Radio interference issues
 

Are there any inherent risks of radio interference due to close proximity of pilots while flying? If so please explain.

RE: Radio interference issues
 

Absolutely not, not unless the antennas are touching each other.

RE: Radio interference issues
 

Thanks Rodney

A few guys and I actually have taken the time and utilized a spectrum analyzer to see if we could see any problems. All seemed ok... but... we are not sure if we have evaluated all circumstances or conditions that might give some reason why pilots could/should not stand closer to better communicate while flying with each other. There are a few guys here that practice a sort of formation flying and being close is very beneficial.

For some unknown and un-given reasoning the powers here have determined the practice unsafe...

Rodney I really appreciate your input but can you give the qualifications you have to make such an assessment. Again thank you.

RE: Radio interference issues
 

There have been some cases where multiple transmitters operating in the same area have caused interference. The type of interference is called "third order intermodulation interference" (3IM). 3IM interference is caused when a number of different frequencies come into a receiver and hit the mixer stage. Those frequencies are then subject to the normal addition and subtraction effects that happen with waves. Bascially, if any combination of the channels added together and then subtracted equals the frequency of the receiver, you get interference. Usually though, it works out like this:

(2 x Channel A) - (Channel B) = (Channel C) Channel C gets hit.

With some real numbers: (2 x 72.030) - 72.050 = 72.010 72.010 (channel 11) gets hit.

You can even use the channel numbers: (2 x 12) - (13 ) = 11

This formula works with any 3 channels that are evenly-spaced apart.

If a receiver is far enough away from its own transmitter, and the other two are much closer to it, then there's a possibility it can get a hit. However, most receivers are well-designed enough to do a very good job of rejecting 3IM interference. That's why it's not really considered much of a problem. If multiple transmitters are in close proximity to each other, then the likelihood of 3IM interference is slightly higher than when the transmitters are spaced out across the field. The RC car guys, operating shoulder-to-shoulder present the worst-case scenario for 3IM interference, and it's not been much of a problem at all with modern systems.

RE: Radio interference issues
 

Bax I don't thinki your explaination is correct. It would not make any difference how close the transmitters are if that is the case. The mixing occurs in the receiver.
I think the reason the AMA recommends a minimum space between transmitters is the possibility that one transmitter might pick up signals from the other tranmsmitter. Then the mixing would occur in the transmitter and be retransmitted.

RE: Radio interference issues
 

I forgot about mixing in the transmitter. At any rate, the pricipal still applies, but incidences have been extremely low. The current crop of systems are very good at keeping this type of interference from being a problem.

RE: Radio interference issues
 

Littlecrankshaf I have been involved with a number of airshows with multiple airplanes in things like mock dogfights, group touch and goes, opposing passes, so on. Also as of late been doing considerable glider towing on both ends of the towline and all these activities have required pilots in close proximity with in five or six feet of each other. None has caused any indication of 3IM, although I don,t think I would care to try it with someone using an AM Tx. I might add that three pole AMA Rulebook pylon racing will typically end up with the teams maybe as little as ten feet apart as they spin around and sometimes even playing dueling antennas. Its for this reason I went to duck type antennas.

John

RE: Radio interference issues
 

Distance does make a difference because the interfering signal is very much weaker than the intended signal. However, if the receiver's far enough away from its own transmitter, and the interfering transmitters are close enough, then a hit can happen. With the way almost ALL R/C activities are practiced, though, 3IM is a rather rare event. Modern equipment is very good.

RE: Radio interference issues
 

Thanks to everyone for their input.

RE: Radio interference issues
 

3IM is not a problem with dual conversion receivers, only with the single conversion units. As an aside, a group of flyers, mostly HAMs, did an experiment with having pilots close together and seperated. While no concrete instrumention was used, they could not detect any degredation with the pilots actually touching one another over those seperated by the suggested 20 feet..

RE: Radio interference issues
 

here is something contrary though... (this is with my SINGLE CONVERSION foamy) anytime I have that out flying, I have to make sure to keep my transmitter closer then the people around me. (need to be on the end of the flightline!) if i cross in front of anyone else and their tx ends up closer, it usually just locks out the receiver and down the plane goes. This is HIGHLY repeatable, and I've noticed it happens with frequencie up to around 10-15 channels away.

i'm speculating it is just front end overload in the rx, as I'm usually flying the foamy fairly close in anyway. ANY tx can be screwed up by enough RF bombarding it, doesn't really even matter if the frequency is close or not.