metal to metal interference

[Plummet]

In the olden days there was a lot written and talked about about avoiding metal to metal joints and contacts because it caused radio interference. There is a thread going at the moment about silver soldering a horn/hinge arrangement, with a Z bend on the pushrod,

Is metal to metal interference a thing of crystal radio days and no longer a problem, or was it all just an excuse for pilot error?

Plummet

[John F]

It definitely occurred as flying at the RAF Cottesmore flying club could demonstrate this interference quite adequately. If you flew over the separately fenced off POL store you would lose control until the model flew far enough away. Similarly if you went anywhere near a particular crash gate you would have no inputs until it left that area.

The radio and aircraft engineer qualified members would confirm it was due to the relatively loose fencing in those areas that interfered with the radios.

[Josip Vrandecic -Mes]

G/day Plummet , the tip referred most to the throttle lever at the IC that should have a plastic clevis, allegedly the motion of the lever "cut" the magnetic field. On my models I built a plastic clavis.

Edited By Josip Vrandecic -Mes on 30/08/2017 11:45:25

[Devcon1]

Well, funnily enough I was musing about this the other day. Although not quite the same question that Plummet brings up.

When doing some FPV flying with my Fatsharks and 5.8G video link I never get vision interference from electric powered models sharing airspace but I do seem to get some vision interference that seems to coincide with IC models being brought to the strip, revved up and also whilst sharing airspace.

Not particularly scientific but there does seem to be a pattern.

I always though that metal engines thrashing around generated some sort of electrical, electromagnetic and crude electrical energy bursts. Or am I wrong...

[Plummet]

... and of course there are more and more petrol engines with their electrical sparky bits which might generate radio waves as well.

Plummet

[Peter Miller]

The problem that John F is referring to is radio signals being reflected of metal. In the old days(27 and 35 Mhz) there used to be a chain link fence that caused serious problems.

On one field that we used just in one spot the signal bouncing off a hangar could do it.

Strangely enough flying right beside an WWII hangar did not seem to be a problem.

I have no idea why but these are examples that I have seen.

I think the metal to metal noise referred to by the OP is more that of metal clevises on metal arms. Never had a problem with that myself.

Somehow I doubt if it will affect 2.4Mhz but I could be wrong on that.

[Stevo]

Hiya Plummet! How's Flappit?

I remember in the old 27MHz racing car days that metal to metal was a disaster. Once I moved to 2.4GHz it all seemed to disappear.. however I avoid it where possible.

[Plummet]

She is sat behind me at the moment dealing with a plate of freshly made warm potato salad.

(The recipes is available on request.)

Plummet

[Martin Harris - Moderator]

Hopefully someone properly knowledgeable such as Pete Christy or Phil Green will correct me if I'm wrong but I've always assumed that the noise generated by metal to metal contact relates to voltages generated by dissimilar metals. If the vibration causes these metals to touch and break contact fast enough, a radio frequency signal will be generated which may be a harmonic of the frequency used for our radio control signal. When you compare the frequency of 35 MHz to 2.4 GHz (35,000,000 Hz / 2,400,000,000 Hz) there is a massive difference and the effects will be attenuated as the harmonics become more remote. While our transmitters transmit at far higher powers, the noise generator is very close to the receiver.

2.4 GHz Spread Spectrum also works differently to 35 MHz PPM and transmits data packets (in very simplified terms, imagine a stream of envelopes addressed to the receiver containing a note with all the required servo positions in it which are opened and read many times a second) which can be rejected if not properly received. Due to the complex systems used, it has the ability, within limits, to "hear" data sent to an individual receiver in the presence of other competing signals.

I believe this means that while it is still theoretically possible for metal to metal to cause problems - maybe at extended range - it's far more unlikely with 2.4 GHz.

Edited By Martin Harris on 30/08/2017 13:34:59

[John F]

Posted by Peter Miller on 30/08/2017 12:08:36:

The problem that John F is referring to is radio signals being reflected of metal. In the old days(27 and 35 Mhz) there used to be a chain link fence that caused serious problems.

On one field that we used just in one spot the signal bouncing off a hangar could do it.

That was discounted as there were several compounds with wire mesh fencing that proved to be no problem to fly over, or near, just the fences with loose mesh that gave issues.

I still don't understand why we were not allowed to fly over the golf club area though, the greens were an excellent emergency landing strip for our models!

Edited By John F on 30/08/2017 13:50:03

[gangster]

Two different issues have crept in here. I assume the op refers to fittings on the model. There is probably enough sound evidence to suggest it could be an issue but I doubt it happens very often. I have never been over concerned and have happily used a metal Clovis to tjrottleblever even with a length of Bowden. The second issue that has crept in about the compounds is a well known radio phenomena sometimes called the rusty bolt effect. This is where a poor joint of metals can actually mix two signals or cause reradiation at a different frequency. I have seen this a couple of times involving broadcast transmitters and vhf systems. I really do not imagine it could ever be an issue for model flying and certainly not on 2.4

[Josip Vrandecic -Mes]

@ Hi John F , flying over the golf lawn is their(golfer) fabrication, because they are afraid that our models, in case of emergency landing, will not damage their "priceless''grass....

