Antenna problem

laz · #1 06-02-2016, 09:24 PM

I am just finish up my G3X touch installation on my RV8. I mounted the Remote GX23 transponder in the rear where the rear mounted battery would have been ( mine is on the firewall). All seemed good and I was checking the manual to make sure my distance between the antennas was enough. Gamin says 3 feet. Then i noticed that Garmin says the Transponder antenna needs to be 3 feet form the actual GX23 unit.

I have seen the GX 23 mounted in the rear where I have it. Any one who has done this , where did you mount your antennas.

I am hoping this 3 feet from the is like the requirement that the magnetometer must be mounted 10 feet from any steel. Which is just about impossible on any RV.

I can switch the ADSB with the transponder , but would't the RF interference still be a problem ?

I contacted the G3xpert, but last time it took a week to get a response.

Any comments or help is appreciated.

lr172 · #2 06-02-2016, 09:49 PM

I don't think the 3' distance requirement is an overly conservative thing with Garmin. I have seen this requirement with all of the transponder manuals I have seen, as well as the ADSB units, which use a similar frequency. I have no RF knowledge, so can't speak to why this is required, but it seems to be a widely adopted standard for devices using this frequency.

Larry

Walt · #3 06-03-2016, 05:38 AM

In my own RV7 I used the original antenna location about 6" aft of the baggage bulkhead with the 23ES mounted on the bulkhead, they are probably less than 1' apart with a cable about 3'.
About 2 years in service and I've had absolutely no problems.

n816kc · #4 06-03-2016, 06:31 AM

I just had a new Garmin 327 installed in my RV. the avionics tech was initially concerned about the antenna location just behind the firewall, within 3 feet of the 327 unit. ultimately decided that location was okay as long as the coax run was at least 3 feet. not sure if that's the technically correct answer, but it has been working fine in that location with no issues.

Mel · #5 06-03-2016, 06:51 AM

The only place I've seen this as a problem was on a Pitts where the antenna was about 6" from the xponder. The xponder continually responded to its own transmissions. We lengthened the antenna lead to 3' placing a loop in it and the problem was solved.

dpansier · #6 06-03-2016, 07:04 AM

Regarding the 3? spacing, in a metal aircraft the Transponder box is effectively shielded from the external antenna, I believe the spacing would be more important in a composite or fabric aircraft which would offer no or min. shielding from the antenna.

Another thought on the 3? would be the length of coax, the designers may have calculated in the slight loss and delay of the coax as most Transponder have listed coax lengths of 3? min to 9? max.

I have no confirmation on this from Garmin but it makes sense from a RF engineering point of view.

clutch22 · #7 06-03-2016, 07:45 AM

This all makes me feel a lot better! I've been going through the same dilemma this week as the OP. Thanks guys!

laz · #8 06-03-2016, 08:21 AM

I called Garmin this morning and they indicated that in a metal airplane there would not be any real issue. A composite aircraft would be another thing. No mental shielding.
I will probably mount the ADS b antenna where the Transponder one was going to go. Garmin said since the ADSB is for input only there is no issue with it being close the the Transponder unit. I won't have quite 3 feet but it will be close .. My Transponder antenna will be right behind the rear seat bulkhead and my ADSB will be about six inches behind the rear bulkhead where the battery try is normally mounted. Both my Coms are just behind the spar as far outboard as they will go.