SV-XPNDR-262 Transmitter Power

[Michel from Switzerland]

Installed new Skyview System 10" with XPNDR-262. It's a new installation in an experimental metal aircraft. I had an avionics specialist check the transponder signal. He measured only 2 Watts transmitting power at the antenna. We thought it might be the antenna cable. But there is 0 resistance on the cable +/- and no short circuit. The antenna is a new RAMI AV-22 from ACS. I installed the latest version of Skyview Software, but did not do any updates to the XPNDR. XPNDR cabling seems right because all information is shown on the skyview screen. I was also able to see the aircraft on flightradar24.com.

What other cause could lead to the low transmitting power at the antenna? Does it depend from the update? How can I find the error?

 

[RVDan]

The DC resistance of a cable isn't a god indicator of cable condition as the impedance changes with frequency which is 1030 to 1090 Mhz. One needs to more thoroughly measure the forward and reverse power using a wattmeter if you have one available. The DC resistance simply tells you that the center pin was installed to the wire correctly. It is possible that the center pin is not fully inserted and which may result in some radiated power. I would verify that the connectors are assembled correctly. Then check the ground plane of the antenna to make sure you have a good electrical bonding to the ground plane (<.0000025 ohms) You can't measure this with a regular ohm meter, need a milliohm meter (check Amazon for cheap ones- you want a 4 wire unit). Then if necessary, make a temporary antenna cable and connect to the transponder and antenna without routing through the airframe for testing If that fixes the problem you likely need to replace the cable.
Of course there is always the possibility that the transponder is defective. Hope that helps

 

[rvator51]

I found a low battery can really affect the watts out on a ground check.