A

apatti

Well Known Member
All,
I am chasing some gremlins in my wiring. When I transmit my EFISes show varying degrees of pitch and roll changes. Here is my setup:

- Dual GRT (one HX and one HS)
- COM 1 - 430W
- COM 2 - SL40
- Com antennas - Rami bent whip on belly near centerline (one near wing spar carry-through and one just aft of the rear passenger seat)
- AHRS just aft of rear baggage compartment and just forward from elevator bellcrank
- Both antennas fed with RG400 running down left side of fuse
- DU/AHRS wiring down right side of fuse
- Magnetomer wiring running up right side (with AHRS wires) to just before the main wing spar and then out right side of fuse. The cable exiting the fuse is about 12 feet long.
- Everything is in my garage (not a metal hangar)
- GRT's have not been "calibrated"

Observations:
When I transmit I get varying degrees of pitch and roll changes on DU's. The changes on the two DU's are always in the same direction. However, the magnitude of the change on each DU can vary. The overall problem varies a little with transmit frequency. But, it seems much more sensitive to magnetometer and magnetometer wiring location. When the wiring drapes down below the fuse, the problem is worse. When I place the magnetometer on top of the fuse back at the tail and route the wire along the top of the fuse, the problem is minimized.

Side note: I originally had RG58 going to the COM2 antenna. Thinking this could be the problem I purchased some RG400 and ran it direct between the transmitter and the antenna (i.e. didn't physically remove the RG58). When I switch between the RG400 and the installed RG58, I see no difference in the symptoms.

Hypothesis:
RF energy from the COM antennas is coupling into the magnetometer wiring and causing the pitch/roll changes.

Questions:
Should I not worry about this now and wait until the wings are on and such? Does the wing provide some shielding for the magnetomer and associated wiring? I'd much prefer to clear this up now when the access is better.

I'll be contacting GRT on Monday, but thought I'd check here and see if anyone else had any experience with this.

Thanks,
 
Typicall RFI is due to improper grounding. Check the grounding of the coaxial cable and the ground from the GRT to the radio.

I am going brain dead for a second here; but I think rg400 cant be used for comm's just gps. Correct me if i am wrong....
 
I would do a test outside before you get too worried

I saw exactly the same problem in the hanger (lots of reflected energy) but it completely disappears outside.
 
Don't mess with it now

These problems can be caused by many things including being inside a building/hangar, unterminated wiring, etc. I had the same problem when building and wasted 3 days troubleshooting only to find out that the RF was transmitted out of my antenna on the bottom of the airplane and entering unterminated strobe/position light wiring hanging down the side of the airplane before the wings were attached.

Andy
 
New BNC connectors

Here are two issues that I experienced:

I had the same problem with RF feedback w/ my GRTs and replaced the BNC connectors on both ends. This solved most, but not all, of the problem.

I also had a problem with a wingtip Archer comm antenna. Great antenna, but they need tuning. I had glassed my comm antenna down inside the and left myself no way to trim it for the required tuning. Ultimately, I pulled the coax out of the wing and wired up a belly antenna which solved the rest of the problem.

One other thing to look out for is the effect of the RF feedback on your auto-pilot. Prior to changing the belly antenna, I would get pitch and roll changes each time I keyed to mic.

In the end, a bad BNC connector install can cause grounding problems. This may be worth trying track down.

I hope these tips may help you.

Keith
 
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RF issues - easy solution

RF from a nearby VHF antenna can indeed cause all sorts of problems with nearly any form of electronics (not just EFIS's).
The received energy on various cables (signal and power wiring) can be huge and sometimes several tens of volts of RF can be observed at various points of the cable.
This kind of energy can be difficult to filter inside the various electronics boxes and the result can cause power supply regulators to output the wrong voltage and sensitive analog electronics can be severely compromised.
In bad cases, malfunction of voltage regulators may cause expensive damage to equipment.

Right. What to do about it ?

All the comments posted here are good. Grounding issues, coupling between cables, bad cable connections on the RF cable itself or just a powerful transmission a tad too close to the equipment (the more powerful your radio, the further the antenna should be away - particulary if you fly a plastic aircraft).

We have nifty clip-on ferrites available (check with Matt at MGL Avionics USA). These ferrites have been formulated to be most effective at VHF frequencies (unlike others which typically are formulated for frequencies up to about 30 Mhz and don't do anything above 100 Mhz).
Placed over any cable, they provide a strong attenuation effect for RF on that cable, effectively preventing most of the enegry from going into sensitive equipment.

As they are clip-on, it is easy to use them to find out just where the issue is at its worst. Play around and you will soon find the area of contention.
Then you have two options - change the wiring at that point given the hints the ferrite gave you - or if the ferrite fixes the issue anyway - leave it there.

Rainier
CEO MGL Avionics
 
All,
Thanks for the replies. I plan on waiting until I can pull the fuse outside to try this test again. If the problem persists, I'll get some of those ferrite clip-ons Rainer mentioned and play with them. My install is pretty standard except for the placment of the COM2 antenna being somewhat close to the AHRS. But, since my problem is not limited to the COM2 antenna, I don't think that is being a problem. But, we'll see...

Thanks again!
 
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