Me and a club member found out that that if either of us gets our plane near the others tx, the plane starts twitching. We are both on different channels. Has anyone had this problem before. I guess this is a good reason to switch to 2.4

Seen it before on 72MHz.  Generally you must be very close, like just a few feet.  Sometime your own TX can cause twitching on your RX if the TX/RX are too close.  Normally unless the problem is isolated to one persons TX, or only one RX, I wouldn't worry about it.  Exactly how far was "near"?  Like 3 feet or 10 feet or 30 feet, etc?

+1

That's a long time 72 MHz 'problem'. That's one reason flight stations at events and funflies keep the stations apart from each other.

Stay away from other TX's and you'll be fine.

Very likely a problem called third order intermodulation and occurred when certain different frequencys that were a specific distance apart in the band were operated closely together. Fairly common with AM TX's I cannot be more specific but perhaps one of the RF experts which I am not can give a more specific explination of 3 OIM.

John

It's a problem or a blessing depending on which way you look at it. Two diffrent fquencys will add together to make a third,or subtract from each other to make a fourth freqemcy. That is the way the IF (Intermediate Frequency) part of your receiver works. The receivercrystal generatesa frequency that iscombined with all of the received frequencys the the first stage can accept. The result is two different frequencys, the two added together or the two subrracted from each othe give to greatly different frequencys. The IF stage can narrow down the signals to a more precise frequency. A single conversion receiver does this once, buta dual conversion does it twice and the desiredmix is 425KC IfI remember correctly. Two strong TX signals will bleed into the IF(mixed frequencys in the receivers) to come up withthird frequency.When that mix happens to match one of the receivers IF frequencies, then the signal will get through.This is what you are seeing.

I remember seeing a post on this some time back that detailed theinteractive frequencys, but I don't have a linkfor you

This is the reason most RX's are dual conversion,each mixing helps refinethe mix to a verynarrow possibilty of usable signals. The 425khz is a frequency that gave the least mix of twoTransmitted signals on allbands that woud feed through to the final signal. Ijust noticed that I used a 40 year old term, KC for KilloCycles vs todays KHZ Killo Hertz .It's just a nominclature difference, not a practical one. The 425Khz If is an old holdover from the early AMbroadcast radio days. I suspect that that if you do the math, it is a frenquecy that is less likely to be duplicated with a random mix of frequncys so the true signal will be the result.

Bottom line, there are acouple channelsin the 72mhz bandthat can beat together and over power the RX IFfrequency and cause problems. It is usually only a problem when the TX's are close to each other andproduicingnearequal signal strength. If oneTX signal is dominate, the problem goes away.

I hope this makes sense. It's late and added to that are a couple glasses of old grape juice.

Don

<div style="margin: 0in 0in 0pt"><span style="font-size: 18pt">Johnand Don,

You give a good explanation of mixing, but are slightly missing the 3IM point.

With transmitters, there is sometimes some undesirable power output at multiples of the frequency, which are referred to as harmonics. Sometimes in a receiver, this harmonic is mixed with a fundamental frequency, and produces a third order intermodulation distortion product, aka 3IM. The important point to stress, is that it is generated in the receiver. I have seen on more than one occasion, people using equipment trying to monitor for 3IM, and this is an invalid test. The only place to monitor is inside the receiver that is exhibiting 3IM symptoms, which is not practical in the field. </span></div><div style="margin: 0in 0in 0pt"></div><div style="margin: 0in 0in 0pt"><span style="font-size: 18pt">What can be done? Good quality transmitters, good quality receivers, and spacing are all good approaches. Receivers should be double conversion, or utilize DSP as in the Castle Creations receivers. </span></div><div style="margin: 0in 0in 0pt"></div><div style="margin: 0in 0in 0pt"><span style="font-size: 18pt">Attached is a 3IM spreadsheet that I created. Please note that for any 2 channels of operation, there are 2 possible 3IM products that may be generated. </span></div><div style="margin: 0in 0in 0pt"></div><div style="margin: 0in 0in 0pt"><span style="font-size: 18pt">Dave</span></div><div style="margin: 0in 0in 0pt"><span style="font-size: 18pt">

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A more probable cause of the twitching of the servos when two transmitters are near the receiver is that the AGC on the receiver is being overloaded or a poor AGC circuit rather than 3IM.  Most good receivers will not have this happen even with the two transmitters right next to the receiver being used.  It is more likely to happen when the transmitter that should be in command is more distant  from the receiver being effected and the interferring transmitter is very close to the receiver being effected.   The receiver interprets  the interfering transmitter  frequency as noise and cranks up the AGC to a point that it's selectivity is destroyed.