transponder antenna location

I should be out in the garage working on the -10 tonight, but I just don't physically feel up to it. I've learned that building a plane is a lot like flying. If I don't feel good, I don't do my best work and I tend to make bad decisions. Better to stay on the ground.... eh, I mean, stay out of the garage.



I am rapidly approaching the day when I must close out the forward floor panels F-1050-L/R. So I really need to decide exactly where the transponder blade antenna will be located so I can create an access panel and install the appropriate doubler.

I want to mount the transponder antenna on the belly just a few inches aft of the firewall, offset to right of the centerline. Thinking it over, the exhaust pipes will be exiting just forward of where the antenna looks like it should go. Is that a problem?


Where did you mount your transponder antenna?

If I mount just aft of the firewall on the belly, should I mount it in the inner-most cavity under the forward floor? (Between F-1049D and F-1049A)
The middle cavity? (Between F-1049A and F-1049B)
Or the outer-most cavity? (Between F-1049B and F-1049C)

I have largely ruled out mounting in the center trunk because there is already so much going on in there.

Thoughts?
JW

I located mine under the pilot seat. Easy to access and works just fine

Make it pop up in the tunnel. No access plates needed, no exhaust interference.

Bill.Peyton said:

I located mine under the pilot seat. Easy to access and works just fine

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+1 Me too.

Mine is on the bottom of the tailcone, a few inches aft of the baggage bulkhead and right of centerline. It's back there because my transponder is in the tailcone.

Not a -10 but I didn't want it transmitting all the time under my important parts, just in case.

In my case the Transponder is on the right side, just aft of the firewall and the ADS-B antenna is on left side, just aft of the firewall. Short antenna runs, out of the way, etc. and most importantly, they work just fine there.

On my non-RV, the transponder antenna is under the belly just behind the firewall on the right, but forward of the fixed steel spring landing gear. It's in the stream of exhaust from three cylinders but at least a couple feet, maybe more, away from that.

It's been fine ever since the beginning of the transponder requirement.

Dave

Transponder under the pilot's seat; ADSB under the co pilot's seat. Easy to trouble shoot connection concerns.

I used a simple "lollypop" style antenna, and put it way back behind the battery. The original thinking was keeping distance between two antennas for ADS-B. By the way, the lollypop works great!
John

N941WR said:

Not a -10 but I didn't want it transmitting all the time under my important parts, just in case.

In my case the Transponder is on the right side, just aft of the firewall and the ADS-B antenna is on left side, just aft of the firewall. Short antenna runs, out of the way, etc. and most importantly, they work just fine there.

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My setup is the mirror image of this. Ditto on the pro's, no con's to mention.

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.

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

Min/Max coax length?

Nukeflyboy said:

Keep the coax short.

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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?

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".

Auburntsts said:

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".

Click to expand...

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.