How easy is it to....

[PatMc]

So why does a running engine have no effect, particularly 4 strokes ?

I'm still a disbeliever.

[Biggles' Elder Brother - Moderator]

If there was Gallic shrug emocion I'd use it! I can only speculate that all the contacts in the engine are separated by an oil film?

BEB

[PatMc]

The valve mechanism of four strokes certainly have intermittent metal - metal contact. There must be a degree of bearing, piston/liner contact through the oil or engines wouldn't wear out & by the rusty bolt theory poor contact that sort of high resistance contact is the worse.

[Chris Bott - Moderator]

Maybe it's because an engine casing acts as a pretty effective faraday cage, keeping any spurious RF inside?

There's a wacking great electrical spark inside petrol engines. This is a massive source of wideband RF interference, but this doesn't affect us does it?

[PatMc]

Posted by Chris Bott - Moderator on 18/07/2013 23:14:11:

Maybe it's because an engine casing acts as a pretty effective faraday cage, keeping any spurious RF inside?

There's a wacking great electrical spark inside petrol engines. This is a massive source of wideband RF interference, but this doesn't affect us does it?

Not all metal contact is internal for example some old 4 strokes had open rockers.

I've never operated a model petrol engine but I've seen several threads of people having interference problems from their ignition systems, though probably before 2.4 was prevalent.

[Peter Beeney]

I’d agree with Pat, metal to metal interference is insignificant. Easy to demonstrate with the transmitter 18 inches away from the model, but take it 20 feet away, (or even a lot less), and it all disappears. Some sort of function of the power of the very close range and thus relatively powerful signal of the transmitter. If m-t-m contact had ever been a problem, I’d have thought that 27 and 35 MHz PPM would never have got off the ground, so to speak. I’ve seen a few cases of glitching, which we’ve invariably been able to to identify, but it’s never been metal to metal.

In Mathews’ case, the little spigot bolt that locates the throttle barrel unscrewed and went missing. I’d say it's then quite possible that the barrel moved sideways and allowed the engine to speed up briefly uncontrolled. So was it was simply this that caused his unfortunate crash? Maybe it’s possible it was causing the original erratic throttling, too. Is that a black plastic Irvine carby I can see, with the locating pin screw missing? I must admit, I liked the engines a lot, but I was never inspired by those particular carburettors.

PB

Edited By Peter Beeney on 18/07/2013 23:42:07

[John Olsen 1]

Well, I have not been a radio technician for a long time now, but I can assure you that loose metal to metal contacts can cause interference, and there is actually more than one mechanism possible.

First...one of my former bosses did an investigation back in the 1950s (long before I knew him) into interference caused by ordinary wire fences. Standard number 8 fencing wire as used in those days was galvanised, and it turns out that the oxide layer from two wires twisted or looped together at a join can make a crude diode. In the presence of radio signals this will lead to the formation of intermodulation products. If only one carrier is present, you will just get harmonic frequencies, eg A, 2A, 3A etc and as is usual in such situations, the energy in each harmonic will tend to fall off, and they are well out of band anyway. If there is more than one carrier present then more complex products will be formed. For two carriers the most important one will be 2A-B, which will fall in band. For three carriers A+B-C is also important. Note that the resultant carrier will carry the modulation that was on each input, and that as well as 2A - B you will get 2B - A. These effects are very real and I have obseved the products being produced with spectrum analysers. My last major involvement with this sort of thing was writing a program to calculate intermodulation products in a linear amplifier used for single channel per carrier operation at a satellite earth station. Each voice channel used a separate carrier, and these were amplified to kilowatt levels by a linear amplifier. As any engineer will tell you, linear amplifiers are never quite perfectly linear, and get quite non linear if overloaded, so it was important to be able to predect the level of intermodulation products. Getting closer to our own situation, we don't want any intermodulation products generated that lie within band. In the realities of a flying feild situation, it is going to be really hard to know if this was the problem or not. Bonding or insulating loose metal joints is a good precaution...either will do.

The second mechanism whereby loose metal connections can cause interference is a bit cruder. If there is current flowing through the joint then the making and breaking of the connection will cause momentary arcing. The current flowing through the joint can be DC or AC. For instance, bad connection in power lines can cause interference. (as also can bad insulators) In our situation, the most likely cause of the current would be radio signals, which need not be from the other guys Tx, they might well be from a broadcasting station. So a vibrating connection acts as a crude make and break, and turns the parts into a crude broadband spark transmitter.

Engines are not too likely to cause this sort of problem since most of the motion is taking place inside the metal case. So even if they were generating interference it would be shielded. (Of course spark ignition systems very often require suppressors and shielded leads etc.) In the early days of installing radios in cars, it was sometimes necessary to install special brushes to earth the brake drums, since these could cause interference.

These sort of effects are not likely to be a problem on 2.4GHz for two reasons. One is that the amount of randomly generated interference tends to fall off as frequencies get higher, so there is much less energy to cause interference. For microwave systems internally generated thermal noise is more of a problem than external interference. The other is that the more sophisticated modulation schemes used with 2.4 GHz systems can cope with interference better, eg they can detect a corrupted frame and not use it.

Interference in general is unpredictable and often unrepeatable and so it is usually best to just take what precautions are reasonably possible and then hope for the best.

Erfolg, you can't prove that Pi is 3.142 since it is not! It is of course close enough for a lot of practical purposes.

John

[PatMc]

John, there are plenty of explanations of what causes stray RF & the range of frequencies affected. However in practice I've never come across any solid evidence that it's generated from control linkages or any other passive source within a model to a degree that causes any problems.
I've ignored all the advice on the subject for around the last 35 years & the theoretical consequences have never materialised nor has anyone been able to demonstrate the problem in that time.

[Martyn K]

Interesting.

I use a Z link with metal pushrods on a few of my models to connect servo to carb. Never had an interference problem though (35MHz or 2.4GHz).

The carb in the photo loks like an early Kavan carb. - which I think is a plastic body. I dont think that will be the source of interference. I suspect that the loose carb retaining screw is a far more liekly candidate.

Martyn