Investigations on the Microstripline-Fed Wide-Slot Antennas for Wideband Applications

Introduction

Slot antennas are most common at microwave frequencies. Any slot has its complimentary form in wires or strips so that the pattern and impedance data of these forms can be used to predict the patterns and impedances of the corresponding slots. The theory is based largely on the generalization and extension of Babinet’s principle. In its basic form a λ/2 slot is cut on a flat metal sheet. The width of the slot is much smaller than one wave length. The currents are not confined to the edges of the slot but spread over the sheet. The slot antenna is a very efficient radiator and radiation occurs equally from both sides of the sheet. Slot antennas are very practical in space applications because it can be flush- mounted on the surface of the aircraft or spacecraft. The main advantages of the radiating slots are of wider bandwidth, less interaction via surface waves, better isolation and negligible radiation from feed network. The slot antennas may be square, rectangle, circular, elliptical or any other configurations. The concept of microstrip slot antenna has evolved from slot antenna excited by a strip line. The strip and slot combinations offer an additional degree of freedom in the design of microstrip antennas (Yoshimura, 1972).

Printed wide slot antennas are acceptable now a days because of their wide impedance bandwidths. In addition to that, the printed planar antennas are compatible with active devices or microwave monolithic integrated circuits. In recent years, growing research activities on microstrip line fed printed wide slot antennas has been noticed (Dastranj et al,2010; Sung, 2011; Sung 2012; Chattopadhyay et al, 2013). The fundamental research on microstrip line fed slot antenna was first proposed by Y. Yoshimura in 1972. The technology was further extended to wide radiating slot by T.K. Sarkar et al in the year 1993, involved exhaustive analysis of electric field and input impedance (Sarkar et al, 1993). The wide bandwidth feature of slot antennas with stub of appropriate shape had been reported by many researchers. A square slot fed by a 50Ω microstrip line with forklike tuning stub at an operating frequency around 2GHz was found to exhibit a 1.5:1 VSWR bandwidth of 1GHz (Sze et al, 2001). Wideband Impedance matching over frequency range of 4255MHz was achieved by incorporating suitable tuning of three resonance frequencies in a rectangular slot antenna (Jang, 2001). Wide impedance bandwidths of 120%(1.82GHz-7.23GHz) and 110%(2.42GHz-8.48GHz) were obtained by Y.F. Liu et al in the experimental study with an arc shape slot and triangular slot respectively(Liu et al, 2004). Another excellent design of microstrip line fed printed wide slot antenna with a rotated square slot has been proposed where the impedance bandwidth of 2257MHz has been reported(Jan et al, 2005).

In this chapter a detailed investigation has been carried out on bandwidth enhancement of hexagonal wide slot antenna fed by microstrip line loaded with various geometries of tuning stub. Two such tuning stub geometries are considered for the same. One is forklike shape and the other is hexagonal shape with a straight section. In another study, rotation of hexagonal wide slot has been carried out towards improvement of bandwidth. Impedance and radiation characteristics of all the configurations stated above have also been demonstrated in the foregoing sections.