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07-31-2010, 05:14 PM
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Join Date: Jul 2008
Location: Shreveport, LA
Posts: 46
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Antenna Location
Guys, finishing up an RV-9A. Will have two comms, an IC200 and Garmin 430W. Was planning on mounting both comm antennas on the belly between the wheels but the more I thought about it, the more I figured there may be some intermod or interference.
Anyone else put two on the belly side by side between the wheels? Pros and cons?
I'm thinking now that I will put one on the belly and one on top about midway between the tail and the canopy, with the GPS antenna just aft the canopy. Using two wing-mount (interntal) NAV antennas for NAV and GS, BTW.
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07-31-2010, 07:12 PM
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Join Date: Mar 2007
Location: Auckland, NZ
Posts: 147
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That's where I've got the CI-122 Comm aerials for my GNS430W and SL30 on my RV-7.
I haven't had any issues at all with them in this location, on the ground or flying.
__________________
Cheers
Steve
Auckland, New Zealand
Flying RV-7QB/ZK-NVS/Aerosport 0320/Dual AF3500s/PMA8000B/GNS430W/SL30/GTX327/TT ADIP II
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08-02-2010, 10:44 AM
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Join Date: Dec 2006
Location: Arroyo Grande, CA
Posts: 938
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The counterpoise, ground plane, on these planes is not very long and as a result a 1/4 wave dipole antenna mounted on top of a plane will have its maximum radiation upward at 10-15 degrees and reduced radiation toward the horizon. Only mount an antenna on top if you're transmitting toward a satellite or if no other location is available. A wave-length at average comm frequencies is 93". Having at least 3/4 wave or 70" or more between antennas will reduce mutual coupling. If you want really good Tx-Rx, mount your antennas on the bottom of the wing at mid-chord just past the fuel tank.
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08-02-2010, 11:25 AM
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Join Date: Dec 2008
Location: Fontana, CA.
Posts: 74
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Cessna mounts them that way...
Cessna has been mounting their comm antennas 3 or 4 feet apart on top of the cabins (152, 172,182, 206, 210, Caravans) since the 70's and I have never head of them having any problems with that particular installation.
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Aaron Boulais
Stuck in spam can land for now.....
Will build when I retire in 25 years 
Donation Made 4/13/2015
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08-02-2010, 12:33 PM
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Join Date: Mar 2005
Location: Dallas area
Posts: 10,768
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A rough rule of thumb....
is that no antenna should be closer than the length of the longer antenna. In other words, if you have a 22" long antenna, no other antenna should be closer than 22" from it.
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Mel Asberry, DAR since the last century.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School E-LSA Project Mentor.
RV-6 Flying since 1993 (sold)
<rvmel(at)icloud.com>
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08-02-2010, 08:15 PM
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Join Date: Feb 2005
Location: Victoria B.C.
Posts: 1,266
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I agree with Mel on the spacing.
I would suggest one on the belly and on on the top. You will find some locations on the ground that you can not contact ground or tower with a bottom mounted antenna, if that happens you can use the top mounted antenna. In the air there is practically no difference in operational range.
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08-02-2010, 08:49 PM
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Join Date: Mar 2007
Location: Auckland, NZ
Posts: 147
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Quote:
Originally Posted by sahrens
How far apart are your antennas under the wheels? I am planning to mount two under the baggage compartment. I think I have read somewhere they are suppose to be 21" apart.
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Mine are 29" apart, and I've had no issues (touch wood).
FYI, the GNS430W Installation Manual states "The antenna should be mounted a minimum of six feet from any DME or other COM antennas, four feet from any ADF sense antennas, and two feet from the 400W Series and its GPS antenna. The COM antenna should also be mounted as far apart as practical from the ELT antenna."
__________________
Cheers
Steve
Auckland, New Zealand
Flying RV-7QB/ZK-NVS/Aerosport 0320/Dual AF3500s/PMA8000B/GNS430W/SL30/GTX327/TT ADIP II
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08-02-2010, 09:49 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Mine are both on the belly............with no problems. A Garmin SL-40 & Icom A210.
L.Adamson --- RV6A
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