A communication system that is difficult to jam has a transmitter transmitting successive chirp signals, which start at different initial frequencies and all have the same frequency - time slope. At the transmitter successive selections are made from among the different available chirp signal frequencies in accordance with a predetermined pseudorandom program that is known at the receiving station. The receiver has a chirp matched filter capable of receiving all of the different chirps but, being a distributed filter, it delays the different received chirps by differing amounts depending upon the initial frequency of each chirp. The output of the chirp filter is envelope-detected, and the resulting pulse signal is passed through a time gate that is gated on at a time that is selected in accordance with the predetermined program. The system has advantages for anti-jam communications, multiple-access communications, frequency diversity radio, and coding using multiple frequencies, as examples. The chirp matched filter is conveniently a surface wave device.