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How close can transmitters be

Old 10-16-2006, 07:29 AM
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carlbecker
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Default How close can transmitters be

A couple of us are trying to fly formation and stand about 5 feet apart so that we can talk. One of the older members told us that this can cause a problem. All of the transmitters are very late models, less than 5 years old and of different brands. Is this a problem? Was this a problem with older transmitters. I have done this about 5-6 times so far without any sign of issues.

Carl
Old 10-16-2006, 08:03 AM
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Flying freak
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Default RE: How close can transmitters be

Not an expert in this but i would think that as long as the antanns never come in conntact with each other you will never have a problem.
Old 10-16-2006, 08:39 AM
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Default RE: How close can transmitters be

YOU shouldn't have a problem, but there is a small chance that folks around you might. If enough signal strength from your buddies transmitter comes back down your antenna (the antenna receives as well as it transmits) it will mix with your transmitter signal, causing what's known as Intermod Distortion, or IMD. The IMD frequencies are on either side of your pair of frequencies, separated by the difference in yours.

For example - let's say you're on 24, and your buddy is on 26. IF there's enough signal to cause IMD, there will be smaller outputs on 22 and 28. Your receivers should never know the difference, but if someone on those channels flies close enough to your transmitters, there might be an issue.

That's probably what the other guy was talking about - there were articles in years past in the model mags about this, with some recommendations for how far apart you should be.

Old 10-17-2006, 07:33 AM
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carlbecker
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Default RE: How close can transmitters be

So we will be sending out trash on some multiple of the addition of our two freq's. I guess there is a combination that would have min impact on others then. Its odd that we would not hurt each other. Usually we are the only ones flying but I will be careful about others. Would it be better if one was odd while the other even?

Thanks for your help.

Carl
Old 10-17-2006, 10:06 AM
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Bax
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Default RE: How close can transmitters be

The actual formula is:

(2 x (channel A)) - (channel B) = (channel C) or (2 x (channel B)) - (channel A) = (channel C)

This is called "3rd Order Intermodulation" because a 3rd frequency is being generated. If someone is flying on channel C, and their receiver is closer to the channel A and channel B transmitters than to its own transmitter, a possibility of it getting interfered with exists. This intermodulation is quite weak, and the channel C receiver would have to be very close to the A and B transmitters and rather far from the C transmitter to be affected.

This isn't common at all with modern systems and the fact that the flyers are all in one spot. You'd more likely get it if two people were flying from one side of a park, and someone else a few hundred yards away at the other end of the park had the channel C rig, and their plane was close to you. Then their receiver could get "hit".
Old 10-18-2006, 03:59 PM
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Default RE: How close can transmitters be

Hi Bax,

I found your formula interesting but I am not sure if I got it right.

If:
Ch. A=24
Ch. B=28
Ch. C=???


2 x Ch. A - Ch.B = Ch.C
2 x 24 = 48 48 - 28 = 20 Then Ch. C = 20 correct?


2 x Ch.B - Ch.A = Ch.C
2 x 28 = 56 56 - 24 = 32 Then Ch. C =32


Which would be right? Did I mess up with my math here, or would there be IMD on both Ch.20 and Ch.32

I know this is maybe not a big deal now a days but I saw it and was curious how the numbers would work out. But now I am not sure where or if I mis understood your example or where I went wrong.


Could you explain?

Thanks












Old 10-18-2006, 04:24 PM
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Bax
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Default RE: How close can transmitters be

You are correct. Channels 20 and 32 could get a 'hit', but chances are not particularly likely with modern systems and the way most people fly...but the scenario I mentioned with the 3rd flyer 'way across the park and his plane near you two fiers...well then the chances of the 3rd party's model getting 'hit' does become greater.

Please remember, though, that the 3rd-order intermodulation result is a very weak signal when compared to the transmitter's normal signal, and modern receivers have been made with the awareness of intermodulation interference. That spectre came up years ago when the current channels were set up. A LOT of research and development went into making sure that 3rd-order intermodulation was minimized as a problem source. If it wasn't, there would be a LOT of problems at a LOT of flying fields, contests, and so on. R/C car racers are also affected..

Please note that the formulas work only because the channel numbers are evenly-spaced. You should really work this out with the actual frequencies in the formulas. That will give you the correct answer, but using channel numbers works just as well for our purposes.

One last thing. If one of the channels you come up with is higher than channel 60, it won't affect the surface users. The formula works with their numbers, but they're on 75 MHz...no way a 3rd-order signal from 72 MHz transmitters could affect them. The reverse is also true.
Old 10-19-2006, 11:31 AM
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Default RE: How close can transmitters be

Cool Thanks for the explanation.
Old 10-23-2006, 06:32 AM
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Default RE: How close can transmitters be

I don't know about the US but at our club (Australia with 36Mhz) the pilots stand at a single fenced "flight line" and can get as close as they like to each other... (naturally avoiding antenna contacts). Not a single problem noticed at all.

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