Analytical modelling of microstrip travelling wave antennas

Abstract

Recent advancement of radar technologies and fast growing requirement of automotive and vehicular radars have necessitated the use of antennas capable of beam scanning with frequency as well as scanning at constant frequency. Complications in designing the feed networks of phased array antennas have naturally put travelling wave antennas to the task. In this context, travelling wave antennas based on Quasi TEM mode based slow wave type have become a major candidate. As was discussed earlier, introduction of discontinuities of non-periodic or periodic nature is a must for such antennas. Consequently, bent and non uniform microstrip lines have been used to cater to applications demanding beam steering, polarization agility and wide impedance bandwidth. Our work aims at development of analytical models capable of analyzing such bent and non uniform microstrip line based antennas in an approximate manner while describing the underlying physics in comprehensive detail. To this end, we start with a simple transmission line model for microstrip line antennas with right angle bends and thereafter introduce empirical models for acute and obtuse angle bends and finally interface a simple model for inter-segment mutual coupling into the transmission line model. This leads to a comprehensive transmission line model with very wide applicability. The propagation constant along the microstrip line however becomes spatially varying when the bends become continuous instead of discrete i.e. for an arbitrarily curved microstrip line. There is very little literature apart from some full wave analysis efforts and some analytical developments for curved and twisted waveguides by Lewin. We have developed a mode matching based rigorous technique for analyzing arbitrarily curved microstrip lines supporting both fundamental Quasi TEM as well as higher order modes. As a natural extension, non uniform microstrip lines came up next. Most of the literatures available for such lines have used various techniques for solving one dimensional non linear Riccatti equations thereby ignoring any transverse field variation completely. Such minimal transverse fields are hardly the case for an antenna application. We applied the mode matching technique for analyzing arbitrarily non uniform microstrip lines supporting fundamental or higher order modes. Finally, there has been a very rapid expansion in the leaky wave antenna domain focusing on periodic leaky wave antennas. In recent years several periodic leaky wave antennas in microstrip configuration have been developed based mainly on slot or stub loading. These works have focused on designing leaky wave antennas with multiband or specific beam scanning behavior. Most of the analysis is dependent on either simplified ABCD matrix technique or very tedious full wave technique. The ABCD matrix technique is heavily reliant on some simplified circuit model of the periodic antenna which is not always available or is difficult to formulate (for example, in case of a microstrip line supporting the first higher order mode loaded periodically with open stubs or in case of a half width ladder line). We have addressed this problem by developing a technique combining the mode matching technique and Floquet modal theorem for analyzing the dispersion characteristics of periodically perturbed microstrip line supporting fundamental or higher order modes. In the end, design guidelines are outlined for designing microstrip leaky wave antennas based on dual stubs (open and shorted) with single or dual wideband behavior. Following are the novel aspects aimed at this thesis:  Development of a transmission line model capable of analyzing any bend based microstrip travelling wave antenna.  Development of mode matching based techniques for analyzing arbitrarily curved and non-uniform microstrip line antennas.  Analysis and observation of the dispersion characteristics for periodically loaded, width modulated and non-uniformly periodic microstrip lines and applying the same for perturbing the radiation characteristics of a Rampart line antennas.  Development of a mode matching and Floquet theorem based technique for analyzing the dispersion characteristics of single/multiple stub loaded microstrip lines, and SIW based periodic leaky wave antennas.  Outlining design guidelines for wideband leaky wave antennas with frequency beam steering.