[skyking902001]

I wanted to get some opinions from the board regarding where to mount the transponder antenna. Pics are of the doubler in two possible locations. Comments or suggestions will be greatly appreciated. Thanks in advance,

Steve
Skyking902001
RV-8 working on fuse

 

[Walt]

You need to think about and plan for where all your antennas will go, not just the xpdr.

 

[dpansier]

As Walt mentioned above, planning the locations for all the needed antennas is important to insure reliable radio performance.

With that in mind a few additional things to consider,

Transponder: Because of the frequency of operation, 1030 / 1090 MHZ, the coax from the transponder to the antenna will have the highest loss. Mounting the transponder antenna near, rather then far from the transponder will minimize the line loss. Modern transponders output 150 to 200 watts of energy, high quality coax, good connectors, and short runs insure the max. energy to the antenna enhancing performance.

UAT ADS-B:
You may install UAT in the future, planning a location now for the UAT antenna would save having to shift other antennas around to get the needed antenna spacing. Most UAT makers require a min of 60" from the transponder antenna to the UAT antenna. The UAT operates at 978 MHZ and the spacing insures the transponder does not overload the UAT receiver.

 

[skyking902001]

Antenna locations

Opps, should have mentioned other antenna locations. VOR-in wing tip, ELT under empanage fairing, GPS on top of dash, dual com antennas just behind footwells. First pic would put the xponder antenna about 36" away from the com antenna. Second pic is farther from the com antennas, but the ground plane is not as wide. A UAT antenna needs to be 5' from the transponder--quickly running out of real estate in the RV-8!

Steve

 

[Mel]

but the ground plane is not as wide.

Don't be too concerned about ground plane for the transponder.
Typically if the radius of the ground plane is greater than the antenna length, you'll be OK.
This, of course, assumes the antenna is centered on the ground plane.