Transponder output power

[johnsteichen]

At my last pitot/static/transponder check, the technician reported that the function of the dynon transponder was good, BUT it was putting out very low power. This jives with the complaints I had been getting from ATC and I frequently have to resort to my secondary Garmin transponder.
It is installed in a Glastar, with the transponder remote on the floor of the cockpit, with a 10 inch coax wip connecting to the transponder antenna. There is an 8-10 inch square aluminum ground plane.
Can I send the transponder in for a power output checkup?

 

[Dynon]

Which of our transponders do you have?

Also, have you tried swapping the transponders to see if it follows the installation? Finally, take a look at the discussion at http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1328697560/5 on ground planes. there's a chance that your "in the middle" of the two recommendations (120mm edges are the smallest practical, and it can get worse until it gets better as you get to 700mm+)

 

[johnsteichen]

I cannot swap transponders because the garmin transponder is panel mount and the skyview transponder is mounted on the bottom of the aircraft.
SV-XPNDR-261
although I thought I had the proper groundplane, I will remove the seat bottom and measure the groundplane again. Thanks.

 

[johnsteichen]

My ground plane is a square piece of alum. 120 mm on a side. It was my understanding that the groundplane should be a square, not a round circle

 

[Carl_Froehlich]

I assume you are using two separate transponder antennas. Swap the Dynon and Garmin transponder antenna leads to verify you do not have a bad antenna on the Dynon transponder. Do this swap at the transponder connections, not the antennas. That way you can eliminate a bad antenna coax connection as well.

 

[johnsteichen]

Swapping the antenna leads is easier said than done. Since I have no easy way to test the transponders after I re wire the transponders. The transponders are in two different locations. And antennas are not near each other. Highly unlikely the antenna is actually bad. Might remake the short coax lead but it's Far easier to remove the dynon transponder and. Return it for a test

 

[jakej]

John

Don't discount the antenna - I found one that had 2 of the 4 segments missing that make contact with the pin of the BNC connector.  The result was a poor & intermittent connection to the antenna. This was found after a customer reported the Txpdr was dropping off radar & had reduced range.  A shop had tested the system in situ, removed Txpdr & bench checked it & replaced antenna coax. 

When I had a chance to 'look over' the plane we found the fault in 10 mins - just lucky I guess

Jake J

 

[ckurz7000]

Before doing anything I would do an SWR (standing wave ratio) measurement on both transponders. This is a simple measurement using an SWR meter that is simply plugged in between the transponder and the antenna. The SWR meter is something a ham radio guy or amateur radio operator will most likely have. Try to borrow one, the measurement is really simple and quick. It will tell you if you have a problem with the high frequency radio signal leaving the transponder and getting radiated by the antenna. A bad coax, faulty connector, insufficient ground olane, etc., will all show up as a bad SWR reading. If the SWR is good, you can be certain that the power output of the transponder is at fault.

-- Chris.

 

[flyingboy]

John
FWIW I had a transponder issue with very low power output and did all the suggested things, cables, Connectors Antennas etc etc, I nearly went pear shaped trying to find the problem. No mode C but sometimes it was intermittent. I found the problem by sheer accident the transponder was not sitting back far enough in the cradle, maybe a mil. When I pushed on the Tponder the power would go up on the tester so the issue was the spacers I had on the edge of the cradle. I shaved a very small amount of the spacers and tightened the tponder tested it on the ground and in the air and no more issues, it was fixed. I did not have any power issues just mode C which was power output to the antenna. I am not sure if this helps or is anyway something similar but can be a trap the unsuspecting and one I would not have assumed possible as the tponder sits flush. Food for thought and something to check. Cheers

 

[jakej]

Good try Hugh but the reported low power unit is a Dynon so it isn't the cradle, read his first post :-?

Jake J

 

[johnsteichen]

OK I am going to have a radio amateur friend of mine to a Standing Wave Ratio test on the Dynin unit to see what that is registering. The antenna is a simple 2 inch pole, not sure what can go wrong with that. Cable is a possibility, and the test should be able to detect that.

 

[cessnadriver850]

Here is the deal. You need a special type of swr test to test this as transponders send out pulses of energy not the continuous power output that a normal swr meter reads and it is in 980 mhz to 1080 mhz range. If your friend has a signal generator in that range and a swr meter in that range, he can test the cable that way. All I can say is good luck with this approach.

 

[dynonsupport]

You definitely need to test in the right frequency band, but I'm not sure why VSWR changes over pluses vs continuous duty. I don't think the test needs to be done with a signal that looks like a transponder pulse.

 

[johnsteichen]

Im giving up, taking it to my friendly avionics shop. No one I know has the equipment to test the SWR in the 1 ghz range.