J

jimgreen

Well Known Member
I'm beginning to suspect that my antenna position is causing poor comm performance on the ground.
I'm using a Comant bent whip under the left seat in my -7. A pretty common mounting place.
At a couple of airports I've had weak reception of tower transmissions while receiving just fine from aircraft in the circuit. On these occasions ATC has reported my transmissions as hard to read.
I'm wondering if antenna performance in this mounting location is poor on the ground and whether it changes with aircraft orientation to the other station.

I've tried some basic checking of my system such as headsets, jacks and grounding, but thought I'd ask before I went any further.
New SL40, Zulu 2 headset, PS1000 intercom
 
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Absolutely. Try turning 30 degrees or so, see if transmission improves. In flight my wingtip com antenna is always weaker than the belly whip, but on the ground it is sometimes stronger.
 
For me it's enough of a problem, I'm considering moving my antenna to the top. That leaves me with a problem of where to put my 406 elt antenna. Under the fuse doesn't sound good, and the 406 antennas are probably too big for the baggage compartment. Ideas?
Where are the slider guys putting their 406 antennas?
 
In Addition to what Az Gila said (antenna, especially tip, close to ground equals usually poor performance), can you tell us what other antennas are on the belly of the craft? Generally speaking, if another antenna is within 1/4 wave of a Comm antenna (21 inches or so) that could cause TX problems, or at least cause a null in the transmit pattern.

I have 3 antennas on the belly--Comm 1, APRS, and TxPonder with no problems, but I am using an Icom 210 radio which has a decent power output. How much Tx power does your radio provide? (I know it does not make any difference for such short distances from your taxiing aircraft to the tower, just curious.)
 
Well I'm going to go against other advice here and say that your poor performance is probably because of the "horizontal" positioning of the bent whip.
Before you do anything else, try straightening out the antenna as much as possible. This will place the tip closer to the ground, but it will also give you more vertical polarization.
I have 2 com antennas on the bottom of my -6 and they have been working great for almost 20 years.
 
FAA

This is the quoted info:

" VHF Com 1 should be mounted on the top of the aircraft since this will provide the best unobstructed location. VHF Com 2 can also be mounted on the top, provided there is at least 1/2 wavelength (of the antenna operating frequency) distance available between antennas.
e. If Com 2 is mounted under the aircraft, a bent whip may be required to provide ground clearance. Bent whips may not provide the best performance because of proximity to the aircraft skin. Signal reflection and obstruction is more of a problem with such locations. Extension of the landing gear or flaps may also impact belly mounted antenna performance."
 
Al,

I put mine (bent whip) on top behind the canopy (tipup) and it works very well. Probably adds a bit of drag, but I never worry about snapping it off when cleaning the belly or otherwise working under the plane.

Greg
 
I put mine on the belly, under my left knee. The aft two screws of the Comant bent whip antenna pass through the forward web of the spar. I added a doubler plate that picks up the forward two screws.

This kept me from running an antenna cable aft, through the spar, with its limited passageways.

Regarding its effective range. I have never had an issue on the ground or in the air and have had clear conversations with other aircraft at distances well over 100 miles.

However, when climbing out of my airport, I do have to wait until I?m at pattern altitude or higher to contact Charlotte departure. I?m not sure if this has anything to do with the antenna location or simply that I?m too low to talk to them. Maybe putting it on top would help but I can?t imagine the few feet difference in location is going have that much impact.
 
Thanks for the replies. My comm antenna is under the left seat and tspdr under the right, both behind the spar. Well separate but not close to the fuse edges. Pretty much what you see on lots of RVs .
I think I'll try straightening the bent whip - seems the easiest thing to try first.
 
Before you straighten the antenna

About three years after my first flight, I developed a radio problem.

The solution was to remove one screw at a time and gave each screw hole two quick turns with my deburring tool, and reinstalled the screw. This removed some light surface corrosion in the aluminum base.
 
Thanks Bill. I'll recheck the installation first. I'd already checked for a good ground on the belly skin but didn't think of the screw holes in the antenna base.
 
I have one comm on top and one comm on the bottom.
The comm on the bottom does not work well on the ground but is fine in the air. If you are only going to install one antenna, I would recommend top mounting.
 
Just for the archive.
Never could get my belly antenna to perform adequately on the ground.
Straightening it to 4" above the ground helped,but still not up to snuff, so I installed a straight whip behind tip up canopy. No further problems.
 
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