A first mixer block converts received MMDS channel frequencies to MMDS frequencies capable of being processed by an MMDS decoder. The MMDS decoder, which includes a crystal controlled reference generator, is located in a benign environment. The reference is supplied to a phase locked loop for controlling a fixed frequency local oscillator that supplies the first mixer. The local oscillator output is also supplied to a second mixer that produces a response signal, of a different frequency, by biphase shift key modulation of said reference that is transmitted back to the transmitting site via the antenna. A squelch circuit precludes transmission of the unmodulated reference.