, it would take multi-kilo$ test gear and an antenna test range to detect improvement/degradation with top vs bottom antenna location, except very specific situations. For instance, ground ops (as you know) and trying to communicate with a ground station almost directly under you.

With the fuselage width of a -7, the ground plane will be fine with a top mount. Bigger issue is whether you're building a slider; that might handicap your location a bit.

Do what you like.

Charlie
 
Thanks for the replies. It's a tip up canopy so interference should not be an issue. Being able to customize an experimental is part of the beauty of doing this. It can also lead to unintended consequences. I think either location is satisfactory but we will probably go with upper and lower.
 
Location?

YellowJacket RV9 said:
Single bent whip comm antenna on the belly. Never had a a problem on the air or on the ground.

Chris
Click to expand...

I've had a communication problem with a similar bent whip on the belly my Tiger at Tucson (TUS) - direct line of sight to the control tower but on an old concrete ramp that apparently has lots of rebar in it.

TUS even has a note about it on their taxi diagram - bottom of the page -

http://155.178.201.160/d-tpp/1707/00430AD.PDF


Belly mount problems may be location dependent.
 
I have the most powerful transmitter on the bottom antenna for airborne work. A less powerful transmitter on the top antenna.

Rarely (like 3 or 4 times in 650 hrs), I experience poor communications from the bottom antenna when at the end of runway 35 at Asheville. I suspect it is a 7A main gear leg blanking the antenna view of the tower antenna. There is also some geography that is a possible contributor. A quick flip to the top antenna resolved it.
 
rzbill said:
I have the most powerful transmitter on the bottom antenna for airborne work. A less powerful transmitter on the top antenna.

Rarely (like 3 or 4 times in 650 hrs), I experience poor communications from the bottom antenna when at the end of runway 35 at Asheville. I suspect it is a 7A main gear leg blanking the antenna view of the tower antenna. There is also some geography that is a possible contributor. A quick flip to the top antenna resolved it.
Click to expand...

In the case of the Tiger, it's not the main gear legs since they are fiberglass...:D
 
az_gila said:
I've had a communication problem with a similar bent whip on the belly my Tiger at Tucson (TUS) - direct line of sight to the control tower but on an old concrete ramp that apparently has lots of rebar in it.

TUS even has a note about it on their taxi diagram - bottom of the page -

http://155.178.201.160/d-tpp/1707/00430AD.PDF


Belly mount problems may be location dependent.
Click to expand...

I offer that whatever problem you had, it had nothing to do with the antenna on top or bottom. In a close situation like that, a radio transmission does not need to be line of sight as many propagation paths are present. Simple example - does your radio hear anything in a closed metal hangar?

Carl
 
Carl Froehlich said:
I offer that whatever problem you had, it had nothing to do with the antenna on top or bottom. In a close situation like that, a radio transmission does not need to be line of sight as many propagation paths are present. Simple example - does your radio hear anything in a closed metal hangar?

Carl
Click to expand...

Well it was fixed by switching to my comm 2 radio (KX-170) that I only use as a back-up to the comm 1 KX-155 radio. Comm 2 uses the upper antenna and worked.
Given the very specific note on the FAA Taxi Diagram for TUS I suspect that this is a general problem for many planes.

How many other airports have that same note about using upper antenna only?
 
One bent whip comm antenna on belly, one straight on top. No problems and all bases covered. More important for me was just to have two comms for redundancy.

Bevan
 
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