This contribution deals with a compact substrate integrated waveguide (SIW) diplexer for terminal array antennas at K-/Ka-band. The diplexer connects the transmit (Tx) and the receive (Rx) frontend with an antenna element. It is comprised of a bifurcation and two second order bandpass filters with low insertion loss. The Rx-filter has additionally two transmission zeros in the Tx-band. The structure is implemented in multilayer printed circuit board (PCB) technology. This yields a compact design which can be integrated in an array with half-wave element spacing. The simulated diplexer performance is validated by measurements. The insertion loss in the Rx-band remains below 0.7 dB, while the Tx-/Rx-isolation exceeds 40 dB in a 1.2 GHz band at Tx.
