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Radio Frequency Effecting Electronics International Instruments

Steve Pierce

BENEFACTOR
Graham, TX
Worked on a Super Cub last week with all Electronics International engine instruments that was making the electronic tach redline when the push to talk was pushed. Called EI and they said you are picking up RF. Checked the coax and all checked good but went ahead and changed the single shield RG58 with RG400. Well on his trip home it got worse and started effecting other instruments. My Dad who dealt with radios in the Navy for 20 years says it needs to go to a good avionics shop that can bench test the radio and has a meter to read RF. I remember a thread years ago on here but have been unable to find it. Any insight would be greatly appreciated.
 
Worked on a Super Cub last week with all Electronics International engine instruments that was making the electronic tach redline when the push to talk was pushed. Called EI and they said you are picking up RF. Checked the coax and all checked good but went ahead and changed the single shield RG58 with RG400. Well on his trip home it got worse and started effecting other instruments. My Dad who dealt with radios in the Navy for 20 years says it needs to go to a good avionics shop that can bench test the radio and has a meter to read RF. I remember a thread years ago on here but have been unable to find it. Any insight would be greatly appreciated.

Your coax is routed to close to the tach and it's wiring, or you have the radio and tach mounted to close to each other. The best way to deal with interference is distance. Even an inch or so can make a world of difference. I have taped the coax to the underside of the boot cowl with aluminum tape with very good results. This keeps it away from all other wiring in the panel area and still leaves it 'reachable' for maintenance. Mounting an electronic instrument next to a radio is also a no-no. This one can be a bit of a coin toss as some of the better quality radios are well shielded, internally, where the cheaper radios are not. The shielding does NOT eliminate RF but it does mitigate it, which can be enough for a particular installation to work.

Then there are the very basics. Make sure your coax ends are terminated securely and correctly. Crimp style ends will produce a longer lasting and lower loss end than the screw together types. Check that the grounds (especially for the radio) are squeaky clean and secure. In this case it would be a good idea to seperate the avionics and airframe grounds. Once again, even a little distance between will sometimes make a big difference.

Web
 
Could be a bad antenna or antenna match giving you a high SWR, which means too much of the power that should be radiated is coming back. I don't know what impedance that cable you used was and is, but it makes a difference. Only one radio? IOW, no antenna switching.

I flew a brand new Huskey several years ago. Whenever the PTT was pushed the oil pressure went to Zero on the EI gauge. Got my attention the first time. I don't know what the fix was. Others didn't do it. I flew 3 or 4 different new ones for the dealer at that time.

Rich
 
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I've actually seen this quit a few times. Always in an aircraft with tight panel spaces or with wires bundle together. More annoying than anything as the gauges are not damaged.

Web
 
There's some metal shielding that's used to isolate the whiskey compass from electrical interference. Could you wrap the wiring with that?

Anne.
 
Anne

It does help but there is no stopping RF. Distance between the source and the effected instrument is the only sure cure.

Web
 
The cub has an excessive amount of RF in the cockpit compared to other aircraft due to the skylight/greenhouse window. Maybe a screen across the top, bonded to the sides....?? Also, coax leaks a lot of Rf...foil shielded wire is the best, but not real available for transmitter use..
 
As wireweinie said, proximity is everything. The aluminum tape to the cowl is a good idea, it forms a .006uf cap for every 2 7/8 inch square stick to aluminum.....you might also try some clip-on ferrite filters to the cable at the instrument.
This one is 1/4 inch hole size....http://www.radioshack.com/radioshac...rm=2730105&cid=iP:PLA:RSO:Google#.VHoFoWe9Y38

Those ferrite beads are especially good for installations where you get that loud pop in your headset with the PTT. I've seen it bad enough that it would change the flip-flop display. The beads on the antenna and on the power/ground wires can knock this down to manageable levels.

Web
 
The cub has an excessive amount of RF in the cockpit compared to other aircraft due to the skylight/greenhouse window. Maybe a screen across the top, bonded to the sides....?? Also, coax leaks a lot of Rf...foil shielded wire is the best, but not real available for transmitter use..

In this case the source seems to be the radio/coax. Screening the panel would not keep the signal out. Need to insure that the RF can be minimized and then the source of it is moved away from the affected instrument.

Web
 
Just for clarity, I was referring to screening across the greenhouse window...

The hardest part of building a headset mic preamp is shielding it from RF and creating a feedback howl when transmitting....only had trouble with that in a cub...
 
The radio coax is going up the right engine mount support tube and up the right windshield pillar to the antenna on the right outboard fuel tank cover.
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Steve

Where are your P-leads routed? I'm looking at your pic and if you have routed the tach wires to the mag toggle switches, they are getting close to where the com coax is going, up the right windshield post. Just for a ground check, can you pull the mag switches and tach wires and pull them left of center of the panel? If this is where the interference is injecting itself, then moving the switches/wires will prevent the squirrely tach upon PTT.

