Comm issue with belly mount ant.

RickWoodall · #1 05-05-2011, 07:11 PM

My comm is working great. BUT, twice now after taxi out and run up, I pull up to the hold short, call tower and wait...and wait. Finally I figured it out and it seems with the belly mount it is affecting the transmission. If I swing the plane around sideways, no issue. If I call ready to go, before hold short..no issue.

In the air, never an issue. Anyone else find the plane shielding the receiving? Any suggestions or other things I can change as its kind of goofy just blocking sometimes and tower says he could hear ALL my repeated calls.

gereed75 · #2 05-05-2011, 08:10 PM

I have had similar experience with my KX155 and bottom mount antennae. I my case, I lost the side tone and could not hear the transmission. As I recall, others could hear the transmission, as in your case. I was also getting a lot of RF interference in the cockpit as indicated by crazy readings on my EIS. Most notably fuel flow.

As I recall, someone very knowledgeable about radios said that lots of feedback can overpower the sidetone electronics in the radio and that can disrupt the side tone.

I found a poor coax connection to the antennae. Fixed that and have not had a problem since.

I also have a top mounted ant to my second comm and often use that for ground comms when taxing to avoid airframe sheilding. It will happen with belly antennae, especially with flaps down.

zilik · #3 05-05-2011, 08:17 PM

On my 6A if the wing is between the antenna and the CT the tower does not even hear the carrier wave. I have learned make sure my wing does not block the tower and seldom have any problems.

Jeff Vaughan · #4 05-05-2011, 08:28 PM

Rick,
recently at the hold short line the tower was unable to hear me. I could hear the tower and others in the pattern could hear me. The tower relayed my instruction for take off thru the approaching plane. I have flown into this tower airport many times with no issues. I assume my wing was blocking my transmission toward the tower.

tkatc · #5 05-05-2011, 09:26 PM

Keep in mind that sometimes the ground antennas are not mounted on the tower itself. Might help troubleshoot.

pierre smith · #6 05-06-2011, 04:59 AM

Quote:

Originally Posted by RickWoodall

My comm is working great. BUT, twice now after taxi out and run up, I pull up to the hold short, call tower and wait...and wait. ......:

What kind of radio do you have? In my -10, both comm antennas are belly mounted for the SL-30 and the 430W...never a problem.

Maybe they just put out a bunch more watts?

Best,

RV7AV8R · #7 05-06-2011, 08:34 AM

Rick, do you have two antenna close together? One can load the other and create a directional issue if they are close to each other. I forget how close is too close, I think it is 1/2 a wavelength or 48 inches. A radio expert here will know. I have a friend with them closer on an rv8 and at times he is very hard to hear even though he is not far away.

Mel · #8 05-06-2011, 10:20 AM

Another thing that can affect belly mounted antennas is that most of them are "bent" whips. Com radio signals are vertically polarized. If your antenna is bent so that it is horizontal, transmission will be hampered in two ways.
#1 It is now somewhat horizontally polarized and #2 It is now closer to the ground-plane than it should be.

If you "straighten" the antenna, even just a little, to get it more vertical, that will help considerably.

RVbySDI · #9 05-06-2011, 10:52 AM

I had a problem at Destin, FL when flying to Rain N Pain, uhh, I mean Sun N Fun this year. Before takeoff at Destin one must contact the Eglin AFB tower for clearance. While holding short at the end of the runway I could not hear any response to my radio calls requesting clearance. I finally had to call the Destin FBO from my cell phone. They told me they were hearing all of my calls and Eglin's replies. It turned out the surrounding trees combined with my belly mounted antenna were preventing me from receiving their signals. After communicating with the FBO and getting clearance via cell phone I took off. Once I was just 20 or so feet off of the ground I immediately started picking up Eglin.

I do have the bent whip antenna on the belly of my plane that Mel mentioned. I am sure it contributed to the lack of reception in this situation. So far, this incident has been the only time I have had an issue with this problem but even though I do not regularly fly from controlled airports, as I travel in and out of them in the future, I will have to be cognizant of this problem.

kenora · #10 05-24-2011, 05:04 PM

My comm antenna is behind the canopy (of my T-18), I would like to add another comm antenna for my iCom A14 and was hoping to mount it close to the existing antenna.

Is that possible? I would only have one radio or the other powered up at a time (if that matters).

You can see from the picture there isn't much room on the turtle deck (click the picture to get a bigger image)

http://www.airport-data.com/aircraft/photo/252534.html