R&D Report 1970-11 : Combined effect of several interfering signals J.W. Head When a site is considered for a new radio or television transmitter the best available assessment of interference expected from existing transmitters operating on or near the same frequency is required. This involves calculation of the combined effect of two or more interfering signals simultaneously present. The information available about each interfering signal is its distribution in time, that is, the percentages of time for which the signal can be expected to exceed certain levels. The resultant of interfering signals is best expressed in the same way, ie as the distribution of an equivalent single interfering signal. A general method of finding the resultant of two signal distributions, suitably quantized, is discussed and possible simplifications are considered for distributions approximating to certain standard types. Correlation has usually been either neglected (so that signals simultaneously present are combined by convolution of their power distributions) or assumed to be complete (so that combination is achieved by power addition of the signals). A tentative method of interpolating between these extremes to allow for actual correlation is discussed in Appendix A; very little relevant experimental information is available. When an interfering signal has a sufficiently wide range of variation it is usually accurate enough to regard it at any instant as being either interfering or negligible. This is the basis of the 'probability multiplication' procedure hitherto used in Research Department for multiple-interference calculations.