Q: How are intermodulation calculations done?

A: Intermodulation interference (or "intermod") results when two or more frequency components combine in a way that interferes with another frequency.

As a very basic example, suppose you have two transmitters at a site, one operating on 150MHz and one on 155MHz (all of these examples are for illustration only). The two fundamental carrier frequencies could combine into a resultant (usually called an intermod "product") of 305MHz. If there were a receiver operating on 305MHz in the immediate vicinity, this spurious product could potentially overload the input of that receiver.

Not only the sum but also the difference between frequencies can generate an intermod product. For example, transmitter frequencies of 450MHz and 145MHz could also cause a potential problem for the hypothetical 305MHz receiver (450 – 145).

Likewise, harmonics of the fundamental frequencies can contribute. (Harmonics are multiples of the actual transmitting frequency, often generated by the transmitting equipment. They are unintentional and generally much lower power than the specified transmitter frequency, but can contribute to intermod and other types of interference problems.) For example, the second harmonic of a 100MHz transmitter could combine with the fundamental frequency of a 155MHz transmitter to generate the 305MHz problem frequency (2x100 + 155).