Estimating ionospheric cross-modulation

Abstract

A method of predicting the modulation imposed on a wanted radio wave by a powerful disturbing transmitter is described. It is shown that there are five general cases, depending on the positions of the receiver and the wanted and disturbing transmitters, their frequencies, the time of day and some other factors. Each case is examined in detail and it is decided whether the simple theory of Huxley & Ratcliffe is applicable for the situation. If not, then some modifications are suggested to this theory. More elaborate refinements are also described, but generally they need not be used by the radio engineer who is concerned only with achieving a rough guideline to indicate cross-modulation as a possible source of interference in communications. Some examples of the way in which the simple theory can be easily amended to produce more reliable results are given.