Low-Profile Low-Cost Ultra-Wideband Circularly Polarized Slot Antennas

Aiming for ultra-wideband (UWB) indoor wireless access, three low-profile UWB circularly polarized slot antennas are proposed in this paper, all of which are made by a pair of curved tapered slots. The slots are miniaturized to allow a short direct microstrip feeding. The first antenna is a bidirectional antenna with a measured 10 dB impedance bandwidth of 110 (2.94 - 10.06 GHz), and a 3 dB axial ratio (AR) bandwidth of 89 (3.19 - 8.26 GHz). Its measured peak gain is 7.5 dBic and its size is 0.80 $\lambda _{\mathrm {max}}\,\,\times $ 0.80 $\lambda _{\mathrm {max}}\,\,\times $ 0.5 mm ($\lambda _{\mathrm {max}}$ is the free-space wavelength at the lowest frequency, 3.19 GHz). Its performance is similar to that of a traditional spiral antenna, but it has a much simpler structure with a more complete ground plane, making it possible to integrate with other components (e.g. filter and amplifier) on the antenna body to reduce the overall size of the system. The second antenna is a directional antenna with a simple solid metal reflector placed 21 mm away from the radiator. This configuration increases the peak gain to 8.6 dBic, but reduces the impedance bandwidth to 52 only. The third one is a directional antenna with a meshed reflector. It has a maximum gain of 9.6 dBic, which is higher than that of the second antenna, while its 10 dB impedance bandwidth is 99 (3.37 9.94 GHz) and its 3 dB AR bandwidth is 84 (3.70 - 9.07 GHz), which is similar to that of the first one. Also distance between the radiator and the reflector (antennas height) is reduced to 9 mm only (0.111 $\lambda _{\mathrm {max}}$ ). The proposed antennas demonstrate high efficiency, simple and low profile structures, wide impedance and AR bandwidth, making them excellent candidates for in-door base station and UWB positioning systems.