The territory of Indonesia sea is very wide, The area is very prone to illegal fishing, territorial violations by foreign vessels and prone to sea piracy and smuggling. Due to the limited capacity of TNI Navy and the Police Department to guard all of the sea, they has to used radar (Coastal Radar). In the type of pulse radar, it has only one antenna which is used to transmit and receive signals. In order to send and receive the signal, this radar requires a separator called duplexer. The duplexer serves to isolate the transmitted signal and the received signal and the main component of a duplexer is Branch-Line Hybrid Coupler. The Coupler has parameters that indicate the performance such as: Return Loss, Power Coupling, Insertion Loss and Isolation characteristic
In this thesis, the research was conducted to investigate the effects of optimization dimension to the isolation value of the Branch line coupler. The isolation is very important because it affects to the performance of the duplexer. The smaller value the better isolation performance of the duplexer. This research is carried out by modifying the length and width of the channels of the impedance of the Coupler. In this experiment, Branch-Line coupler is designed in the form of microstrip and fabricated using the FR-4 substrate that has a dielectric constant of 4.6, thickness of 1.3 mm, and 3 GHz operating frequency. To get the optimization value of the isolation characteristic, impedance channel must be modified, which are the length and width of the arms series (Z0 = 50 ?), the length and width of the arms series (Z0 = 35.35 ?), and the length and width of the arms shunt (Z0 = 50 ?). The optimized results of the isolation is -67,786 dB is got.