Wideband monopolar square-ring patch antenna

The design of a monopolar square-ring patch antenna with wideband operation is presented. The antenna consists of a parasitic square-ring patch antenna that is shorted to the ground plane through two shorting walls and is excited by a top-loaded coaxial probe. The key design parameters to obtain 50 /spl Omega/ input impedance and optimum bandwidth are investigated experimentally. The effects of the inner slot size of the square-ring patch on the operating frequency of the proposed antenna are also studied. From the obtained results, a 10 dB-input-impedance bandwidth of more than 50% can be achieved while the side length of the square-ring patch is about 0.3 free-space wavelengths and the antenna height is less than 0.1 free-space wavelengths. Also, the antenna can provide stable monopole-like conical radiation patterns across the impedance bandwidth.

I. Introduction

For designing an antenna used to present wireless communication systems, three requirements are often considered: omnidirectional radiation pattern, wide impedance bandwidth, and compact size. A traditional monopole antenna can provide uniform radiation fields across all of azimuthal angles; however, its antenna height is about (here is referred to the center frequency of 10 dB-input-impedance bandwidth) which is too high to be applied to mobile units. Several substitute antennas with similar radiation characteristics and lower height have been proposed, mainly including planar inverted-F antennas (PIFA) [1], monopolar wire-patch antennas [2], and circular patch antennas operated at mode [3]. The PIFA inherently has a less bandwidth as compared to the traditional monopole antenna. Several methods have been proposed to improve the bandwidth more than 25% [4]– [6]. However, the obtained radiation fields are not uniform enough in the -plane and it is difficult to provide stable radiation patterns across the enhanced bandwidth. The monopolar wire-patch antenna can be considered as a monopole antenna top-loaded a shorted patch; consequently, the antenna height can be moderately reduced if a proper size of the loading patch is used. Moreover, the antenna still has the conical radiation patterns. As regards the circular patch antenna operated at mode, it can be implemented on a thin substrate to produce the conical radiation pattern. However, it would be difficult to be installed into a limited space because the resonant frequency of the mode is about 2 times the frequency of the same-size circular patch antenna operated at mode. According to the above discussions, the monopolar wire-patch antenna could be a better candidate for the omnidirectional antenna design.