Quasi Frequency-Independent Increased Bandwidth Planar Log-Periodic Antenna

A novel antenna composite with greatly extended bandwidth of a baseline printed 4-arm log-periodic (LP) topology is introduced. Bandwidth increase by accomplishing a miniaturization factor >2 is achieved by coupling the currents from the booms of the LP to an annular ring, thereby providing an efficient well-matched radiator at frequencies below the cut-off frequency of the LP. The annulus, the turnstile (booms of the LP), and the LP form the radiators of this novel quasi frequency-independent (FI) ring-turnstile-LP antenna. A 160 mm diameter antenna is designed and fabricated to demonstrate the proposed concept. Measured VSWR and extensive far-field characterization verify multi-octave dual circular polarization (CP) operation and a size reduction factor of 2.25. Also, the ring-turnstile-LP configuration is integrated with two planar impedance transformers/feeds to demonstrate small size, simple beamformer and dual-CP operation over bandwidths as wide as those of more complex sinuous and singly polarized spiral antennas. Detailed computational studies are conducted for better understanding the impact of various structural parameters on antenna's unidirectional performance and relevant conclusions are provided.