Coplanar Capacitive Coupled Probe Fed Ultra-Wideband Microstrip Antennas

Abstract

Modern wireless communication systems call for ultra wideband operations to meet the continuous growth in the number of users of these systems. Since antenna is an integral part of any wireless communication system (transmitter or receiver), designing antennas with good gain over large bandwidth needs to be considered first. To meet the popular demand, wireless communication systems should be as cheap as possible which require antennas with small size, light weight, low profile and low cost, and that are easy to fabricate and assemble. A type of antenna that satisfies most of these requirements is the microstrip antenna. Most of the wideband techniques for microstrip antennas utilize complicated geometries such as stacked multiple metal/dielectric layers, complicated feed arrangements etc., which elude the primary attraction of microstrip antennas. On the other hand, single layer suspended configurations are considered the best choice as these are simple to fabricate and assemble. The objective of this research is to investigate simple microstrip antennas with large bandwidth. A single layer suspended microstrip configuration was chosen for the purpose. In the first part of the research, the bandwidth was increased to about 50% with linear phase characteristics by optimizing the feed configurations while retaining the overall simplicity. This study has resulted in proposing a criterion for obtaining maximum bandwidth in the suspended microstrip configuration. An analytical model has been developed for such an antenna configuration. Although several analytical tools are available for the microstrip antenna analysis, equivalent circuit based approach proves to be a simple one and offers convincingly accurate results. Another advantage of the proposed equivalent circuit modeling scheme is that it is suitable for computer aided design (CAD). In order to make this approach even more useful, the antenna designed in the first part was modified to meet desired specifications such as reduction in the air gap to make the antenna compact, symmetrical patterns, making antenna circularly polarized (LHCP or RHCP) without changing the feed configuration. Nearly symmetrical patterns were obtained throughout the band of operation by modifying the profile of patch close to the feed strip. Circular polarization (CP) operation has been obtained from the basic antenna by cutting a diagonal slot on the radiator patch. Here the slot orientation decides the type of CP i.e., LHCP or RHCP. In this work obtained of 7.1% axial ratio (3dB) bandwidth with other characteristics unaffected. The overall height of the antenna is reduced by 55% by cutting a slot and re-optimizing the feed strip dimensions. These studies emphasize flexibility offered by the design approach in realizing practical antennas for various applications.