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  #1  
Old 02-03-2019, 04:31 PM
rdamazio's Avatar
rdamazio rdamazio is offline
 
Join Date: Dec 2013
Location: Santa Clara, CA
Posts: 217
Default COM antenna on tailcone

I know COM antennas ideally need a 23" radius for ground plane, and that the best place for them at the bottom is the fuselage skin, but I'm wondering if anyone has had success with having one on the forward bottom part of the tailcone, and what kind of performance they got from it.

Context: I originally planned to have two antennas at the bottom of the tailcone - transponder and ADS-B, and added the appropriate doublers and holes for them. With the latest avionics, I no longer need two separate antennas (and have no other antenna that needs to be placed at the bottom), so I have an extra hole in the tailcone, so I'm wondering if I can just use that.
You can see the hole location in this (old) picture.
Lesson learned, don't finalize antenna placement until you actually buy the avionics

I can of course just plug the hole and make another one further forward for the antenna...
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  #2  
Old 02-03-2019, 05:51 PM
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Bill Boyd Bill Boyd is offline
 
Join Date: Sep 2006
Location: Landing field "12VA"
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Default

I doubt the lack of a flat 46" of ground plane would be any issue at all. The cross-section of the fuselage at its more narrow aspect would look like "drooping radials" under a classic quarter wave monopole, and this configuration is used to get feed point impedance closer to 50 ohms (a theoretical vertical over a flat ground plane is 36 ohms, a dipole in free space is 72 ohms). So VSWR should be closer to ideal in your case with RG-400 or similar feed line than on a flat large ground plane-mounted quarter wave whip.

If it were me, I'd try it. Good matching gets the most power from the transmitter, and the line-of-sight nature of VHF make the rest of the story pretty much shooting fish in a barrel.
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  #3  
Old 02-03-2019, 06:23 PM
MConner MConner is offline
 
Join Date: Mar 2018
Location: Snead Island, Florida
Posts: 205
Default Com 2 in the baggage compartment,

I didnít want any more holes or drag so I used a surplus ELT antenna in my baggage compartment. It does everything I need for com 2. It receives ATIS, can easily monitor CTAF while I am on with ATC on Com 1 and transmits at least as well as ant handheld backup in case #1 fails.
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