A Compact Parallel-plane Perpendicular-current Feed for a Modified Equiangular Spiral Antenna and Related Circuits

This work describes the design and measurement of a compact bidirectional ultrawideband (UWB) modified equiangular spiral antenna with an integrated feed internally matched to a 50-Ohm microstrip transmission line. A UWB transition from microstrip to double-sided parallel-strip line (DSPSL) soldered to a short (1.14 mm) twin-line transmission line feeds the spiral. The currents on the feed travel in a direction approximately perpendicular to the direction of the currents on the spiral at the points where the feed passes the spiral in close proximity (0.57 mm). Holes were etched from the metal arms of the spiral to reduce the impedance mismatch caused by coupling between the transmission line feed and the spiral. This work also describes a low-loss back-to-back transition from coaxial line to DSPSL, an in-phase connectorized 3 dB DSPSL power divider made using three of those transitions, a 2:1 in-phase DSPSL power divider, a 3:1 in-phase DSPSL power divider, a radial dipole fed by DSPSL, an array of those dipoles utilizing the various power dividers, and a UWB circular monopole antenna fed by DSPSL. Measured and simulated results show good agreement for the designed antennas and circuits.