RF Issue - possible resolution

[PhantomPholly]

EDIT 2009-10-11 - So you don't have to read through this entire thread.
Less expensive solution found here (USA). (http://www.l-com.com/productfamily.aspx?id=902)
European customers try this link. (https://www.distrelec.ch/ishopWebFro...ries/is/1.html)
End Edit.
__________________________________________________ ____________________

Hi all,

I know that over the years there have been some folks (including me in my previous Lancair) who had an issue where keying the mic caused sudden pitch changes (up or down depended on whether the reverse jumper was used).

One of the regulars on the Lancair Mailing List believes he has found a solution, and I thought it would be a good idea to post it here so one or more people including the TruTrak folks can verify it works. If it does, perhaps TruTrak may wish to incorporate similar circuitry in their future designs.

I am happy to report that the RF problem I was having with the Trutrak autopilot has been solved. One of the posted suggestions pointed to a filter adapter that is installed between the controller and the harness. I ordered part number FCE17-B25AD-250 from Digikey (http://parts.digikey.com/1/parts/1599616-filter-male-female-adpt-25pos-fce17-b25ad-250.html) for about $25 and it cured my system. Not a hint of interference from either radio.
thanks to all who wrote in.

Chris Zavatson
N91CZ
L360std
www.N91CZ.com

Cheers all,

Bill

[TrutrakTech]

Hi Bill,

Thanks for the tip. I'll p*** this on to the engineers and we'll take a look at it. I'll post back with what we find.

[PhantomPholly]

Ok, I went and tried it. In my current airplane, the pitch change was not particularly noticeable in the air as it was in my prior plane, but on the ground if I engaged the autopilot and then pressed the transmit button the stick would move a couple of ticks on press (forward) and release (back).

I put one of those filter thingies in and voila' - no movement.

May not work for everyone.

Cheers,

Bill

[PhantomPholly]

TruTrak folks -

On most forums there is an option to make a post "Sticky" (stays near the top). It is probably limited to admin/moderators.

If this forum allows that, you may wish to make this post "Sticky".

Cheers,

Bill

[n737z]

I tried all the suggested solutions except moving my antenna. Then I followed the link as previously posted on the D sub 25 pin filter supplied by Digi-Key, purchased it, installed it in 5 mins, and it solved my pitch up problem with my PTT. I believe my problem originated with my using a single shield coax instead of a double shielded coax like RG400 on my radio antenna. Although properly routed and grounded at all points the RG58 leaks to much RF. Apparently the Garmin SL40 puts out a lot of RF. Here is the link for the part which cost $27 plus shipping. Good luck!:D

http://parts.digikey.com/1/parts/159...b25ad-250.html

Don Pierson
RV7
PHX

[TrutrakTech]

We have one on order right now to do some testing with it. Our problem so far has been that we are unable to duplicate the PTT pitch change problem here. I will keep this thread updated with whatever info we obtain. If it continues to show good results, we'll definitely make this a BIG sticky! Thanks to everyone who's helped!

[N91CZ]

We have one on order right now to do some testing with it. Our problem so far has been that we are unable to duplicate the PTT pitch change problem here. I will keep this thread updated with whatever info we obtain. If it continues to show good results, we'll definitely make this a BIG sticky! Thanks to everyone who's helped!

Lucas,
FWIW, before I tried the filter in my Lancair, I performed a few tests to see if the root cause was radiated or conducted energy. First I checked all ground connections with an ohm meter, such as ground to panel to avionics stack to antenna shields etc. Everythign checked out fine. My pitch up interference was caused by the #2 com antenna which sits in the tail cone on an aluminum shelf. The entire unit is removable. By moving the #2 antenna, I could move the interference source from the #2 radio to the #1 radio. Placing the #2 antenna off to the side of the aircraft the completely moved the RF problem from transmissions on #2 to #1.
Much of the equipment wiring is not ideal for preventing RF energy from getting into the various circuits. Some equipment in the panel has plastic housings, ground and power line pairs have to split and go on different paths, the ground bus and power bus is too far apart.
I suspect one or more of these deficiencies is allowing the RF energy to get into the power/ground wiring and the auto pilot pitch gyro happens to be sensitive to this. I saw two options. First was to redesign all the panel wiring to make it less suseptible to RF energy or trying to catch the RF energy at the autopilot. The second option was much more appealing. The good news is that it completely cured the system.
Since posting my results a few weeks ago, other Lancair owners have also been having success with this approach.
Regards,
Chris Zavatson

[TrutrakTech]

Chris,

It's curious that everyone is having good luck with this "filter". Ours finally arrived. When we dissected it, we found nothing but a male to female adapter. We will try to look into it further as I'm thinking perhaps the metal casing around the adapter is helping to "filter" out the RF going in on the wires. I'll post more info when we have it. Thanks!

