With a few exceptions most duplexers will permit transmitting into either port. Many units are marked "Hi Freq" and "Lo Freq" rather than "Tx" and "Rx". The only real difference between a base station unit (Tx=Hi) and a mobile unit (Tx=Lo) is the physical size. The larger cavities in a base station duplexer provide greater power handling capability and increased selectivity for greater Tx/Rx isolation. The high frequency section of a duplexer is designed to be tuned to the higher frequency whether that happens to be either the transmit frequency or the receive frequency. Occasionally duplexers are even used as two-channel transmit combiners but the total average power cannot exceed the product's rating.