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  #11  
Old 02-05-2017, 06:45 PM
Dean Pichon Dean Pichon is offline
 
Join Date: May 2006
Posts: 532
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I have the same problem in my -4. I have a belly mounted bent whip antenna (RAMI AV-17) and have never had great performance. This past annual, I removed the antenna, replaced the star washers, scuffed the interior surface of the belly skin and reinstalled everything. After all that, I measured the SWR at 7-8.
The person I spoke with at RAMI thought I should be able to get a much better installation (2-3 SWR) given my aircraft and antenna, though they could provide no specific means to attain this. One thing that was suggested was to measure the SWR using a handheld radio and a different coax (and the installed antenna) to eliminate radio or wiring issues.

I have yet to try this, but it is on the list... Please keep us posted. I would love to improve my installation.

Dean
Bolton, MA
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  #12  
Old 02-05-2017, 06:50 PM
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dpansier dpansier is online now
 
Join Date: Jan 2006
Location: Green Bay, WI (GRB)
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The VSWR on a VHF belly mounted antenna will change when going from ground to flight conditions, I have confirmed this thru testing.

The ground plane where I test antennas has the ability to vary the height over the ground surface, the antennas can be mounted on top or the bottom to simulate top or belly mount.
With a belly mounted antenna on the ground plane the VSWR will change as the ground plane distance to the ground surface is changed.

The freedoms of ?Experimental? offer us much latitude on the type and we select to mount our antennas.
Reflect back to the 60?s and 70?s when most of the Certificated aircraft were produced, Cessna and Piper would call in the RF engineers with their expensive test equipment and work with the structures guys to find suitable antenna positions.
Once the positions were determined, they were standardized and all the antennas were mounted in their place. Most if not all VHF antennas were mounted on the top of the aircraft with good reason.

A long winded explanation to say, if users are looking for optimal antenna performance, they will not see it with a belly mount, bent element antenna.
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  #13  
Old 02-09-2017, 08:06 AM
Kellym Kellym is offline
 
Join Date: Aug 2005
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Quote:
Originally Posted by az_gila View Post
Try it on the ground and in flight and see if there is a difference.
I worried about belly performance on the ground. However, I found having a type certified aircraft with all of the com antennas originally on the top of the airframe, lined up in a row caused problems with ELT transmitter excitation that interfered with regular com radios. So I replaced one com antenna with a Comant CI-122 on the belly. Not only was the above problem mostly eliminated, but the belly antenna worked equally well on the ground and in the air. No problems calling ground or getting ATIS while taxiing.
So I installed two CI-122 antennas on the belly of my RV-10, and they both work great on the ground and in the air. Using GTN-650 and SL-30 for com radios. Never had any problem communicating with ATC. Have not attempted to find someone to check SWR. I used all RG-58 with crimped BNC connectors for nav and com equipment, RG-400 for GPS and transponder.
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  #14  
Old 02-09-2017, 08:12 AM
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flightlogic flightlogic is offline
 
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Location: Prescott, AZ
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Good data. Thanks Kelly. Comant has robust construction as well.
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  #15  
Old 02-09-2017, 08:34 AM
humptybump humptybump is offline
 
Join Date: Feb 2011
Location: USA
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This is a "YMMV" reply ...

I tried DeltaPop "bent" whip on the belly of my RV-8 and had less than stellar results. I have "enough" clearance that I run the DeltaPop "straight" whip (aka slightly angled) on the belly and do so with better results.
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