Induced voltages, trim switch issue, etc in a Slingsby T67C

[CFR]

1. When COM 2 (a Bendix King 155a) transmits it affects the engine gauges and most predominantly the fuel level gauges. It is worse at the low end of the spectrum (118.00) but does affect it across the band. We have done a number of diagnostics (swapping radios, swapping antenna's, temporarily relocating the antenna to various locations around the aircraft, and have replaced the antenna, VSWR checks are normal) and have concluded that it is RF induced voltage. The condition can be duplicated with a handheld radio held near the instrument panel. We have tried a trimmer capacitor in the instrument gauge wiring with no success. The COM 2 antenna is located on the underside of the AC below the seat area, around 3 feet from the panel. If the COM 1 radio (Garmin 300XL) is fed to the COM 2 antenna the problem is less but still there (the Garmin puts out around 5 watts, the Bendix around 10). Confusion reigns on this one !?!!?!?

2. When the electric trim is first pressed, it breaks the squelch for an instant on the comm radio's producing a momentary hiss in the headset. We are going to disconnect the trim motor tomorrow and see if the problem is at the controlling relay. If not it may be a capacitor issue on the motor leads. Any other thoughts on this?

thanks

[single_swine_herder]

Have you considered ferrite torroids on the leads to suppression potential common mode interference?

Although this link deals with audio applications, the same concept will hold true for the problem you describe ....

The best suppression in this case would come from a "Type 61 mix" for VHF/UHF applications, and a snap-on bead would be the most convenient and least intrusive to install. I make no suggestion as to the regulatory compliance aspect of doing this though, only the potential technical solution.

http://www.audiosystemsgroup.com/SAC0305Ferrites.pdf‎

Info on split-bead torroids .... http://www.k0bg.com/beads.html

The momentary trim switch issue is likely able to be suppressed with three .001 ufd capacitors, one from each terminal to ground, and another across the + and - terminals. This is a common issue in ham radio installations where DC motors of all types cause havoc with sensitive receivers.

[ettw]

Very interesting read. Thanks for bringing that up.

Cheers

ETTW

[CFR]

Have you considered ferrite torroids on the leads to suppression potential common mode interference?

Although this link deals with audio applications, the same concept will hold true for the problem you describe ....

The best suppression in this case would come from a "Type 61 mix" for VHF/UHF applications, and a snap-on bead would be the most convenient and least intrusive to install. I make no suggestion as to the regulatory compliance aspect of doing this though, only the potential technical solution.

http://www.audiosystemsgroup.com/SAC0305Ferrites.pdf‎

Info on split-bead torroids .... http://www.k0bg.com/beads.html

The momentary trim switch issue is likely able to be suppressed with three .001 ufd capacitors, one from each terminal to ground, and another across the + and - terminals. This is a common issue in ham radio installations where DC motors of all types cause havoc with sensitive receivers.

Thanks for the suggestions. We will give them a try and follow up here with the results.

[single_swine_herder]

Looking forward to hearing about the cure.

The folks at Amidon would be able to fix you up with the proper items in the correct sizes and have proven to be very helpful on the phone or by email.

Have you also discussed this with the recently retired "four alpha yankee?" As you know, he is a wealth of knowledge.

/www.amidoncorp.com

73

[captcrunch2013]

I'd say it is most likely to be grounding, it's not jinxed, it's RF floating around, is it possible that
the instruments and more likely, the radios are not not grounded internally or externally correctly?

Did you try any of the instruments in an RF field? Do they behave like FS meters?

Putting toroids on the power supply is curing the effect not the cause, and by hiding
the cause the problem will rear itself later in other ways.

I'd suspect RF leaking from the coax line, and while transmitting I'd check the field strength
along the coax, inside and outside. Then check the field strength in the vicinity of all the DC
wiring.

[CFR]

I'd say it is most likely to be grounding, it's not jinxed, it's RF floating around, is it possible that
the instruments and more likely, the radios are not not grounded internally or externally correctly?

Did you try any of the instruments in an RF field? Do they behave like FS meters?

Putting toroids on the power supply is curing the effect not the cause, and by hiding
the cause the problem will rear itself later in other ways.

I'd suspect RF leaking from the coax line, and while transmitting I'd check the field strength
along the coax, inside and outside. Then check the field strength in the vicinity of all the DC
wiring.

We did a significant amount of testing to ensure the coax is correct, radio output and VSWR were all good (field strength in various locations is a good plan and I will have to see about a field strength meter - not certain if it is an issue but I suspect the antenna to wiring harness distance is within the near field). We did find two marginal connectors (one on each radio) which were replaced. As well a coax made up of a number of jumpers in it, was replaced with a single coax. We thought as you did, that the coax or antenna ground were bad and reflected RF was getting into the system, however the results could be duplicated with a handheld using its small whip antenna. We're with you on the grounds, but it is a composite airplane with a significant number or grounding locations and the test points are very numerous. Testing them all is on the list, but some require removal of seat panels and other access points that make it a lengthy process (read expensive). There is some question as to the location of the COMM 2 antenna. It is on a metal taped area that provides the ground plane on the belly of the aircraft, that does not appear to be the correct size for the antenna. Additionally it appears it is not the original location of the antenna, as when we removed it (I replaced it thinking it was bad - $$$) the base was covering the outline a a small fin antenna. Toroids and capacitors are not the best solution, but as an interim they will hopefully clean up the issue until we find the culprit, most likely as a result of stumbling across it, while doing something else!

