Transponder low wattage

[Paul_Havelka]

So we just installed an Avidyne axp340 in our piper Cherokee and took it to the avionics shop for a vfr transponder check. Everything worked out great and was well within tolerances but the wattage was on the low side at 160 watts. This install was just a temporary one u til we get the harness for our full avionics upgrade as our plane had an empty tray where an AT50 used to reside. The coax was changed out with a new run of rg400 that was 7’6” long, maximum for rg400 is 8’4”. Anyone have any idea on what could be causing the low output wattage?


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[denverpilot]

Bad antenna, problem in the new coax, VSWR problem, bad transmitter... all pretty easy to troubleshoot with the right test gear.

Any reason why the person testing didn’t hook up theirs and look to see? Most modern transponder test gear has test modes for testing the coax/antenna and transmitter output.

If it’s a shop with old test gear, it’s separate boxes, but they should have had them.

 

[Paul_Havelka]

Not sure but possibly for a few reasons. I did tell him that this was only temporary and that within the next 2 months we’d be redoing it all. Currently have a kma24, kx155 and now the axp340 and we’ll end up with amx240, gns430w, kx155, and the axp340. I’ll for sure make sure everything is good to go and at peak performance with the new setup. We were just happy to get it certified so that we can go on a trip on Monday to Kansas City


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[denverpilot]

Roger. Yeah they should have been able to tell if it was the transponder itself or the antenna system. Hopefully they can sort it out when you get back.

 

[Paul_Havelka]

@denverpilot I think we may have found at least a large, if not the sole, contributor to our issue. On the 3rd flight after the transponder check we lost the alternator. If it was weak or intermittent then our battery was likely a little drawn down. Headed to Tulsa now to get a plane power unit and new voltage regulator


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[denverpilot]

@denverpilot I think we may have found at least a large, if not the sole, contributor to our issue. On the 3rd flight after the transponder check we lost the alternator. If it was weak or intermittent then our battery was likely a little drawn down. Headed to Tulsa now to get a plane power unit and new voltage regulator


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Cool!

 

[derekbox]

CFR 43 Appendix E: (i) For Class 1A and 2A ATCRBS transponders, verify that the minimum RF peak output power is at least 21.0 dbw (125 watts).
You pass the inspection per the regs. Sign off and move on.