Web
 
The airplane is back in Oklahoma so I will have to pass this on. I am guessing that the P-leads are routed to the left side of the boot cowl.
 
This airplane was built in 2002 by CC. This problem started every now and again about a year ago. After we started messing with it it got worse. They moved stuff around yesterday and got it better. I will let them post on what they did and what it changed.
 
A little history on this airplane;

Purchased three years ago, no known problems at the time. Still has same instruments, comm, and battery, no electronic modifications. (Batt at least 3 years old) All engine monitoring instruments are EI, these were added after it left CC, but had been in the plane for some time before my friend purchased. A few months after purchase, the over volt light was observed coming on momentarily and intermittently, maybe every 5-10 flight hours (still does). This correlates with no particular cockpit function or repeatable circumstance such as pressing the PTT. I checked the MFGs voltage regulator output specs, if memory serves spec was 14.1, he was never able to see any spike or indication, other than the light, that there was an actual problem. After some time and research we decided that it could simply be that the light is set internally and a little on the low side for the regulator.

Over the last 12 months the new problem began to build, occasionally at first, growing in frequency until it happens every time the PTT is pressed. The EI digital tach red lines with all lights lit, and display shows 2700 RPM, no matter the power setting, and returns to normal as soon as the PTT was released. This became annoying enough to have Steve take a look at it during this year's annual, his findings and actions are posted above. To be clear, we had to depart to meet a schedule so Steve did not have the opportunity to test and troubleshoot as he normally would. But did take the time to add a few zip ties to try and clean things up, was a bit messy under the panel, not your typical everything nice and neat aircraft style wiring job.

During the ride home several new problems were observed. Now when the PTT was pressed, in addition to the tach redlining, both, oil temp and oil press (same instrument) redlined in the same manner. Also, upon RELEASE of the PTT, a new squeal was present in the headset. The squeal also happened when I would release my PTT after transmitting to him from 100-300 yards away, more distant transmissions did not produce this squeal. Additionally, now the fuel totalizer would flash and reset, again, only upon RELEAE of the PTT, not during the transmission.

Having more electrical experience than the owner, the next day I went over to take a look. I removed a number of wire ties to manually trace all connections and check grounding for all components. I wouldn't have wired it the way it was, but all connections and grounds were consistent with safe and acceptable wiring practices, so I didn't want to start hacking away just to make it look good to my eyes. It was also discovered at that time that the oil temp would drop about 5 degrees and the CHT would drop from 10-30 degrees while transmitting, but both would go back to base line when the PTT was released. All of these problems were repeatable every time, whether the engine was running or not, with the exception that the tach was only affected with the engine running. All grounds showed full continuity back to the battery (< .1 ohms).

After about 4 hours of testing and head scratching we decided to button it back up so he could fly if the chance presented itself. I took the time to untangle what I could, straighten out the rat's nest somewhat and restore a little order under there. Upon completion one last test revealed that all problems had now disappeared with the exception of the temp drop in both gauges. We did not run the engine again, it is unknown if the tach or other problems would act up if running. This all seems to support the proximity theory mentioned above. But we'd still like to have confidence that it will work under all circumstances or if further work is ever required under the panel that would necessitate removal and rearrangement of the wires.

For good measure another friend of mine who has a little RF experience asked the owner to temporarily lay aluminum foil across the top of the aircraft on both sides of the antenna and over the skylight, windscreen, and glare shield in an attempt to shield the instruments from the known RF source just to see if there was any change. This test was completed this morning with no changes noted, both temps still drop. This friend also suggests replacing the battery, due to age, for good measure. In our industry we've seen some weird stuff with older batteries that would indicate full voltage, but not be able to supply sufficient current, in particular when solid state components are utilized.

We had considered the ferrite doughnuts, but seemed a little hard to get on without cutting and splicing due to the size of the plugs, which would have opened the door for more potential problems. Thanks for making us aware of some that clamp around.

We would still like to have a fix, as this seems like a, "hold your mouth just right, tilt your head to the right side and cross your fingers on your left hand," kind of a fix. IOW no fix at all, just a little luck with no confidence that things will (sort of) stay working.

Thank you for all comments and suggestions so far, and in the future. They are helping to paint a more complete picture for a couple of guys who have little to no RF experience. We are kind of stumbling around in the dark right now.

The owner may respond to correct or elaborate on what I've stated.
 
After posting, it's not showing up as a new post. Not sure if I did something different, but this is to try and bring it to the top so the smart kids in the room will know there is something new to look at.
 
