8 GHz Vital Signal Sensing Doppler radar using Isolation-improved Branch-line Coupler

8 GHz Doppler radar which is wireless sensor to collect the respiration and heart rate using feed-forward type branch-line coupler is developed. Even though the wireless sensing systems using a circulator or branch-line coupler have been developed, these systems have low sensitivity because of low isolation and bulky system due to Tx/Rx two independent signal paths. In this paper, proposed Doppler radar has improved the isolation by using feedforward type branch-line coupler, which has -55 dB isolation between transmitter and receiver. As a result of achieving high isolation, irreversible vital signal due to the low isolation of conventional branch-line coupler can be detected. So, sensitivity of the system is increased drastically