A diplexer, for coupling a first radio frequency (RF) signal corresponding to a first carrier frequency and a second RF signal corresponding to a second carrier frequency is disclosed. The diplexer includes a first port arranged to couple the first RF signal; a second port arranged to couple the second RF signal; a third port capable of connecting an antenna; a first impedance unit coupled to the first port and the third port; and a second impedance unit coupled to the second port and the third port; wherein the first port, the second port and the third port are coupled to a direct current (DC) ground; wherein the first impedance unit is arranged to provide an first open-circuit impedance against the second RF signal, and the second impedance unit is arranged to provide a second open-circuit impedance against the first RF signal.