The selectivity and intermodulation problem in UHF communication equipment

The problem of intermodulation is not new. With the increasing use of uhf and vhf and the attendant close channel spacings, it is merely emphasized. Despite the fact that it is a very old problem, a casual review of the old reliables in the handbook and textbook fields shows that a few of the backs mention the word “intermodulation” none give it any attention. In fact, one would gather the impression that the subject is unimportant. The purpose of this paper is to review the subject of intermodulation, with particular emphasis on the elements of the problem at ultra high frequencies. In the presentation, I am sure, you will find nothing new. The purpose of the talk is merely to describe some of the methods which can be employed to reduce intermodulation, and their advantages and disadvantages. As you doubtless knew, intermodulation is not a serious problem at low and medium frequencies because adequate protection in the way of receiver pre-selection can be achieved. Likewise, I am sure, you have experienced cases of external intermodulation where spurious signals are produced by rectification which occurs at poor or corroded junctions in current carrying systems of wires, steel structures, and the like, which are located near the receiving antenna. At vhf and uhf, external cross modulation, while it is occasionally observed, is not the principal offender. The principal offender is the receiver itself. Why then, you may ask, isn't something done about it? It can be safely stated that much has been done, but we can also add that there is much that remains to be done if the maximum use of the closely spaced vhf and uhf channels is to be obtained.