Design of constant width branch line directional coupler for the microwave sensing application

This paper investigates the design of directional branch-line coupler using constant width multiple sections of series and open microstrip line stubs. This device is operating at the center frequency of 2.45 GHz. The designed coupler is having good performance for the frequency band 2-3 GHz. The proposed design is compared with a typical branch line coupler and shows the improved performance. The design is simulated and fabricated on the Fibreglass-resin laminate (FR4) substrate of height 1.5 mm. The results of simulated and experimented circuits are discussed and analyzed. The circuit is measured using Combination Analyzer N9918A Field Fox Handheld Microwave Analyzer. The proposed topology is easy to design and fabricate with a planar microstrip line technology. The designed coupler shows the good results at 2.45GHz i.e. S 11 = -61.26 dB, S 21 = -45 dB, S 31 = -2dB and S 41 = -4.5dB. The main target of this work is to design a coupler to differentiate between transmitted and reflected signals in the split ring resonator (SRR) sensor for Bone Density Measurement Analysis (BDAS) system.