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What is Intermod?
Article by Alan Hill, N5BGC

What is this thing they call Intermod? And how do I know if I have it? And will penicillin get rid of it?

Intermod is the common term for Intermodulation Distortion. It occurs when more than one signal is mixed in a nonlinear device. This device could be a lot of things. Intermod occurs in electronic circuits, diode type devices and even rusty pieces of wire left on a radio site. When a study is done to determine if there is a problem, many studies look at the effects of 2 to 4 transmitters and the receiver frequency and possible image frequencies of the receivers.

For several years the 146.82 repeater on Tesuque Peak suffered with it. It was so bad that we had to remove the autopatch we had on it. The problem, as it worked out, was mostly with our transmitter. This transmitter was the GE Master that had a tube type final. It was mixing with the Santa Fe district State Police transmitter and getting into the autopatch receiver that was not tone protected.

Here is the math (Hit your browser's back button it you don't like math):

Ham transmitter 146.82 times 2 (Second harmonic)
SP transmitter 155.565
Produced mixing frequencies of 449.21 and 8.84 MHz

The frequency of the autopatch receiver was 449.20. The mixing product was only 10 KHz off and could get into the bandpass of the inexpensive receiver that we were using. (Another reason for using commercial grade equipment on a mountain top.)

The repeater would stay quiet until someone "tested" it by keying it up briefly. If, while the transmitter was keyed, the State Police transmitted, the intermod product of our second harmonic and the fundamental of the State Police transmitter would be received by the autopatch receiver (10 KHz off frequency, but still mostly readable). That particular controller had a very long squelch tail, so there was ample time to hear the problem. Of course, this was cured by removing the 442/449 MHz autopatch equipment.

Could we have changed frequencies and avoided it? Maybe. But, there are over 100 transmitters on the mountain top, and we could have run into the same problem with someone else. Or, even worse, we could have been the "bad guy" by getting into a commercial service. If that happens, we would have to cure our problem and possible shut down until we solved their problem.

We are in good shape right now as far as audible intermod on our equipment. The CTCSS tone knocks most of it out. I do believe that we do have some occasional problems with desense. What is that? Wait until the next article.

Alan N5BGC



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