Image Frequency Rejection in a Radio Direction Finder

An analysis is given of the bearing error in a radio direction finder due to simultaneous reception of two signals, referred to as target and spurious signals. The carrier frequency of the spurious signal is assumed to be the image of the target-signal frequency around the local-oscillator frequency, so that both signals are passed by the IF amplifier of the direction-finder receiver. The analysis applies to any DF receiver having a carrier-suppressed double-sideband input, such as is obtained using a mechanical or electronic goniometer. The results can serve as a basis for specifying the image rejection requirements on the RF amplifier of the receiver. In particular, it is shown that in order to have less than one degree of error, the spurious signal must be attenuated to at least 35 db below the target signal amplitude at the RF output of the receiver.