In this type of radar pulses only have one antenna that is used to transmit and receive signals, it requires a duplexer that will function as a separator for both signals, this duplexer will isolate the transmitted signal and the received signal. The main component of the duplexer that plays a role in the regulation of isolation is the Coupler which is abbreviated as coupler. The performance of the Coupler can be seen from the characteristics of the parameters it has, namely: Isolation Characteristics, Insertion Loss, Return Loss, and Power Coupling. In this study, changes in the dimensions of the Coupler, changes in this size will affect the value of the isolation. If the isolation value gets smaller, the duplexer performance will be better. Coupler in this study is applied in the form of microstrip and substrate used FR-4 with a dielectric constant of 4.6, 1.3 mm thick and 3 GHz operating frequency. To do this work is done with the 2014 CST simulation software, from the results of simulation experiments obtained the isolation value of-67.786 dB.