🚀 go-pugleaf

> 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?  

That's it precisely.

IM can occur in any nonlinear circuit; even if it's just two pieces of
metal with a corroded junction between them. However, a transmitter is
one of the most common places, since a nonlinear (Class C) amp is  
connected directly to an antenna. Commercial TV transmitters often have
ferrite isolators to prevent this.