Radio interference from Mobile phones

On 24 Sep 2005 05:41:28 -0700, snipped-for-privacy@iprimus.com.au wrote in :

The latest I've read (RC Report?) is that the problem is verifiable with some synthesized transmitter modules.

Keep the cell phones a few feet away from the transmitter, and all should be well.

Marty

I have never seen any interference caused by a mobile phone, and I always fly with mine in my pocket, and it has rung while I'm flying and I've not seen any adverse effects. I have heard anecdotal stories from other people, but then it turns out they all refer to the same 2 or so incidents where it simply wasn't clear whether the mobile phone had anything to do with it.

That said, given that the internal control signal (before modulation) is in the audio band, and you often hear interference from (GSM) mobile phones on audio devices, it certainly isn't beyond the realms of possibility.

However, in your circumstances, given you say you've moved to a new field and are getting some kind of interference, and I assume you had mobile phones at the previous flying location, I'd say the most likely explanation is someting about the new location. There could be a strong microwave signal running through the airspace for example.

Thanks guys for all the interesting and helpful comments. They are very informative and the links given helpful as well.

We think we have narrowed the problem down to an electric fence. This coming Thursday we will have a computer and radio ham guru on site, to test all the signals and identify the source of interference. I'll let you guys know the results.

We are also having each Radio checked for band width(?).

DN

If anyone has a spare Hitec Feather receiver (which nobody sane would ever put into an aircaft) you can do a very simple modification to use it to listen for interference on a particular channel (by sticking the associated RX crystal into it). Basically all you do is connect an audio amplifier (although I've heard of a crystal earpiece being used by itself) to pin 9 of the decoder chip - you'll then hear anything on that band.

| If anyone has a spare Hitec Feather receiver (which nobody sane would ever | put into an aircaft)

Oh, it might be OK if you need to add a few grams of ballast to a plane. :) (Just don't hook it up to anything.)

| you can do a very simple modification to use it to listen for | interference on a particular channel (by sticking the associated RX | crystal into it). Basically all you do is connect an audio amplifier | (although I've heard of a crystal earpiece being used by itself) to | pin 9 of the decoder chip - you'll then hear anything on that band.

Nice idea, though since the Feather is such a crappy receiver, it probably will pick up interference that better receivers would ignore.

And even worse, since it's not very sensitive, it would probably ignore interefence that better receivers would receive.

But the idea of using a R/C receiver to look for interference is a very good one. But ideally you'd use a receiver similar to the ones that you're using in your planes -- in particular, making sure that the conversion (single vs. double) matches what you're using in your planes. Ideally, you'd use the same model receiver, but that certainly can become expensive ...

The real reason is that if your RX is listening to channel 33, 72.450 MHz, that doesn't mean that you only have to worry about signals from

72.440 to 72.460 MHz. Depending on your receiver's construction, there are other frequencies that can cause it grief, and any receiver will be desensed by sitrong signals, especially those on nearby frequencies. These factors vary from design to design, so you either need to 1) understand the limitations of the design you're using (and test against those) or 2) use the same design in your testing device.

Ultimately, if you're looking for why your R127DF receiver is glitching, there's _nothing_ better than another R127DF modified to listen to the signal to see if there's interference. It's better than a spectrum analyzer, than a scanner, or anything else.

I believe a good monitor receiver should be designed to have poor image rejection so I can hear both the primary frequency and the image frequency. The monitor receiver will need to have the same frequency hierarchy as the plane's receiver (matching the number of conversion stages and IF frequency). I would probably give the image frequency out of the converter, it's own, broad bandpass, IF strip and detector/agc circuit.

What we all need is to have manufacturers of RC receivers, provide specifications for overall RF/IF bandwidth and image response charts. In the drive to make receivers smaller and lighter they seem to have forgot about better and better. With better technical info, you can avoid the crappy receivers.

Don

That's why we use hitec feather recievers to make simpole montors :-) :-)

Far too sensible to ever catch on.

We had the field fences checked without any result regarding radio interference. However, there may be a clue here, with a gate which may be carelessly locked and which MAY with windy conditions cause some radio chatter.

We have decided to have an amplified AM radio between stations, monotor for radio interference from now on and keep a history of time etc if we detect alien signals.

Thanks for you input guys - it is most appreciated.

DN

I get your point. But with that type of monitor, you cannot listen to the image frequency while the tx is on.

Sad isn't it. It makes me wonder if these manufacturers really know much about their receiver specs.

| > I believe a good monitor receiver should be designed to have poor image | > rejection so I can hear both the primary frequency and the image frequency. | > The monitor receiver will need to have the same frequency hierarchy as the | > plane's receiver (matching the number of conversion stages and IF | > frequency). I would probably give the image frequency out of the converter, | > it's own, broad bandpass, IF strip and detector/agc circuit. | | That's why we use hitec feather recievers to make simpole montors :-) :-)

Except that being single conversion receivers, they'll have different image frequencies than dual conversion receivers (which I'm guessing that most of our receivers are) so they'll 1) give false positives and

2) false negatives.

| > What we all need is to have manufacturers of RC receivers, provide | > specifications for overall RF/IF bandwidth and image response | > charts. In the drive to make receivers smaller and lighter they | > seem to have forgot about better and better.

