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This paper reports a portion of the work done in the investigation of spurious signals associated with broadcasting in and about Seattle, Washington. By suitable equipment and procedure the influence of such spurious signals as were generated in either the transmitting or receiving equipment was avoided and such spurious signals as originated external to the equipment and, hence, were presumably due to nonlinearities in the transmission path, were evaluated. The phenomena here involved are commonly termed "external cross modulation." It was found that spurious signals of this kind were observed in those locations where a suitably computed product of the field strengths of the associated real signals, termed the "field product," exceeded a critical value. It is suggested that the evaluation of "field products" and "susceptibility factors" provides the basis for predetermining the probable incidence of spurious signals due to external cross modulation.