ANALYSIS AND DESIGN OF BROADBAND AND DUAL BAND MICROSTRIP ANTENNAS

Abstract

The concept of using microstrip antennas to provide printed radiating structures, which are electrically thin, lightweight, and low cost, is a relatively new and exciting development in antenna engineering. The application of this phenomenon to design useful antennas started only in the early 1970s when conformal antennas were required for missiles. Today, rectangular and circular microstrip resonant patches are used extensively in a multitude of array configurations.

Microstrip antenna elements have a number of useful and interesting features, but there is one serious limitation of this technology- the narrow bandwidth. While competing antenna elements such as dipoles, slots, and waveguide horns have operating bandwidth of 15-50%, the traditional microstrip patch element typically has an impedance bandwidth of only a few percent. For this reason much of the large volume of research and development in the area of microstrip antennas in the last twenty year has been devoted to various techniques for the enhancement of microstrip antenna bandwidth.

Although bandwidth is a dominant topic in the microstrip antenna literature, there are sometimes confusing and misleading conclusions presented due to a lack of clear definitions of bandwidth. So in the first part of the thesis, several different types of bandwidth that are relevant to microstrip antenna elements and arrays, and their limitation factors are discussed. Then the categories are given to classify those broadbanding techniques currently available.

Two interesting structures, namely log-periodic patch and SSFIP ante1mas are under investigation. Theoretical properties and empirical design formulas are briefly discussed, with simulation results given after dimensional optimization. It is seen that quite good impedance bandwidths can be achieved. But these structures normally take a large space as compared to other microstrip antennas. Also they are more complicated for fabrication.

Recently, U-slot patch antenna, a novel microstrip antenna which has a quite large bandwidth, is published. In the author's investigation of this antenna, an empirical formula is found to calculate the initial structure dimensions. And then a design method is obtained which can be used to optimize those physical parameters. It is found with additional modification an even larger bandwidth can be achieved. Another improved version is the modified double U-slot patch ante1ma. Despite having more physical parameters, the new antenna is seen quite easy to design as a very broadband antenna. Measurement result shows that a bandwidth of 54.4% is obtainable.

To be used in a real application, U-slot patch antenna is further discussed for array fabrication. The full wave simulation of a big array structure is quite time consuming. So at first MDS models are used to design the matching network, followed by full wave analysis. A 2x2 array is made finally with a bandwidth of 25% ( S11 < -10 dB and Power gain> 8 dB).

Possible use of U-slot antenna in dual frequency application is also studied. Two slots are added to induce the second resonance. This dual band U-slot antenna is found to have a good frequency agility.