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09-16-2012, 05:01 PM
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Join Date: May 2006
Location: Rochester, MN
Posts: 420
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Transponder antenna in wing?
Anybody mount a transponder antenna in one of the wing access panels (the wing NOT containing the autopilot roll servo)? See any problems with this?
Also, is the "minimum of 3' from the transponder" rule a "line of sight" rule, or a "cable length" rule?
Thx.
__________________
Rich and Cindy Macrafic
Rochester, MN
Flying
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09-16-2012, 08:00 PM
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Join Date: Feb 2008
Location: Austin, TX
Posts: 179
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It's an antenna-to-antenna line of sight issue.
You should find cable length restrictions in your transponder installation instructions, and it will depend on what kind of cable you use. That restriction will be likely be more than 3 feet. It was 7 feet for my choice of RG-400 cable and Mode S transponder.
As we all add ADS-B antennas, the 3 foot antenna placement restriction is going to be a problem. My transponder antenna is nearly in the center of the usable real estate on the belly, and a 3 foot radius from there doesn't leave many choices that I like. Out on a wing might make sense.
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09-16-2012, 08:05 PM
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Join Date: Feb 2008
Location: Austin, TX
Posts: 179
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I answered a different question. I didn't have to worry about the minimum cable length issue, but I remember Stein saying I had to have the cable be at least 3'.
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09-17-2012, 07:25 AM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,122
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Quote:
Originally Posted by chipf
I answered a different question. I didn't have to worry about the minimum cable length issue, but I remember Stein saying I had to have the cable be at least 3'.
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Interestingly, Dynon has specifically said their transponder coax will not need a min/max length.
__________________
Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now. 
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09-17-2012, 07:28 AM
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Join Date: Aug 2006
Location: Gulf Breeze, FL
Posts: 408
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Here's a Table
This is from MicroAir depicting MAX lengths of various coax for transponder installs.
http://www.microair.com.au/admin/upl...lation01R1.pdf
Depending upon your routing you will likely have a longer cable than the recommended maximums. I've seen several aircraft with coax exceeding these lengths with no reported problems.
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Rocky McKiernan
Navarre, FL
N767JM RV-7 Flying
N1011D (res) RV-10 Building
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09-17-2012, 09:05 AM
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Join Date: Oct 2008
Location: Utah
Posts: 8,145
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Quote:
Originally Posted by macrafic
Anybody mount a transponder antenna in one of the wing access panels (the wing NOT containing the autopilot roll servo)? See any problems with this?
Also, is the "minimum of 3' from the transponder" rule a "line of sight" rule, or a "cable length" rule?
Thx.
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I've seen in on an access panel you describe. Might have a picture of it will look tomorrow. |
09-17-2012, 02:26 PM
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Join Date: Aug 2007
Location: Ontario, Canada
Posts: 2,280
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Since transponder is "line of sight" and these aircraft have some dihedral, mounting it too far out on the wing will result in poor coverage off the opposite side of the aircraft as the fuselage will effectively be placed between the antenna and ground station and lower operating altitudes.
As I watched my hangar mate working on tires and tubes I realized that one also wants to place the antenna where working on a flat tire isn't going to result in the antenna lacerating one's noggin...
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09-17-2012, 02:35 PM
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Join Date: Mar 2005
Location: Dallas area
Posts: 10,762
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If you have a minimum distance between the transponder and antenna, the distance would be "line of sight".
I have had an experience with a Pitts where the transponder continued to "reply" to itself because it was too close to the antenna.
__________________
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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09-17-2012, 04:19 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,767
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Minimum cable length:
You may be thinking of a 430W/530W. When the waas versions came out Garmin just bought an OEM antenna to go with the radios. The antenna turned out to be too sensitive. Rather than fix the real problem, Garmin required that you use a minimum cable length (they actually specified a minimum loss, which many installers met by using extra long cables!).
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