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  #11  
Old 01-29-2017, 10:47 AM
Nukeflyboy Nukeflyboy is offline
 
Join Date: Aug 2007
Location: Granbury, TX
Posts: 103
Default Xpdr antenna

The real question is which transponder do you plan to use? If panel mount, then under the seat or in the tunnel works. If remote mounted xpdr, then consider putting it and the antenna behind the baggage bulkhead. Keep the coax short.

Also if you install ADS-B, you will need two xpdr antennas and they should be at least 3 feet apart, if I remember correctly.
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  #12  
Old 01-29-2017, 02:27 PM
jpowell13 jpowell13 is offline
 
Join Date: Nov 2008
Location: Baton Rouge, LA
Posts: 671
Default

I installed mine just aft of the baggage bulkhead on my 6A. Sometimes ATC can't see me on arrivals until I lift the nose or make a turn to better expose my antenna.

Also, I am installing a Trig TT31 transponder for ADSB out. I have an iLevil handheld for ADSB in. With this setup only one exterior antenna is needed.

John

Last edited by jpowell13 : 01-29-2017 at 02:34 PM.
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  #13  
Old 01-30-2017, 06:43 AM
mikeyj350's Avatar
mikeyj350 mikeyj350 is offline
 
Join Date: Jan 2014
Location: Cedar Rapids, IA / USA
Posts: 159
Default Min/Max coax length?

Quote:
Originally Posted by Nukeflyboy View Post
Keep the coax short.
I seem to remember at least for the G3XT system, the manual specified a minimum length for the coax, stating that that length may be longer than the physical run between the transponder and the antenna... so it may be a good idea to keep this in mind as well when planning your runs.

Related question on this: is there a *maximum* distance that the transponder coax should be? Due to some last-minute changes in my panel, I'm being faced with a slightly longer transponder antenna run than I was planning for, since my GTX45R is mounted in the stack in the panel, and the antenna is in the tail just aft of the elevator bellcrank/battery tray. That makes for a ~15-18 foot run of coax... Any concerns with that, or anyone else do something similar?
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RV-10 In progress! (N165MJ reserved)
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  #14  
Old 01-30-2017, 06:59 AM
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Auburntsts Auburntsts is offline
 
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Location: Tampa, FL
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The Garmin install manual for their GTX 23 doesn't specify a minimum cable length. Instead it talks in terms of maximum dB loss and the maximum cable length by cable type that will not exceed the dB limit (which for the 23 is 1.5dB)--this is all depicted in a table. For example the maximum length for RG400 (M17/128-RG400) is listed at 6' 1.3".
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  #15  
Old 01-30-2017, 08:31 AM
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mikeyj350 mikeyj350 is offline
 
Join Date: Jan 2014
Location: Cedar Rapids, IA / USA
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Quote:
Originally Posted by Auburntsts View Post
The Garmin install manual for their GTX 23 doesn't specify a minimum cable length. Instead it talks in terms of maximum dB loss and the maximum cable length by cable type that will not exceed the dB limit (which for the 23 is 1.5dB)--this is all depicted in a table. For example the maximum length for RG400 (M17/128-RG400) is listed at 6' 1.3".
Interesting... I had to spend a minute to go find what I was trying to remember. I was looking in the Garmin GTX35R/45R manual, rev 2:
  • Section 3.13 Antenna Considerations, final bullet: "The antenna should be attached a minimum of three feet from the GTX35R/45R to prevent RF interference"
  • Section 4.5 Note: "Make sure the length is set for the necessary cable loss. Some antennas have minimum cable loss specifications, that may need a cable longer than the physical run in the aircraft".

    Also:
  • Section 1.9 Transmitter power 125W min at antenna, with max 2dB cable loss

So it seems there is *at least* a 3' minimum length (I'm interpreting it as a physical separation, so the cable likely needs to be a bit longer). The documentation for my CI-105 antenna does not seem to say anything about minimum loss per what was mentioned in the Garmin manual, so perhaps it is safe to assume I don't need to go any *more* than 3'.

What it seems I missed is that there is also a maximum length so as to not exceed the 2dB loss. However, I seem to have gotten a slightly different number for this, using the calculator here it seems I hit 2dB loss at around 12' 6". So, if I'm understanding this correctly, I need to be between ~3ft (physical sep) and 12'6" (cable length)... yes?

EDIT: Sounds like at least some of the difference between your numbers and mine is the 1.5dB and 2dB max loss... 9'6" and 12'6" respectively according to the calculator linked above.
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Mike Jimenez & Sarah Hammonds
EAA Chapter 33 Prez & Soc Chair
Marion, IA USA

RV-10 In progress! (N165MJ reserved)
-Emp & Wings complete
-Panel wired up, working on Fuse & Finish
-Blog horribly outdated (sorry)
-Too many distractions, we will finish the plane someday!!!

http://mikeandsarahrv10.blogspot.com/
http://www.eaa33.org/

Last edited by mikeyj350 : 01-30-2017 at 08:47 AM.
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