[N91CZ]

Lucas,
These adapters have filter capacitors between each line and the case. Not fancy, just conveniently packaged. Different capacitances are available - see spec sheet for values. FCE17-B25AD-250 uses 1000pF.
Chris

spec sheet is available here:
http://www.amphenolcanada.com/ProductSearch/drawings/AC/FCE17XXXADXX0X.pdf

[TrutrakTech]

Lucas,
These adapters have filter capacitors between each line and the case. Not fancy, just conveniently packaged. Different capacitances are available - see spec sheet for values. FCE17-B25AD-250 uses 1000pF.
Chris

spec sheet is available here:
http://www.amphenolcanada.com/ProductSearch/drawings/AC/FCE17XXXADXX0X.pdf

Well, I might have to have a talk with the engineers then....

Thanks for the info Chris, I'll print that and give it to the engineers that told me it was nothing. :)

EDIT: Apparently they gave it a quick visual inspection and that was it. A more thorough test is coming soon.

[TrutrakTech]

We continue to have more and more reports that this filter is resolving the problems that some customers are experiencing. We have a small batch ordered now to keep on hand.

Thanks so much to everyone who gave it a shot. We appreciate the feedback!

[TrutrakTech]

A customer of ours did some searching and found this filter to do the trick also. It's cheaper, slimmer, and available in several different pin configurations without being on factory backorder.

http://www.l-com.com/productfamily.aspx?id=902

[flyvans.com]

we also had the pitch down emi issue in our rv-7a when transmitting on com1.

we can confirm that a db-25 filter solved the issue completely.

https://www.distrelec.ch/ishopWebFront/catalog/product.do/para/artView/is/true/and/node/is/DC-70623/and/productNr/is/125806/and/id/is/01/and/series/is/1.html

for all european customers, this might be a good source as well... also, the unit costs only approx 20$.

rgds,
bernie

[Erbman]

Like many others, I had an excessive pitch-up from interference on my ADI Pilot II autopilot in altitude hold mode when transmitting on my COM1 radio. However, the same filter on the DSub connector that everyone else says fixed their problem had no effect on my installation. I left the filter installed since it didn't make anything worse and may have some small positive effect.

I have now found the solution to my problem, which was a need for better shielding. I won't bore you with all of the other unsuccessful fixes I tried, but suffice it to say that I determined that my COM1 antenna was placed in what seems to be exactly the right spot to cause interference with the autopilot control head in the instrument panel. In my Bearhawk, this antenna is directly over the ****pit between the wings. RV builders don't put an antenna that close to the instrument panel because that area is a transparent canopy on RVs. If I could have moved the antenna about a foot closer or farther away the problem was greatly reduced. However, since this is a tube and fabric airplane, I can't just drill another hole in the skin to move the antenna.

Part of the final solution was sending my control head back to Trutrak to have the plastic box replaced with a metal box. By itself, however, this did not fix the problem. Eventually we would find the problem was that the metal box did not cover the entire outer surface. The plastic faceplate left about a 3/4" gap between the metal box and the aluminum instrument panel. With this gap, the aluminum instrument panel and the metal box on the control head responded to radio frequency energy (radio transmission) in such a way as to funnel that energy into the control head in such a way as to cause a large pitch up.

The solution was to complete the shield between the metal box and the instrument panel. To do this, I purchased some copper adhesive tape (www.amazon.com, part number CFL-5CA/COP07536). This is not just any copper tape--the key point is that it has a conductive adhesive. This is critical--if the adhesive on the tape does not conduct electricity then the tape will have no effect. There must be electrical continuity between the tape and the metal box of the control head.

I wrapped the copper tape from touching the metal box, over the plastic faceplate, and over on to the front of the control head where it touches the instrument panel when installed. I did put a piece of paper over the seal that says "Warranty void if seal not intact" underneath the tape so that if I ever need to prove that the case has not been opened I can remove the tape and the seal will be intact.

I sanded the paint off of the back of my instrument panel where the control head touches when installed to ensure good electrical continuity. I then re-installed the control head.

To test the system on the ground, I engaged the NAV and ALT modes of the autopilot with the airplane sitting in the hangar. Prior to the fix, when I transmitted on the COM1 radio the pitch servo would make much noise and pull the stick back about 5 inches. After the copper tape fix, pressing the transmit button caused absolutely no response from the autopilot, not even one clunk on the servo. I have yet to test this in flight but I expect the same results--no interference.

[oscilentinc]

I don't know if this will help or not but I think its a solid rf filter site - http://www.oscilent.com/catalog/Category/rf_saw_filter.htm