From: Roger.Wilfong@umich.edu (A. Roger Wilfong)
Subject: Re: Radio Interference for R/C flyers
Organization: U of Michigan - Information & Networking Services
Date: Fri, 4 Sep 1992 12:58:03 GMT

In article <1992Sep4.022156.8118@SSD.intel.com> nm@ssd.intel.com (Nicholas
Mati) writes:
>In article <1992Sep03.210721.5306@bmerh85.bnr.ca> mkfeil@bcrki9.bnr.ca (Max
Feil) writes:
>>
>>  3) 3IM
>>
>>     While not a big problem, 3IM is still an issue, as it has always been.
>>     The best protection for this problem is to ensure that people always
>>     stand in their pilot's box when flying so that no two transmitters with
>>     their antennas up come closer than 15-20 feet to each other. This is
>>     because intermodulation products increase in power if the two sources
>>     are close together. Another rule to keep in mind is not to walk too
close
>>     to somebody else if both your antennas are up.
>
>What is the mechanism by which two physically adjacent radio control
>hobby transmitters could cause a 3IM problem presumably for a third reciever?
>
>It is common practice to maintain this transmitter spacing, but does anyone
have
>a precise explaination as to how the intermodulation products are created that
>would depend on the transmitter spacing?   Why do "intermodulation products 
>increase in power if the two sources are close together"?
>
>Generation of intermodulation products by direct interaction of the E-M fields
>would require the presence of a non-linear media or interface boundary. It
seems 
>like air has a pretty linear u and e and should not support such a phenomena. 
>
>Is it possible that the mixing occurs when the signal from one transmitter 
>is induced on the other transmitter's antenna? This signal finds it's way 
>through the RF amplifier output filter and into the RF amp. where it mixes 
>with the native transmitter frequency.  The resulting cross products are 
>then amplified and transmitted?  
>
RF has always been my weak area (it's all FM - f*cking magic - to me) but
here's my two cents. 

The IM does require a 'non-linear media or interface boundary'.  While we tend
to normally think of linearity and interface boundaries in relation to
amplifiers, filters and such, the same exists for wires and connectors.  In the
early days of digital radios, 'noise' was a large concern.  At the time,
metal-to-metal contacts such as a metal clevis on a metal bell crank were
identified as source of RF noise, so they were to be avoided.  Experimenting
at home with a single transmitter, I could never produce these noise problems;
however I had a couple of unexplained glitches at the flying field that
disappeared when I replace a couple of metal clevises with nylon ones.  I have
always suspected that these glitches were caused by IM and that the metal
connections were infact the source of the IM - being located close to the
receiver, they could easily get through the wide RF and IF amps of the day. 
Newer RX designs have eliminated alot of the 'noise' interferance - again I
suspect by fltering the IM products before they hit the detector; however, with
more channels stuffed into the same bandwidth, this problem may still be
around.  This theory would agrue that source of the IM is close to the receiver
and therefore the mutual proximity of the two transmitters has little effect. 


I am not currently active in RC and have only taken passing interest in the
frequency allocation and control issues; however, I remember an AMA developed
frequency control system that was intended to reduce and/or eliminate IM
interferance.  The system involved an electronic frequency pin board that would
indicate any conflicts as the pins were removed from the board.  I do not
recall if that was only for the older 'broad-band' systems or also applied to
the narrow-band systems.

>Is moon-bounce a better explaination? :<)

I've always subscribed to the Allan Hinek's LGM theory myself (LGM = Little
Green Men).
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