Re-printed with kind permission by Brad Adams, Communications Service Engineer, The Nashville Network.

Intermodulation, or the "mixing" of two or more RF signals can be a tremendous source of problems for the wireless engineer. When two or more signals "mix" the by-product of this mix WILL BE the sum (f1+f2) and the difference (f1-f2). So, for example let's say that we have two frequencies in operation. Frequency #1 = 513.300 mHz ( a popular wireless mic frequency) and Frequency #2 = 684.500 mHz (a common wireless intercom or wireless mic frequency). So when these two frequencies are operating we will see both of the fundamental frequencies (F1 & F2) and also we will see the sum, 1,197.800 mHz and the difference, 171.200 mHz. Now let's say that we have a wireless IFB operating on 171.200 mHz.........we have a problem!

Mixes can occur in receiver front ends, transmitter output stages and even on a corroded screw or bolt in the metal framework of a sound stage. Our example uses only two fundamental frequencies. In a given television production such as an awards show, there may be as many as 50 to 60 wireless frequencies in use!

Take some time and do the math on that one! The number of mixes that can occur increases dramatically as more frequencies are placed in service. For this reason, care must be taken to provide adequate physical separation of wireless microphone transmitters and intercom systems and to properly coordinate all RF frequencies in use at a given venue.