GRTs with RF interference

KiloWhiskey1 · #1 12-22-2006, 09:32 PM

I have recently powered up my panel and am currently debugging a few minor items. My panel has 3 GRT screens with dual AHRS, 430, SL40, 330, PMA-8000B and a 496. I wired the panel myself and so far everything works as designed. The most troublesome issue is an RF interference issue that I am experiencing with the GRT system. When I transmit over the 430 the GRTs react with a 5-10 degree heading change and a momentary loss of altitude followed by a gain in altitude (+/-50ft variance). I have rerouted one of the AHRS wires which took care of the bulk of the problem; however I am still seeing changes in heading and altitude readings.

Has anyone experienced this problem? If so, could you please provide some insight in to how you sorted things out? Did all the problems go away, or were you unable to get all the interference out?

Thanks in advance for your help.

Keith

Radomir · #2 12-22-2006, 09:52 PM

have you tried re-routing your coax for the 430? I'd play with that as well.

Ironflight · #3 12-22-2006, 09:57 PM

Keith -

Where is your AHRS mounted? Anywhere near the Comm antenna? I'd suspect the end of the Comm antenna cable closest to the AHRS of having a "leak"....

I have never had a bit of interference with my GRT - I had some definite problem with my LED trim indicators when I had a lousy, unshielded connection to my first (cheap and lousy) Comm antenna...

Paul

KiloWhiskey1 · #4 12-22-2006, 10:07 PM

Paul,

My AHRS are mounted just aft of the firewall on opposites sides of the fuselage. Your point about the bad connector may be the issue; however it would probably be the connector at the 430 end as that is the one closest to the AHRS. I did change the connector on the antenna end because my magnetometer wires run within a few inches of the antenna connection. I will change the connector on the 430 end as you suggest and I will probably reroute the magnetometer wires away from the comm. antenna.

Did you change to a more RF friendly connector?

Thanks for the reply.

Keith

hevansrv7a · #5 12-23-2006, 06:19 AM

When transmitting, I get downward spikes in the EGT readings, often enough to trigger the cooling alarm. I also get a pitch-up from my TrueTrack AP. I have had the antenna installation checked for reflected energy and it's very good. The answer appears to be the proximity of the COM antenna to some of the wires. I don't think it's the AHRS, at least not in my case. Two options exist: 1] re-route the antenna wire and 2] replace with transponder grade coax. Both may be needed. The TrueTrack guys suggested twisting the power and ground wires and I haven't tried that yet. All of these are difficult to implement and I wish I had been smarter about this while building.