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  #11  
Old 11-06-2017, 01:08 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 4,686
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Quote:
Originally Posted by Carl Froehlich View Post
Gil,

Again, build as you like but top vs bottom comm antenna performace is not, in my opinion, a factor.

Carl
I have my #1 com on a belly antenna; #2 on a wingtip (I know, many report awful performance there. But mine seems okay - not as good as the belly, but generally useable). I have had 3, but only 3, occasions where the tower either heard nothing, or said ‘weak and unreadable’, in response to the belly antenna. Twice on the ground, once in the air. In all three cases the tower reported the wingtip as loud and clear. And, in all three cases, they reported the belly antenna as loud and clear after turning 10 or 20 degrees. So I think there may be a small advantage to a top mounted antenna. But small enough that I’m not going to rush out to the plane and put one on.
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  #12  
Old 11-06-2017, 02:54 PM
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az_gila az_gila is offline
 
Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 8,810
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Quote:
Originally Posted by BobTurner View Post
I have my #1 com on a belly antenna; #2 on a wingtip (I know, many report awful performance there. But mine seems okay - not as good as the belly, but generally useable). I have had 3, but only 3, occasions where the tower either heard nothing, or said ‘weak and unreadable’, in response to the belly antenna. Twice on the ground, once in the air. In all three cases the tower reported the wingtip as loud and clear. And, in all three cases, they reported the belly antenna as loud and clear after turning 10 or 20 degrees. So I think there may be a small advantage to a top mounted antenna. But small enough that I’m not going to rush out to the plane and put one on.
Then it simply becomes atheistic since I have none of the reinforcing plates made yet. An upper antenna does not bother me.

However, ask any RF engineer what he/she would think of an inverted (or bent) 1/4 wave antenna at 120 MHZ with the tip of the antenna a few inches above a ground surface.
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EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ

Last edited by az_gila : 11-19-2017 at 08:45 AM.
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  #13  
Old 11-06-2017, 04:10 PM
jwilbur jwilbur is offline
 
Join Date: Dec 2010
Location: Culpeper, VA
Posts: 585
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As a point of reference the C172 I used to rent had 2 comm antennas on top. I never had an issue in the air or on the ground. On my RV10 I put COM 1 on top and a bent whip for COM 2 on the bottom. So far I have detected no difference. My audio panel permits split TX which requires the antennas to be separated or I probably would have put them both on top.

I mounted the top one offset from the center with a flat doubler. No way anyone would know (much less care) unless I told them.
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First flight - 8/2017
Pink slip - 7/2017
Start - 11/2010
RV-10 Build Log
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  #14  
Old 11-18-2017, 11:58 PM
Kellym Kellym is offline
 
Join Date: Aug 2005
Posts: 70
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Quote:
Originally Posted by az_gila View Post
Then it simply becomes atheistic since I have none of the reinforcing plates made yet. An upper antenna does not bather me.

However, ask any RF engineer what he/she would think of an inverted (or bent) 1/4 wave antenna at 120 MHZ with the tip of the antenna a few inches above a ground surface.
Gil, I was going to put one on top and one on bottom. My CFO vetoed that plan on appearance. So I have both on belly. Only place I have ever had a little issue was Prescott at the north end of 12, which is depressed below the runway. I've had no issues with range, including over the NW corner of the TX panhandle, which is usually difficult at lower altitudes. Even at Prescott, just switching to #2 with antenna on opposite side of belly took care of the issue. Both my antennas are under rear seat, near the front of the compartment, as far apart as possible laterally.
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  #15  
Old 11-19-2017, 06:52 AM
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Bill Boyd Bill Boyd is offline
 
Join Date: Sep 2006
Location: Landing field "12VA"
Posts: 935
Default As a reminder

If you haven't gotten this far in your build yet,
I see no reason why this wouldn't work for Com 1. My Com 2 will be a belly-mount actually on a an inboard wing inspection plate. Obviously not yet flying - but in theory a quarter wave stuck to an RF-transparent radome should get out just fine. That's a quarter-wave of copper foil in the left foreground. Feedpoint will be grounded on the right longeron.



While we are at it, why not put the VOR/ILS Vee in a similar spot and feed it with a coax balun? No one will ever poke their eye out on this dipole, nor get tangled with it while cleaning the belly. It's a resonant half-wave dipole and extends a ways down the doorway B-column areas. 26" per leg.


I haven't yet trimmed the comm "whip" to resonance, needing an antenna analyzer to do so, and with the cosmetic paint in place I hope it won't need trimming (yeah, right). The antennas are completely invisible and zero-drag.


One thing I'd do differently: cut the antenna shorter than formula length and bring it to resonance with a little base-loading inductance once installed. As it it, if I'm long, I will have to shorten it with a razor knife through the paint "by feel," or with a series capacitor at the base. Finding a suitable silver-mica cap to electrically shorten this thing could be a bit of work, and I wish I'd thought to go too short to begin with
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RV-10 - N130YD reserved - under construction

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