Dich

3.1 Introduction

A microstrip patch antenna consists of a very thin metallic patch (usually gold or
copper) placed a small fraction of a wavelength above a conducting ground plane,
separated by a dielectric substrate. Microstrip antennas have numerous advantages, they
are light weight, they can be designed to operate over a large range of frequencies (1-
40GHz), they can easily be combined to form linear or planar arrays, and they can
generate linear, dual, and circular polarizations. These antennas are inexpensive to
fabricate using printed circuit board etching, which makes them very useful for integrated
active antennas in which circuit functions are integrated with the antenna to produce
compact transceivers. Microstrip antennas can be in various shapes and configurations
but for the purpose of this project only rectangular microstrip antennas are of interest.
This chapter includes an overview of microstrip antennas radiation mechanism, modeling,
design, and feeding techniques. The material covered in this chapter known as microstrip
antenna theory is from numerous books and articles ([1]-[3], [12]-[24]).
3.2 Rectangular Microstrip Antennas
Rectangular and square patches are the most commonly used type of microstrip
antennas. They can be used in numerous types of applications including circular
polarization, beam scanning, radiation pattern control and multiple frequency operation.
The basic antenna element is a thin conductor of dimensions L × W on a dielectric
substrate of permittivity e
r and thickness h backed by a conducting ground plane. This
configuration is shown below:
Figure 3.1: Rectangular Microstrip antenna configuration
3.2.1 Radiation Mechanism
Radiation from a microstrip antenna is determined from the field distribution
between the patch and the ground plane. This can also be described as the surface current
distribution on the patch. A patch, which is connected to a microwave source, has a
charge distribution on the upper and lower surface of the patch as well as the ground
plane. The patch is half wavelength long at the dominant mode, which creates the
positive and negative charge distribution shown in Figure 3.2 [11].
The repulsive nature of like charges on the bottom surface of the patch, pushes
some charges around the side to the top causing current densities Jb and Js. The ratio h/W
is small, therefore the strong attractive forces between the charges cause most of the
current and charge concentration remains underneath the patch. But also the repulsive
force between positive charges creates a large charge density around the edges. The
fringing fields caused by these charges are responsible for radiation. In order to achieve
better radiation efficiency, thick substrates with lower permittivity are better suited for
these types of antennas. Figure 3.3 shows the fringing fields in a microstrip patch [12].
3.3 Microstrip Antenna Analytical Models
There are various ways to model a microstrip patch. This modeling is used to
predict characteristics of a microstrip patch such as resonant frequency, bandwidth,
radiation pattern, etc. In this section the transmission line model and the cavity model are
presented. These models are based on some assumptions, which simplify the calculations
at the cost of less accuracy. There are other models that provide more accuracy such as
the full-wave model but are also more complicated to analyze.
3.3.1 Transmission Line Model
This is the simplest model and is restricted to rectangular microstrip antennas.
This model considers the patch as a transmission line of width W with two radiating slots
on each end. For a desired frequency f0, the width W can be estimated using [12]:
1
2
2
0
+
=
r
W
e
l
(Eqn. 3.1)
In this model the input impedance of a patch is the same as that of a transmission
line with length L and admittance Yc. Each slot has an admittance of Ys = Gs + jBs where
the values for conductance Gs and susceptance Bs are given by:




=  - 2
0
0
( )
24
1
1
120
k h
W
Gs l
(Eqn. 3.2)
(1 0.636ln( ))
120 0
0
k h
W
Bs = -
l
(Eqn. 3.3)