Wideband Circularly Polarized Applications: Design of a compact, traveling-wave-fed loop antenna

A novel compact wideband circularly polarized (CP) directional loop antenna is presented in this article. The antenna is formed by an inner loop with periodic capacitive loading and an outer concentric loop, both of which are placed on the top side of a dielectric substrate and fed by nonradiating slots. A traveling-wave feed network is adopted to excite the slots, thereby providing the loops with currents of the same magnitude and appropriate phase delay. Thus, stable symmetrical directional CP radiation is achieved. In addition, the feed network is wrapped on the lower cylindrical dielectric substrate, so the size of the antenna can be made very compact. A prototype is implemented to verify the design concept. The measured results indicate that the antenna reaches a compact size of Ø0.11λ0 # 0.075λ0 ( λ0 is the wavelength corresponding to the lowest CP operating frequency), an available bandwidth of 31.3% (1.375-1.875 GHz), maximum broadside gain of 5.03 decibel above the gain of a isotrope antenna (dBic) and stable wide-beam directional radiation patterns covering the whole CP passband.