A diplexer apparatus comprises a diplexer 111 coupled to a transmit port 112, a receive port 114 and to a horn 116 which is arranged to substantially simultaneously transmit a transmit signal 118 or receive a receive signal 120, the transmit and receive signals 118 and 120 being co-linearly polarised signals. The diplexer 111 has one or more internal surface variations that are arranged to allow substantial all of the transmit signal 118 to pass from the transmit port 112 whilst prohibiting the receive signal 120 from passing. The receive signal 120 is reflected by the diplexer 111 and passes to the receive port 114. The diplexer 111 acts as a frequency discriminator of co-linearly polarised signals 118 and 120.