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06-14-2015, 04:07 PM
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Join Date: Apr 2006
Posts: 290
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Correct coax for GTX-327 antenna
Just wondered if anyone could shed some light on the Coax people are using for the antenna on the transponder. The manual States rg400 is ok with a max length of 8.8 feet. My installation routing exceeds that yet I'm finding it a challenge to find the next recommended coax of RG304 in anything less that 100 foot spools.
Many thanks
__________________
Simon Hitchen
Port Perry, ON, Canada
7 Tip Up, Titan XIO-360, Dual P-Mags, Airflow Performance matched Injectors, Sensenich FP Prop, Dynon Skyview, GTR-200, GTX-327
FLYING!
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06-14-2015, 04:11 PM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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Call Stein. http://www.steinair.com/
He may even have a scrap of RG400 that you could buy cheaper than a "cut to length".
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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06-14-2015, 07:17 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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How much longer than 9 feet? I wouldn't worry about a few extra feet. Just use good quality connectors.
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06-14-2015, 08:55 PM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,686
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A better solution would be to relocate the antenna closer to the transponder. The 327 is a great unit but typically it does not have much power to spare. Any more than the specified 1.5 db cable loss and you risk not being able to pass the req'd biannual transponder test.
Another option (not as good as moving the antenna closer) would be to use solid core RG142 which is pretty common and has a little less attenuation than the RG400 (12.8db vs 14.7db/100 ft) is the same size and uses the same connectors as RG400, this would allow you about 10ft vs 8.8ft.
__________________
Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
Last edited by Walt : 06-14-2015 at 09:27 PM.
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06-14-2015, 09:03 PM
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Join Date: Mar 2010
Location: KSGJ / TJBQ
Posts: 2,039
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Quote:
Originally Posted by Walt
A better solution would be to relocate the antenna closer to the transponder. The 327 is a great unit but typically it does not have much power to spare. Any more than the specified 1.5 db cable loss and you risk not being able to pass the req'd biannual transponder test.
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FWIW I had the same situation with my transponder installation and always had issues with ATC not seeing my transponder from any significant distance. When I installed a new panel, I relocated the transponder module a lot closer to the antenna (from 15ft to 4ft) and all the issues went away.

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Galin
CP-ASEL-AMEL-IR
FCC Radiotelephone (PG) with Radar Endorsement
2020 Donation made
www.PuertoRicoFlyer.com
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06-15-2015, 07:54 AM
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Join Date: Mar 2006
Location: SoCal
Posts: 626
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Been there
Installed RG304 when my install was around 10 feet. Hard to find and was somewhat difficult to get connectors. It is also a FAT cable to run through the center section. Did it just in case the margin was needed for the new Mode S stuff at a later date.
Using RG304 I have never had a complaint from ATC
If I was to do it today I would get a remote transponder like the trig to keep the coax run to a minimum.
__________________
JD
----------------------
RV-7 N314SY (KWHP)
IO-360-B1B
CANbus based trim/flaps and electrical
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06-16-2015, 08:29 AM
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Join Date: Apr 2009
Location: Calhoun, GA
Posts: 49
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Aircraft Spruce will make cables custom to the length you want with whatever tip you want. Works awesome.
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06-16-2015, 01:46 PM
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Join Date: Apr 2008
Location: Tennessee
Posts: 440
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Walt, the electrical gurus always say to never use solid core wire in airplanes and boats (basically anything that moves) due to the vibrations and possibility of breaking. Has this changed??? Inquiring minds want to know!
Quote:
Originally Posted by Walt
A better solution would be to relocate the antenna closer to the transponder. The 327 is a great unit but typically it does not have much power to spare. Any more than the specified 1.5 db cable loss and you risk not being able to pass the req'd biannual transponder test.
Another option (not as good as moving the antenna closer) would be to use solid core RG142 which is pretty common and has a little less attenuation than the RG400 (12.8db vs 14.7db/100 ft) is the same size and uses the same connectors as RG400, this would allow you about 10ft vs 8.8ft.
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06-16-2015, 02:46 PM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,686
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Quote:
Originally Posted by JDanno
Walt, the electrical gurus always say to never use solid core wire in airplanes and boats (basically anything that moves) due to the vibrations and possibility of breaking. Has this changed??? Inquiring minds want to know! 
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Which internet electrical Gurus are you referring to? Both are MIL-C-17 approved cables and both are used extensively in aircraft.
For example Garmin (and many others) call out 3C142B coax (or something similar) in their approved install manuals, this cable is basically RG142 made by ECS and Carlisle.
__________________
Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
Last edited by Walt : 06-16-2015 at 03:02 PM.
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06-19-2015, 08:57 PM
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Join Date: Apr 2006
Posts: 290
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Thank you
Thanks everyone, I managed to get the cable length down to 10 feet and the experts seem to feel that regular RG400 will be just fine.
Thanks for the help
__________________
Simon Hitchen
Port Perry, ON, Canada
7 Tip Up, Titan XIO-360, Dual P-Mags, Airflow Performance matched Injectors, Sensenich FP Prop, Dynon Skyview, GTR-200, GTX-327
FLYING!
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