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35mHz interference poser

Real poser for radio boffins

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Martin McIntosh16/10/2007 23:45:00
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Can anyone who is well up on radio please try to answer this. At my local patch I found that under certain conditions I could not not use ch73 if someone else was on ch67. It caused a total lockout of my gear with all servos driving to one end even with the model in one hand, the Tx with full aerial in the other and the offending Tx with the aerial fully down. Which (JR) Tx/  model/ PCM, PPM, IPD, any Rx I cared to use made no difference. The offending Tx`s were probably Futaba, but that is beside the point. There is a gas pumping station a field or two away which I believe transmits telemetry information. This lasted for up to 10 secs at a time, mainly noticeable on Saturdays. A little unnerving when you are in the air at the time! For it to happen it seemed to require one or two other Tx`s to be in use at the same time on whatever other channels (not close ones). Needless to say I do not use ch73 now if anyone is there on 67, but otherwise it is OK. I am a radio coms. engineer, I know quite a bit about the causes of interference but I am blowed if I can anwer this one. All of the gear performs immaculately on any other channel.
FlyinBrian25/10/2007 21:49:00
644 forum posts
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Have you tried a different set of Ch 73 xtals?. Other possibilities include two txs close together causing a harmonic on the 73 frequency.

35 Meg rz detect the 35meg signal then add a locally produced (in the rx)  frequency to it to provide an intermediate frequency (IF) which is then decoded . It is possible for a tx a certain number of channels away to also produce the correct IF so totally blocking the correct signal. To overcome this Dual Conversion Rx were developed, these in effect produce 2 IF frequency stages but require a different rx crystal. Sorry but my technical knowledge is not good enough to explain it better than that.

 Why not just move channels?

 This is an interesting site for learning a bit about RC > http://www.torreypinesgulls.org/radios.htm

Regards

Brian

Martin McIntosh25/10/2007 22:31:00
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Thanks for your comments Brian, but as I said in the post I have a boxful of crystals and different rx`s and it does not come down to what I am using on the day. Could have been due to sunspot activity at the time I suppose. Like I said, it is a real poser. MM.
Eric Bray27/10/2007 22:59:00
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Sounds like harmonics from the gas station transmitter.

All I can suggest is you find someone with a radio scanner that will accompany you, or loan it to you, have a listen on the 35 meg band, and be horrified at the amount of hash there is present!

It is also possible that the gas station is using a beam antenna, and your field happens to be in line with it! 

(Look for something like a telly aerial or a satellite dish aimed across your flying field). 

Martin McIntosh29/10/2007 23:26:00
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3471 forum posts
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Eric, already checked the band with a spec an. You would not fly at all if you saw the results, and that was 5 miles away!
Eric Bray31/10/2007 22:48:00
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My hf ham trx has a spectrum scope built in, and tunes 35 megs, so I've seen the hash on the band with my own eyes! (That is one reason why I took up 459 Megs, when it was available, but nobody seems to make any gear for that band any more). It looks like 2.4 Gigs is the best option, now.
Martin McIntosh01/11/2007 18:59:00
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3471 forum posts
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Definitely.
Peter Christy12/11/2007 09:22:00
1829 forum posts

I've only just stumbled across this thread - sorry for the late arrival!

One vital piece of missing information here is what kind of receiver are you using - double or single conversion?

The image rejection properties of a single conversion receiver at 35 MHz are pretty abysmal - about -6dB at best! 

Its quite possible that there is a strong (and legal) signal sitting in the image band, that is mixing with the "interfering"  ch67 signal and producing a difference signal that falls inside the 455KHz pass band of the IF strip.

I seem to recall that there was a similar problem some years back when our 35 MHz band got expanded. The right combination of channels (it needed 3 transmitters) could produce an interfering signal within the front end of the receiver, if the third order intercept point wasn't high enough!

Just a thought!

--

Pete

Martin McIntosh12/11/2007 21:49:00
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3471 forum posts
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Pete, yes, that is about the right situation, three or more Tx`s with mine on 73, one on 67 and one or more on some other frequency. I have never used dual conversion, only PPM or PCM single conversion. I wonder how many other combinations of frequencies could cause a similar problem? The fact that I could be holding the model with full Tx aerial and the interfering TX had the aerial collapsed and still caused a complete lockout is rather worrying. Thanks for your input. MM.
Peter Christy14/11/2007 14:47:00
1829 forum posts

Putting my engineer's hat on for a moment, we really shouldn't be using single conversion receivers on 35 MHz! The image rejection simply isn't good enough!

Most of the time we "get away with it" because the "image band" (usually 910 KHz below the transmitter frequency, but if the receiver uses high side injection, it would be 910 KHz above) has little or nothing on it! However, there will always be the odd instance where there is something in the image band that will cause an issue, and this is the problem that double conversion receivers address.

The difference between ch 67 and 73 is 60 KHz. I'm guessing that there is a transmitter somewhere in the image band that is overloading the front end of your receiver, and producing an IF of 390 or 510 KHz. This then mixes with the 60 KHz difference signal producing something close to 455 KHz, which is, of course, your receiver's natural IF frequency.

Just a guess, but it is something I have come across before!

Cheers,

--

Pete

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