Edited By Josip Vrandecic -Mes on 30/08/2017 19:05:24

[Peter Jenkins]

I remember reading somewhere that most noise generated in model aircraft - metal to metal connectors, ignition systems, electric motors and many others - all ceased having any effect above 300 mHz. That would put both 27 and 35 mHz at risk but not 2.4 gHz. This fact is what drove the take up of electric powered bnf aircraft as well, of course, of vastly improved battery technology.

[Cuban8]

Find it improbable that the miniscule voltages that might be generated by dissimilar metals would have affected the radio directly. I remember generating sparks from an old rx battery by shorting the leads together close to a receiver and not observing any glitches from the servos at all. Sometimes I could get a very small response if I slid two lengths of piano wire together and wondered if the wire was somehow interacting with the aerial's characteristics?

[Geoff S]

Although I earned a living as an electronics design engineer and been involved in electronics all my life (one of my earliest memories is being given old pre war radios to take apart for spares in the early 1940s) I'm no RF guru. I would be surprised if there was significant interference from slight metal to metal vibration.

However, I was once involved in the design of electronic fruit machines and did some tests to see how susceptible they might be to cheating by users by using a battery and car ignition coil to generate big sparks. I managed to get the machine to empty its coin hopper because I blew up a number of the diode/transistor logic (DTL) devices remotely! But that's far more severe than anything likely to be generated by passive metal parts rubbing against each other.

Geoff

[Former Member]

[This posting has been removed]

[Peter Christy]

There appear to be two different issues being discussed in this thread. 1) Internal metal linkages in the model, and 2) external metal structures.

Lets deal with (2) first, as its the simplest. When a radio wave hits a metal structure - typically a fence, in our case - it will get reflected, just as sunlight gets reflected off a damp road. However, unlike sunlight, which contains many different frequencies, 27 / 35 MHz systems use one very specific frequency. Because of the different path lengths taken by the direct and reflected signals, at certain points, these will arrive at the model exactly out of phase with each other and cancel out! This will cause a momentary loss of signal - or "glitch" as we used to call it!

Glitches were less common with PCM gear, not because the signal was not being interfered with, but because the PCM system disguised it. The glitch was still there, but the PCM system papered over the cracks!

At 2.4 GHz, the wavelengths are MUCH smaller, and so ,correspondingly, are the dead spots. Further, frequency hopping ensures that the dead spots are in a different place with each hop, and finally, the method of modulation used , like PCM, disguises momentary losses of signal.

Moving on to the internal metal to metal noise problem, the reasons behind this are less clear. However, scraping two long pieces of piano wire together near a VHF radio will often produce a crackle of interference. As our models invariably operate in close proximity to the transmitter (in radio terms), it is highly likely that the transmitted signal will induce small electrical currents in a wire pushrod. If this pushrod is in intermittent electrical connection with another piece of metal - say a z-bend into a metal throttle arm - then the radio resonant length of the pushrod will change, and produce a slight frequency modulation onto the signal reflected from it. Since it is very close to a sensitive receiver, it is not really surprising that the receiver might get upset by this! And before anyone says "but we had it in the AM days as well", remember that an AM receiver will pick up FM signals, if slightly off-tuned!

In addition, an AM receiver requires a constant level of signal at the detector stage, a feat achieved by an Automatic Gain Control or AGC circuit, which attempts to "iron out" fluctuating signal levels. However, it can only do this for variations that occur more slowly than the actual modulation, so any fast fluctuations will get through, and cause a glitch.

Peter Jenkins is right in that most ignition type interference tails off over 300 MHz, but it is a gradual fall off, rather than an instant disappearance. The same is probably true of metal-to-metal noise, though I have no idea if this has ever been investigated. And again, the shorter wavelength of a 2.4 GHz signal, combined with a robust modulation system, should make 2.4 GHz pretty much immune to such things.

Hope this helps!

--

Pete

[Don Fry]

Peter, thank you. I understood that, and anything which improves my zero knowledge of radio systems is good. I have many years of ignorance behind me, working with rule of thumb use and installation rules.

I once got badly bitten by metal to metal interference on a throttle of an Extra. I had re-engined it, and made an error when I did not replace the metal clevis on the throttle linkage, having ignored the fact that the old engine had a plastic arm, the new, metal.

The control throws were large. The glitches threw the beast from wingtip to wingtip. I got it back, when I found it behaved at a fast tick over. On testing , when the effect occurred was very unpredictable. System used was 35 meg.

I am on 2.4 now, and still have not got away from no metal to metal on the throttle, as a rule of thumb.

[Former Member]

[This posting has been removed]

[John Emms 1]

Like Don, I DID experience metal to metal interference.

In the days of 27meg, I ignored the advice to use a nylon clevise with a metal throttle arm, and the metal parts vibrating against each other did cause interference. The interference was cured by fitting a nylon clevise.