You're painting the industry with a pretty broad brush.

Certain receivers ARE better and better. The Berg and FMA receivers, for example, are top notch. Futaba and JR make some really good ones, and some iffy ones. Hitec makes some reasonably good ones, and some crappy ones (like the Feather.)

As far as I know, all the PCM receivers are good, though Futaba just came out with a limited range PCM receiver -- that doesn't sound good.

The DSP receivers are good too -- FMA, Berg, Plantraco, Sombra Labs and others. Single conversion or dual conversion, it doesn't matter.

Beyond that, the dual conversion recivers (without PCM or DSPs) are generally OK, but not great.

Most JR receivers are single conversion and are OK, but not great.

Beyond that, most single conversion receivers are bad, and the Hitec Feather is one of the worst.

| > With better technical info, you can avoid the crappy receivers.

Actually, it's not hard to find out which receivers are good and which aren't -- it's all online, if you know where to look. For example, searching for `Hitec Feather' will tell you very quickly what the world thinks of them.

| Far too sensible to ever catch on.

Well, the way it would catch on would be a receiver maker who thinks they make the best receivers posting their detailed specifications first. I can see that happening, if they think it'll improve sales ...

Better and better in what manner? And, according to who?

Single conversion receivers do not need to be bad, and double conversion receivers can be junk.

Everything you have said is vague; lacking substance.

I don't want subjective opinions on what's good or bad. I would like to see technical specifications for all the characteristics of the equipment. That's not too much to ask, considering what risks we take by putting the equipment in our planes. Purchasers should be able to compare specs to help make a choice of equipment types.

Agreed.

Don

| > Certain receivers ARE better and better. The Berg and FMA receivers, | > for example, are top notch. Futaba and JR make some really good ones, | > and some iffy ones. Hitec makes some reasonably good ones, and some | > crappy ones (like the Feather.) | | Better and better in what manner? And, according to who?

According to me.

As for the manner, I'm referring mostly to the ability to reject interference not directly on the channel in question, and the ability to not glitch when interference is detected.

(The two are different. PCM receivers should never glitch, but that doesn't mean your plane won't crash when it goes into failsafe or position-hold mode.)

Oh, and you need some decent range. I've personally had my planes at

1/2 mile up at least once (I measured it with an altimeter) so I'd really want a mile range or so.

| > As far as I know, all the PCM receivers are good, though Futaba just | > came out with a limited range PCM receiver -- that doesn't sound good. | > | > The DSP receivers are good too -- FMA, Berg, Plantraco, Sombra Labs | > and others. Single conversion or dual conversion, it doesn't matter. | > | > Beyond that, the dual conversion recivers (without PCM or DSPs) are | > generally OK, but not great. | > | > Most JR receivers are single conversion and are OK, but not great. | > | > Beyond that, most single conversion receivers are bad, and the Hitec | > Feather is one of the worst. | | Single conversion receivers do not need to be bad, and double conversion | receivers can be junk.

I'm not aware of any dual conversion receivers that are junk. The stock Futaba 7 channel and similar receivers aren't very impressive, but the range is good, so I wouldn't call them junk. And I thought I was pretty clear about the single conversion receivers. Berg makes some good ones, JR makes some OK (like the Futaba dual conversion receivers) ones. But most of the rest are not good -- limited range, limited ability to reject interfence.

| Everything you have said is vague; lacking substance.

Well, it's about the best that you'll find now. | > | > With better technical info, you can avoid the crappy receivers. | > | > Actually, it's not hard to find out which receivers are good and which | > aren't -- it's all online, if you know where to look. For example, | > searching for `Hitec Feather' will tell you very quickly what the | > world thinks of them. | | I don't want subjective opinions on what's good or bad. I would like to see | technical specifications for all the characteristics of the equipment.

It would be nice. But I hope you're not holding your breath waiting for it ...

| That's not too much to ask, considering what risks we take by putting the | equipment in our planes.

Right now, our gear uses a narrow little 10 KHz signal. It's so simple for even a local pager tower (not to mention another guy who forgot to get the frequency pin) to totally swamp that signal and send your plane crashing to the ground, no matter how good of a receiver you have, that you really need to consider that every flight might be your last, merely due to radio problems.

Spread spectrum could be the solution to most of those sorts of problems, but it's still not really available, though there are some home-brewed systems out there.

(Another common problem is that batteries can fail at any time. If you don't have two packs, you're flirting with disaster right there.)

Thinking about it can make you not want to even go up :)

| Purchasers should be able to compare specs to help | make a choice of equipment types. | > | > | Far too sensible to ever catch on. | > | > Well, the way it would catch on would be a receiver maker who thinks | > they make the best receivers posting their detailed specifications | > first. I can see that happening, if they think it'll improve sales | > ... | | Agreed.

... and I didn't say it, but the low end receivers will never post their specifications. But eventually that'll be OK -- you just won't buy any receivers that don't post their specifications.