Metal-to-metal RF interference

[leccyflyer]

Got a question that the radio and electronics gurus here might be able to answer.

It's been suggested that, with modern radio gear, the old bug bear of RF interference generated by vibrating metal-to-metal contacts (such as between a metal clevis and a metal control horn, or a metal clevis and a worn pushrod threaded section) no longer constitutes any problem to model flyers.

It has also been suggested that the amount of RF generated is determined by the mass of the objects in contact and that big objects will generate more RF radiation/interference than smaller, lighter objects, to the extent that making and breaking metal-to-metal contacts in a box of aluminium foil strips would not represent a source of metal-to-metal radio noise.

Link to original thread here

http://www.rcgroups.com/forums/showpost.php?p=3989148&postcount=28

Any comments on the controlling factors on metal-to-metal noise and the risk this represents to modern radio systems?

[vintage1]

Frankly Brian that post is utter bunk.

I've had the worst interference from a metal clevis rattling in a 1/" throttle arm on a (gasp) glo motor..

I forget what it is that causes it, but the size of the lumps is not an issue as far as I can remember.

As for modern radio gear - well it still happens. Maybe with PCM and suchlike you wouldn't notice it, but on litle cheapo S/C sets you certainly does.

ANY interference is bad news.

I do seem to remember you need dissimilar metals to cause it though, so a bunch of identical foil strips wouldn't do it. And inside a box they would be screened anyway.

[Andy W]

I agree that it is still a problem. Any slop in metal - metal contacts/linkages is bad news in my experience, especially brass-steel.. Aluminum, however, is not so bad..
..a

[Tom Harper]

Yep.....the laws of physics haven't changed much in the last fifty years. And the products available make tempting fate unecessary. Used to be that tin snips were standard tools and we cut control horns from tuna cans. Don't need to anymore.

Good point Vintage. Think I'll put a plastic clevis on the throttle.

[Tom Harper]

Interesting link. The writer is wrong that there is metal to metal contact in an engine. The parts float on a film of oil. The old Thor engines relied on electrical conduction through the crankshaft to the case. Worked fine until the engine attempted to start. Then the shaft floated on oil and broke the ignition circuit. The engine would pop all day but could never run.

[leccyflyer]

Yep.....the laws of physics haven't changed much in the last fifty years. And the products available make tempting fate unecessary. Used to be that tin snips were standard tools and we cut control horns from tuna cans. Don't need to anymore.

Good point Vintage. Think I'll put a plastic clevis on the throttle.

Tom

I've always used a nylon clevis on the throttle, for precisely that reason, unless the throttle horn itself was a nylon one. Usually used a steel bowden cable with a nylon clevis at each end. I do recall articles advising the soldering of a bridging link between metal components where that sort of metal-to-metal intermittent contact may cause RF. If the two pieces are linked by a short soldered piece of thin wire that is sufficient to ensure that no makeing or breaking occurs.

[JMP_blackfoot]

ANY interference is bad news.
Agreed
I do seem to remember you need dissimilar metals to cause it though, so a bunch of identical foil strips wouldn't do it. And inside a box they would be screened anyway.
I had an idea that the effect of rapid changes in conductor length such as caused by two metal parts rubbing was actually changing the signal strength at the receiver input (much like a rapidly changing antenna length). This would explain why AM receivers, which use AGC, are more prone to such interference than FM receivers. Also, I do not see how different metals would be required, unless they were wetted with some electrolyte (fuel maybe ? :rolleyes: )

[Rodney]

Yep, vibrating metal, even similar metals, are still an rf source and always will be. Avoid them.

[KillerWatt]

Even with modern radios (single conv or dual), simple metal-to-metal aluminum tube bushed plastic wheel hub sliding or turning on music wire landing gear will cause erratic type Rx hits.......... kw

[Muxje]

Yup, it's easy to see that modern radio gear is susceptible to this: switch on your Tx and Rx, then run a screwdriver along a metal part (e.g. a metal control rod, or the engine). You'll see the servos twitch :)

I'm wondering about aluminium though... one of my planes has a tail wheel with an alu hub, that is clearly designed to accept a metal shaft. I've never had any interference from that, even though I have tried to induce interference by wiggling the wheel.

The same plane also has an aluminium cowl, which worried me a bit even though there's no metal to metal contacts there either. I tried running a screwdriver along the cowl: nothing. As soon as I touched the engine with the screwdriver: twitch twitch.

[Vette]

I have a large high wing plane that has functional wing struts that are a pain to install at the field because they are bolted on. A couple of years ago I was looking for an easier way to install the struts and happened to find some aluminum rods with a turnbuckle that are used to correct a sagging screen door. After shortening the rods a little and bending some hooks on the ends,
the rods hooked into the original strut mounts and were easily tightened to support the wing. I was happy with the ease of assembly and made one good flight, on the second flight the nearly new Futaba radio starting glitching and after landing I found one turnbuckle had loosened and that allowed the strut rod end to vibrate where it was hooked to the mounting plate. After installing locknuts on the turnbuckles and some shrink tubing on the rod ends the glitching stopped and has not been a problem on that plane since. I think with metal to metal vibration there will be a possible glitching problem no matter if your radio is new or old.
Joe

[Bill Mixon]

It will also happen with carbon fiber.
Recently I put a plane together that had two carbon brace rods that formed an "X". Not thinking much of it, and in a hurry to test fly it I left these rods free to move on each other. The "noise" cause by those carbon rods drove the RX wild with constant glitching.
I wraped the rods with some kevlar, and bonded it with CA. No more glitches.

[leccyflyer]

Thank you gents, that is pretty much in line with my own thoughts and experience.

Still interested in hearing more about the actual science behind metal-to-metal interference, so any radio professionals feel free to pipe up.

[vintage1]

wll in any sort of field (electrical) bit will be at different potyentials, its like e.g. two aerials picking up slightly different signals - then touch them and there's a leetle spark.

I also think there is some electrochemical issue with dissimilar metals. You ae in fgact making tiny laiitle batteries.

[leccyflyer]

Well the latter is certainly the case with thin metal foil and amalgam, as any careless Kit Kat eater could confirm, once they have been peeled from the ceiling :) .

I'm not sure that dissimilar metals are required to generate the RF interference though, a steel clevis on a worn, or loosely fitting, steel threaded pushrod is one of the oft-cited causes for metal-to-metal noise.

[KillerWatt]

Who knows conductive metal touching metal in the Rx's near-field ......back when real hi Z dynamic headphones were popular, you could "hear" various single snaps and pops when just one headphone wire lead was touched to any kind of electrical conductive material .......almost like an pre-existing "electrical charge state" was sudendly being discharged and the momentary electron rush flow thru the 'phones winding coils resulted in an audiable pop...this and the pre-posted comments about the near-field Tx and RX antenna Rf fields being momentarly affected certainly has some kind of servo (and Rx pulse train framing error decode) malfunction effects......... kw

[vintage1]

Well the latter is certainly the case with thin metal foil and amalgam, as any careless Kit Kat eater could confirm, once they have been peeled from the ceiling :) .

I'm not sure that dissimilar metals are required to generate the RF interference though, a steel clevis on a worn, or loosely fitting, steel threaded pushrod is one of the oft-cited causes for metal-to-metal noise.


I didh;t say iot was requirted: just that its one mechnism - not the only one.

Oxidation plays a part I am sure too.

I've just forgotten all my phsical chemsitry..its been a long time..nearly 40 years...