Intermodulation Interference in Radio Systems: Frequency of Occurrence and Control by Channel Selection

Intermodulation interference becomes a serious factor in frequency usage when a block of consecutive channels is provided for a given type of radio service in a confined area. Formulas are presented which show the number of potentially interfering 3rd and 5th order intermodulation products that can be formed in a band of n consecutive channels. The probability of encountering interference when a number of operating channels are picked at random from this band of n channels is developed, and the number of interference free operating channels that can be obtained by careful selection in this same band is also derived. When a block of consecutive radio channels is used in a confined area to provide a given type of service, interference becomes a serious problem. The situation is aggravated by the fact that whenever energy at two or more radio frequencies combines in a nonlinear circuit, as in transmitter output stages or in receiver input stages, products at other than the original frequencies are created. These are called intermodulation products, and they are capable of causing serious interference within the block of channels assigned to a given type of service as well as in other bands assigned to other types of service. It is important in engineering a service to know something about the nature of these products in order to evaluate their interference potentialities and to study means of controlling or minimizing that interference. The numbers and locations of various types of intermodulation products are susceptible to mathematical computation.