Berchmans

Berchmans

Well Known Member
I have an electric interference problem that I hope someone might be able to shed some light on. I am constructing an RV 8 and have installed a Dynon FlightDEK 180, an ICOM 210 and a KT76 Transponder as well as a PSE intercom system. Tonight I was doing a radio check with everything on and at least have success in that everything appears to work. Here is my delema, when I press the push to talk button on either control stick the Manifold pressure reading on the Dynon starts to go up and the elevator trim indicator (Ray Allen) immediately indicates full nose down trim. When I power down the Dynon and again press the push to talk button the trim indicator remains at the neutral trim reading. Intercom on or off has no affect. If I leave Dynon on and turn radio off and push PTT button there is no affect.
None of these systems is interconnected and I have run all the communications using shielded wires.
Any suggestions on where to start looking? Oh, and on the radio transmissions the tower said all was 5 by 5.
 
Burke,

Problems such as these are typically caused by "something" not being grounded properly. Finding that "something" is the hard part.

I'd start with making certain your radio transmit antenna is properly grounded.

After that you need to make certain that the Dynon D180 and the manifold pressure sender are properly grounded.

Are all your avionics grounded to the same point?

The book Aeroelectric Connection offers many helpful hints here... and also a way to create a very sensitive ohmmeter to measure quality of grounds.

Do you have more than one transmitter? If so can you swap antennas?

Don
 
Berchmans said:
Here is my delema, when I press the push to talk button on either control stick the Manifold pressure reading on the Dynon starts to go up and the elevator trim indicator (Ray Allen) immediately indicates full nose down trim.
Any suggestions on where to start looking? Oh, and on the radio transmissions the tower said all was 5 by 5.
Click to expand...

Typically this is an RF issue, especially if your plane is in an enclosed area, the RF bounces all over the place causing 'other' systems to go haywire. This is especially true with Vans older style round gauges and in many cases with those gauges it doesn't go away. If the plane is inside and you can put it outside and repeat, that would be my first check.

Good Luck!
 
Rick is right on - you're reflecting RF power back into the system. If it doesn't go away when you are outside, check for "leaks" in the shielding inside the airplane. A common cause of this in Rv's is using a simple bent-rod whip antenna without a BNC connector. They are inexpensive, but very hard to install in a way that doesn't put RF back into the cockpit.
 
Agree, this is an RF interference problem; something is acting as a transmitter in the cockpit that shouldn't be. It's not related to any lack of wire shielding though, that pertains to a different propagation mode (unsure if Paul was indicating that to be the case or not in his post below). If you have access to a handheld transceiver, turn it on in the cockpit while reproducing the problem and you will likely be able to hear the interference. You can then get a sense of its source by moving the transceiver around to see when it gets louder. Probably will just tell you the source is behind the panel though, probably at or near the com itself. If you have any slack in your coax lead to the exterior antenna, try temporarily moving the antenna away from the airframe and see if it makes any diference.
Irritating and hard to pin point I know, but nothing to get too excited over. In my -7a, the fuel pressure goes up whenever I transmit. I don't even notice anymore.

Erich
 
RF most likely huh.

Thanks for the replies, this gives me some places to start. I do have a bent antenna and the aircraft is in the metal hangar currently when I tried this. I have used BNC connectors. I will move it out doors tomorrow and test. Actually I plan on moving the project tomorrow from Merrill Field to Birchwood Airport for final assembly. Not allowed to make my first flights out of Merrill. Either way I will report findings back. Hopefully successful findings.
 
Success

Moved the plane to Birchwood airport for final assembly today and tested the radios...no interference when outside...thanks all...save me from pulling my hair out.
 
I have the same problem -thought it might be unique to my installation. When I hit the push to talk my Ray Allen aileron trim indicator deflects. It sure helps to now that it might just be RF. No need to do a troubleshoot on the wiring til I check it out side as suggested.
 
RF inside the aircraft....

After reading all your replies and inputs, they are all valid but please hear my argument.

Reflection within a hangar can happen - yes, but it has to get back inside your aircraft with sufficient RF energy to cause other equipment to misbehave.

Using a BNC connector is not the end-all be-all to solving the problem assuming the coax cable braid is making good good contact with the "body" of the BNC plug..... both at the antenna and radio ends.

If the antenna has a BNC socket on an aluminium flange or base-plate, as in some Comant and similar models, make sure the antenna mounting screws which make electrical contact with the flange/base-plate also make good electrical contact with the fuselage/aircraft skin.... this may require scraping away paint/primer and using star-washers under the screw head or nuts (depending on the installation).

Using the bent whip antenna with its ceramic mounting hardware also requires that a star-washer on the inside fuselage/aircraft skin makes good electrical contact.

Failing these above precautions can cause RF energy to flow on the outside of the coaxial cable braid back into your cabin, hence defeating the purpose of the shielding nature of the coaxial cable. This RF energy flowing back into the cockpit environment will cause much more interference than reflections from hangar walls.

Hope this wasn't too much of a mouthful and was of some help.

I am quite willing to offer more advice on antenna & feed-line questions.

Peter - R&D engineer for www.laser-antennas.com ( Australia)
 
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Does the problem go away if you disconnect the antenna from the radio? If it does, that confirms that the transmitted RF from the radio is the probable cause, if not, then you may have a problem on the ground side. If all of your equipment doesn't share a good solid common ground, then you can get common mode RF effects on the ground side that can upset your avionics due to differential grounding.
 
yikes Dean, the original post says the problem occurs when activating the PTT. Not a good idea to disconnect the antenna and then start transmitting.

erich
 
Dean_aeroleds said:
Does the problem go away if you disconnect the antenna from the radio? If it does, that confirms that the transmitted RF from the radio is the probable cause, if not, then you may have a problem on the ground side. If all of your equipment doesn't share a good solid common ground, then you can get common mode RF effects on the ground side that can upset your avionics due to differential grounding.
Click to expand...

DO NOT DISCONNECT ANTENNA AND TRANSMIT!!! IT WILL DAMAGE UR TRANSMITTER
 
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