Compact Millimeter-Wave Antenna Array with Low Sidelobe Level for Vital Sign Monitoring Application

In this paper, an antenna array fed in series by a novel substrate-integrated waveguide (SIW) power divider is proposed for the millimeter-wave band vital sign monitoring application. The proposed antenna is printed on a single-layer substrate, with the advantages of low profile, low cost, and ease of fabrication. By branching microstrip lines alternatively at both edges of the SIW, an in-phase power divider is first constructed. Then, by appropriately adjusting the depth of microstrip branches into the SIW, the amplitude of the output power can be tuned to obtain the desired amplitude distribution. Patches are connected to the microstrip branches, respectively, and thus, a radiation pattern with a low sidelobe level is realized. More importantly, series patches can be cascaded to enhance the array gain without the additional feeding network. This leads to a compact configuration. Based on the proposed idea, a prototype is designed, implemented, and tested. The results indicate that the proposed array achieves a maximum gain of 17.8 dBi with a low sidelobe level of -25.5 dB. The attractive performance shows that the array is suitable for the millimeter-wave vital sign monitoring application.