[iflyforpie]

I've seen this on Cessna products for years. Usually the fuel, oil temp, or flap indicators will be affected by PTT.

I always thought that was just a norm...... didn't really feel like messing around with things that otherwise work normally.

[CFR]

I've seen this on Cessna products for years. Usually the fuel, oil temp, or flap indicators will be affected by PTT.

I always thought that was just a norm...... didn't really feel like messing around with things that otherwise work normally.

Induced voltages can cause instrument damage, especially to electronic instruments. In a small A/C with multiple radio systems, there is limited space available to locate antenna's and some spill over is possible. However when we first noticed the glitch, TX on COMM 2 pegged the left hand fuel gauge. Changing out a poor connector and a new antenna has reduced it significantly, but still more than I would like. If we don't find a bad ground, or the beads/capacitors don't fix it, the only other option is to relocate the antenna further back (there are antenna's specifically designed for mounting within a composite aircraft that don't require a ground plane [basically a folded dipole] - but that could simply start a new set of interference problems). If all that fails then living with it is an option, but not one I want to do. I will modify my mental checklist to "When TX'ing on COMM 2, look at attitude indicator"

[NeverBlue]

it's a bonding problem...100%

Antenna, or panel it's mounted on. or ground plane for the antenna, or the coax itself.

check the VSWR

[CID]

I have a few suggestions.

1. Believe it or not, the microphone may be the culprit. Over-modulation can cause radios to emit frequencies beyond the normal spectrum. The KX-155/165 series have pretty decent protection against this but occasionally a microphone with it's level cranked up and/or any pre-amp fault can cause this. An SWR test won't uncover a fault like this. You would need a spectrum analyzer. Or you can just try a different microphone.

2. Corrosion near or at the faying surface of the COM antenna can also cause the transmitted signal to "splash" across different frequencies. Metal oxides can form structures that act like diodes. If you expose these diodes to enough RF energy they can "clip" the RF and start emitting all sorts of frequencies. Look for corrosion not only under the antenna, but in structures and joints near it.

3. Double check the coaxial connectors along the entire run. A broken shield right at the connector can cause the problem you're describing without being detected by an SWR check.

4. Cable separation can also be the culprit. Is the antenna coax run near any of the wires that feed the instruments? It may be well worthwhile to check it and add some separation.

5. Physical damage to a coaxial cable can also cause this. Check the run for any heat damage or pinching or crushing. This sort of thing can throw off the characteristic impedance of the cable and cause a mismatch.

How were you able to verify it's RF being induced and what frequencies did you detect? It may be worth borrowing a spectrum analyzer from someone to do a little troubleshooting. Is it the COM frequencies that are interacting with the instruments or some other spurious frequency that is caused by over-modulation or mismatch or something like that?

A pure sine wave can become a mathematical endless source of other frequencies when it's clipped or "squared off". A pure square wave contains the fundamental frequency and an infinite number of odd harmonics.

[CFR]

I have a few suggestions.

1. Believe it or not, the microphone may be the culprit. Over-modulation can cause radios to emit frequencies beyond the normal spectrum. The KX-155/165 series have pretty decent protection against this but occasionally a microphone with it's level cranked up and/or any pre-amp fault can cause this. An SWR test won't uncover a fault like this. You would need a spectrum analyzer. Or you can just try a different microphone.

2. Corrosion near or at the faying surface of the COM antenna can also cause the transmitted signal to "splash" across different frequencies. Metal oxides can form structures that act like diodes. If you expose these diodes to enough RF energy they can "clip" the RF and start emitting all sorts of frequencies. Look for corrosion not only under the antenna, but in structures and joints near it.

3. Double check the coaxial connectors along the entire run. A broken shield right at the connector can cause the problem you're describing without being detected by an SWR check.

4. Cable separation can also be the culprit. Is the antenna coax run near any of the wires that feed the instruments? It may be well worthwhile to check it and add some separation.

5. Physical damage to a coaxial cable can also cause this. Check the run for any heat damage or pinching or crushing. This sort of thing can throw off the characteristic impedance of the cable and cause a mismatch.

How were you able to verify it's RF being induced and what frequencies did you detect? It may be worth borrowing a spectrum analyzer from someone to do a little troubleshooting. Is it the COM frequencies that are interacting with the instruments or some other spurious frequency that is caused by over-modulation or mismatch or something like that?

A pure sine wave can become a mathematical endless source of other frequencies when it's clipped or "squared off". A pure square wave contains the fundamental frequency and an infinite number of odd harmonics.