You have good coax (RG 400) so double check the installation of the BNC connectors. The shield braid needs to be evenly and securely crimped at each connector. I know the center conductor is crimped correctly as you state that you are able to transmit. You can check that the base of the antenna is secured to a good ground plane. If it's mounted on metal, like a wing root fairing, make sure the paint is removed at each mounting screw. A bonding wire brush works good for this. If you are using something else for a ground plane, disconnect the antenna coax and make sure the ground plane is grounded to the airframe.

If you use the ferrite beads there are two types. One is made in halves that snap together over the coax. The other, more common type is just a bead. This type is slipped over just the power and ground wires, usually at the back of the unit effected.

From what you have described, I'm still thinking that the coax needs to be separated from some other wires. If you haven't already done so. Find the com coax at the back of the 430. Make sure it's not tangled with ANY other wires. Chase it up the windshield post for the same reason. Bundle any other wires in the post area. Untangle the coax from any wires in the wing root area and separate it when closing this compartment. Starting at the lower right corner of the instrument panel push the coax up and away from the other wiring. Tape it in place with aluminum tape. Now route it around to the back of the 430, staying as far away from other wiring as possible, including other coaxes. Keep taping it in place as you don't want it to move as time goes on.

Test the set up to make sure there is no interference.

Web

Something I forgot to ask earlier: Are the P-leads and tach wires as short as possible or are they coiled up under the panel. Long wires will pick up more interference than short wires. Just a bigger target.
 
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I'm kinda leaning towards a common(?) dirty ground connection that the instruments/ptt use??

Won't rule that out, but just from experience with this in the past and their description of most of the issues going away with new coax/sorting wires, I'm still thinking true RF interference. With that being said it's always a good idea to use a separate ground for avionics. And preferably one closer to the battery ground than the airframe grounds. Also the trick with wiring the magneto ground through the P-lead shields and back to the switch helps.

Web
 
It might be appropriate to bring back a previous post: QUOTE: An engineer bud of mine worked for Lockheed. He was a newbe and knew nothing about the F-106 mods that they had just put into the field. His 'trainer' was sick, and he was called out to an air force base somewhere and presented with their newly (avionics) modified F-106. he was told by a USAF officer of high rank that nothing worked, everything interacted with everything else, and fix it or else. He said that he was sweating bullets and contemplated a run for the fence. He had absolutely no clue what to do. He then noticed that every box on the airframe was grounded to the airframe in a dozen different places...he thought that well, his stereo wouldn't work well like that, so they ran all the grounds to a "ground mecca" fastened to the airframe in only one place. It worked, and he was a hero.
The bottom of the panel is usually the ground mecca for avionics in the cub....
 
I believe all the instruments and avionics are centrally grounded where the controllock usually goes.The coax runs alone except for a nav light wire from the radio to the outboard end of the fuel tank cover.
 
It might be appropriate to bring back a previous post: QUOTE: An engineer bud of mine worked for Lockheed. He was a newbe and knew nothing about the F-106 mods that they had just put into the field. His 'trainer' was sick, and he was called out to an air force base somewhere and presented with their newly (avionics) modified F-106. he was told by a USAF officer of high rank that nothing worked, everything interacted with everything else, and fix it or else. He said that he was sweating bullets and contemplated a run for the fence. He had absolutely no clue what to do. He then noticed that every box on the airframe was grounded to the airframe in a dozen different places...he thought that well, his stereo wouldn't work well like that, so they ran all the grounds to a "ground mecca" fastened to the airframe in only one place. It worked, and he was a hero.
The bottom of the panel is usually the ground mecca for avionics in the cub....

Good idea but bad location. I usually run the grounds to the tabs that are used to attach the old style stick lock. Clean the tab and install a ground stud (studs) as per the AC43-13 and you have a ground point that matches well to the battery ground. That way the entire fuselage frame becomes your ground point as it is, electrically, one welded piece. The panel itself is not a good point as it attaches through screws, to the fuselage. The panel and attach points are painted or powder coated in most cases and even if cleaned, over time the corrosion will build up, increasing resistance to ground.

Web
 
My bud had the base machinists build a copper plate with threaded holes all over it for the (insulated) extended ground cables, and one to go to the airframe. The idea of a ground Mecca is to 1. make sure that all grounds are the same potential, and 2. the airframe is effectively out of the picture. The airframe is made out of steel, and is resistive. The idea is to avoid transferring any current (especially signal currents) through the airframe...
 
Chasing Rf is always a problem even for techs that worked in the field. Some really key points in some of the above posts: Check for returning RF from a high SWR at the antenna. I think this is the #1 cause of RF interference in a prior working system! Use a central grounding points, not multiple grounds.

Squeals: You could be dealing with a bad transmitter stage creating multiple frequencies or a spread of frequencies. We used to call that an oscillating transmitter (generally in the final amplifier taking the original frequencies delivered to it from the driver and mixing them together to form a complex but very dirty signal. Open bypass capacitor sometimes) Many times squealing in the audio will be the result as will strange RF problems in nearby devices. The best way to determine this is with a spectrum analyzer brought directly to the offending "installation". An advanced procedure for an avionics shop.

