Low-Profile Steerable Loop Antenna With Capacitively Coupled Feeds

A novel application of capacitively coupled feeds for making a switched beam square loop antenna (SLA) matched to 50 Ω at its inputs is presented. The capacitively coupled feeding mechanism introduces capacitance into the antenna input impedance, which cancels out the high dominant inductance due to the probe and the ground plane, and helps in the realization of low-profile SLA. The net thickness for an antenna developed for the test frequency of 4.7 GHz is 3.04 mm (λ
₀
/21) and has four feeding ports. In order to obtain a tilted beam by this antenna, there must be a gap in the middle of side lengths of the antenna. When one of the ports is excited and the three remaining ports are open-circuited, the antenna generates a linearly polarized tilted beam directed toward the quadrant opposite to the excited port. Thus, by exciting one port at a time and open-circuiting the other three, the antenna can steer the beam in four different quadrants and scan the entire space in front of antenna. The antenna developed has an impedance bandwidth of 270 MHz and an average directivity of 7.6 dBi.