Thanks for all your suggestions. We are working to isolate many of the items you have suggested. Just as a point of reference while I am a HAM radio operator, I am new breed which requires less technical knowledge, however the work is being done by an avionics tech and I have input from a friend who is both an old school HAM and worked as senior tech maintaining TV and radio broadcast stations. So we are not exactly shooting blindly, but we can certainly use all the suggestions we can get!

1) Pure carrier without a microphone attached causes the issue. We have tried with the microphone as well to see if we can see modulation on the gauges and there is no detectable fluctuation. As well when we hooked my COMM 1 radio to the COMM 2 antenna we had the same problem although it was less (COMM 1 radio output ~5 watts, COMM 2 radio output 10 watts). As well we could induce the problem using a hand held TX at various distances from the gauges.

2) The antenna was replaced and when it showed the same issues, we removed the new one and used an aluminum plate to make a ground plane for it. We then used a brand new jumper cable directly from the tray to the antenna. When we located the antenna near its normal mounting point we had the same problem. I then moved the antenna around the aircraft. When we were on the right side, the right side fuel gauge was most affected, when on the left side it was the left gauge. Moving around towards the tail we found a sweet spot that had no interference, and may be one of the options to correct it, move the antenna.

3) As noted we did find some suspect connectors in the permanent run and as well bypassed the whole thing with a new jumper.

4) This is a possibility, and we may be able to make some changes here, however this is not a large aircraft, leaving very limited options for cable routing, in some places, none.

5) again we have used a jumper and as well used the primary radio and saw the same failure.

As usual the antenna had its best match at the middle of the band, with 1/4 to 1 watt reflected at either end. The gauge fluctuation was highest when VSWR was lowest, meaning the more energy radiated, the more the gauges were affected. Your thoughts on a spectrum analyzer are helpful and I think we can get our hands on one. I am getting a field strength meter to see if we can map the radiation pattern of the antenna.

During our latest attempt yesterday, we tried trimmer capacitors on the electrical system which did not solve it, and a ferrite toroid (unknown composition, just one that was sitting around the bench) around the wiring bundle. This gave an interesting result in that it made the situation worse! One of the references I have says that if the ferrite is not the correct composition and not matched to the frequency it can make things worse. I have the correct ferrite snap on beads on order, so that is our next try, before relocating the antenna.

As to the comment about poor grounds, we have run temporary grounds from the antenna, the radio tray, the bus bars,etc with no change. We are eventually going to work our way through all the grounds, but being a composite airplane with a large number of radios, Nav aids and other systems, there are a serious number of grounds to check in some difficult to access locations, including all the lighting protection bonding straps! The issue now is becoming one of paying for tech time vs the impact of the snag itself.

Thanks again for all your suggestions, we now have a few more avenues to follow up on.

[torquey401]

Things that I didn't notice mentioned. Did this problem just crop up or has it been a long time issue? Has it been getting worse? Did it start after a certain event? Have you tried different combinations of radios or other electrical items either on or off? Is it temperature related? What is common to the engine gauges and the fuel quantity?

[NeverBlue]

Check the instrument panel bonding.


Great suggestions here though and it appears as though you're trying everything you should be.
Ditto on Torquey's questions.
CID knows his stuff but it appears as though he's been given a holiday.

Good luck!

[CFR]

Things that I didn't notice mentioned. Did this problem just crop up or has it been a long time issue? ...

That is an interesting question that I have been struggling to answer myself. The aircraft originally had the Navcom radio as COM1 and a VHF COM radio as COM2. There was a DME system etc.
When I got the airplane the DME was removed, the COM1 radio had been changed to a Garmin 300XL and the original Navcom had been moved to the lower radio position in the stack and is wired to COM2 on the COM panel. We assumed that the COM1 radio was connected to the antenna in the vertical stab and the COM2 radio to the belly mounted antenna as shown in the manual. It wasn't until I had my tech do some work on the system that we discovered that the COM 1 was to the COM2 antenna and vice versa.

With respect to the TX affecting the gauges, I was on a x-country with a friend and was calling an RCO for an altimeter update when my friend pointed out that the needle pegged on the left hand fuel gauge when I tx'd. This led to the current run of diagnostics which found two poor coax connections which we replaced. This appeared to resolve it BUT we used the Garmin radio and I think that we tested it on a frequency that gave the worse VSWR and so it didn't affect the gauge. Last week, I had some more work done, some of which was to change the antenna's back to their correct radio's and the issue appeared again. What I think occurred is that when the work was done for the original purchaser, they had the same problem. The King radio puts out 10 watts, while the Garmin puts out 5. This leads me to believe that their solution was to swap antennas so the the higher powered emission was further back. I became aware of it because (a the coax connection was failing and b) I had a pax who happened to be looking at the gauge while I was transmitting. All this to say that I suspect it was there from my purchase, but only came to my attention recently.

Of interesting note, the wiring diagram for my aircraft was not properly updated when all the comm work was done and cut of wire and coax from removed systems were left in the plane. My tech is really pissed at this kind of work, and over time we are updating the diagram and removing dead wire (I have a 3 foot section of coax from the DME that we took out last weekend).

[NeverBlue]

Just wondering if you've had any resolution CFR?

It would be interesting to know what you've found since...if anything.