Analysis of Broadband L-probe fed Microstrip Antennas

Abstract

suspended microstrip antenna on thicker substrate is realized by using non-contact L-probe feeding technique. The guidelines for selecting total L-probe length are not available in the reported literature. In this paper, an analysis of broadband L-probe fed variations of microstrip antenna like, circular microstrip antenna, annular ring microstrip antenna, rectangular microstrip antenna, E-shaped microstrip antenna and half E-shaped microstrip antenna, is presented. It was observed that, total L-probe feed length nearly equals quarter wavelength at the resonance frequency of equivalent patch. Further using this quarter wavelength approximation, L-probe fed rectangular and circular microstrip antennas were re- designed at different frequencies which results into the broadband response with formation of loop inside the VSWR = 2 circle. The comparison between two non-contact feeding techniques of microstrip antenna, namely proximity feed and L-probe feed is presented. The two have similar performance in terms of bandwidth and radiation pattern, with proximity feeding technique to be the simplest method to be implemented in thicker substrates. Keyword microstrip antenna, Circular microstrip antenna, Annular ring microstrip antenna, E-shaped microstrip antenna, Broadband microstrip antenna, L-probe feed upon the ratio of vertical strip length to its horizontal length for the given patch substrate thickness. The design guidelines for properly selecting the strip vertical and horizontal lengths are not reported in the available literature. In this paper, various reported broadband designs of L-probe fed RMSA, U- slot cut CMSA, annular ring MSA, E-shaped MSA and half E-shaped MSAs are discussed. Further using their optimized designs the formulation for total L-probe length in terms of operating wavelength of the resonance frequency of the radiating patch is proposed. The total L-probe length was found to be around quarter wave in length at the patch resonance frequency. Using this approximation, RMSAs and CMSAs were re-designed at different frequencies. This results in broader BW with formation of loop in the impedance locus inside the VSWR = 2 circle. For RMSA, the comparison between the non-contact proximity feeding and L-probe feeding technique is presented. It was observed that in terms of operating BW and radiation pattern characteristics, both these techniques yields nearly the same performance, however as against L-probe feeding, the proximity feeding is simpler to implement especially in thicker substrates. The proposed study was carried out using IE3D software (18) and to validate the simulated results, measurements were carried out using finite square ground plane of side length 40 cm. The input impedance response was measured using R & S vector network analyzer.