A little theory may help. RF of the VHF variety travels only on the surface of a conductor so wide, thin conductors are the best (coax works on a different principle. Talking grounds here). An ohmmeter won't tell you much about how your grounds are functioning at RF but of course low resistance is important but that's just the start. If you attach a RF ground to a stud that penetrates a plate, the RF will not follow the stud through the plate to the other side but will instead leave the stud connection, cross the plate, go around the edge of the plate, go back across the plate on the other side of the plate and then follow your ground wire attached to the other side of the stud to ground or wherever it goes. While it's running around on the surface of the plate it will cause all kinds of problems. One way to deal with this is attach your ground return to the stud on the same side as the other wires you are trying to ground, NOT the opposite side. Another trick I used to use is to take a long piece of 2 inch wide copper sheet strap and first attach it (wide connection) to battery minus and then in as straight a line as possible, attempt to poke around (remove and bypass other grounds) and see if I could reduce/eliminate the problem and that gives a direction to go in (sometimes). Moving the battery end along the grounding path can help to locate problems also. Never believe you have solved the problem until all configurations and time tells you it's cured. The act of troubleshooting can effect the results unfortunately. I used to have a dummy load connection (just a 50 ohm non reactive resistor of the proper wattage) I could substitute for the antenna at the antenna (Motorola NMO mount) and this could "help" tell if the problem was direct radiation from the antenna getting back into some wiring or if the coax/radio/ grounding was the problem. One thing no one mentioned is wire length: If wire is the "wrong" length it can act as an efficient antenna at the frequencie the transmitter is radiating at. If "wrong" it will efficiently couple a good signal right into some other device overloading it's protections on say the ground wire or maybe the power wire or perhaps some sensor wire. Sometimes cutting or adding a few inches of wire to a lead solves the problem.?

The suggestion of shielding over wiring can be good but can sometimes make things worse or maybe a band aid masking the original problem such that it just shows up somewhere or some when else. The shields can re radiate the energy or contain it and intensify "crossover" to other wiring.

1. Double check power output and SWR from the antenna and repair/correct as needed.
2. Attach dummy load at the antenna (replace for antenna)and check for interference (direct radiation from a properly functioning antenna causing problem? Not a radio or coax problem)
3. Attach dummy directly to the antenna connector on radio (eliminates antenna and coax)
4 Isolate radio with a separate battery and remove all aircraft wiring attached to transmitter. Reattach wires one at a time to see where RF is getting into aircraft wiring/devices. (Not necessarily conclusive)
5. Bypass grounds with copper strap one at a time to look for direction to problem.
6. If you can get your hands on a spectrum analyzer (fancy RF voltmeter) a probe can show up hot spots and I find mine very useful. A lot of hams have them as the surplus market is fairly cheap. Maybe SC.org should own one as a loaner :)

I have to run but I know there are many things to discuss and maybe post or edit more later. RF problems are a B---- for anyone and that is why their called gremlins! The only worse problems I had were intermittents and worse yet intermittent RF problems!

Remember if you use double shielded coax ONLY ground one end of the shield! Coax works to deliver a signal to the far end and both ends are grounded. Putting braid over say a power wire, if you attach the braid to ground at both ends you create a ground loop and that will couple whatever is outside to the inside and vice versa! Don't do it.
 
I've been following this thread with true interest primarily because of the voodoo nature of it all but also because of all the radio's I've had over the years that I've paid a bunch to make work and I want to say while I greatly appreciate the knowledge being shared but that my brain is now saying this is akin to spelling rules (all over again)... So, here goes; RF before ground after except coax, right?
 
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It's really not as complicated as it may seem. Just some basic rules that apply to any installations. Good, clean grounds on a spot on the airframe that does NOT have a bolted or riveted connecting between the ground stud and the battery ground. Separate avionics and airframe system grounds as much as practical. Never bundle an active coax cable with any other wire or coax. Active means it receives AND transmits. This includes all comms and transponders. P-leads are always separated from all other wires. Shielded audio wires can be bundled together but not with other wires. Items that produce 'electrical noise' such as strobe power supplies or cooling fans, need to be physically located as far from audio wires and components as practical.

And the last rule is that even when you do all of the above, you will still run into situations where you have noise or interference problems. Then it is just process of elimination. Pull breakers until you can isolate the system causing the problem. Keeping the above rules in mind figure out what can be done to eliminate the problem. There will be times when you may have to move a ground point or re route a wire to stop interference. A simple fix for noisy audio is to move the avionics ground point all the way to the battery ground. This extends the ground wires but can eliminate a ton of issues.

And if something eliminates interference or noise, it's not a band aide, it really is a fix, lol.

Web
 
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