13-06-2012, 05:04 PM
THE CIRCULAR MICROSTRIP PATCH ANTENNA – DESIGN AND IMPLEMENTATION
[attachment=24402]
INTRODUCTION
Microstrip antennas basically consist of a radiating patch on one side of a dielectric substrate, which has a ground plane on the other side. The patch is generally made of conducting material such as copper and gold (Wikipedia, 2010). The patch is very thin (t<<λo where λo is free space wavelength) and is placed a small fraction of a wavelength (h<< λo usually 0.003 λo ≤h ≤ 0.05 λo ) above the ground plane. The microstrip patch is designed so its pattern maximum is normal to the patch (broadside radiator). This is accomplished by properly choosing the mode (field configuration) of excitation beneath the patch. There are numerous substrates that can be used for the design of microstrip patch antennas and their dielectric constants are usually in the range of 2.2 ≤εr≤ 12. Those desirable for antenna performance are thick substrates whose dielectric constant are in the lower end of the range due to better efficiency, larger bandwidth, and loosely bound fields for radiation into space but at the expense of larger element size. Microstrip patch antennas radiate primarily because of the fringing fields between the patch edge and the ground plane. The radiation increases with frequency, thicker substrates, lower permittivity, and originates mostly at discontinuities (Lewin, 1960)
METHODS OF ANALYSIS
There are three popular models for the analysis of microstrip antennas - viz transmission line model, cavity model,
and full wave model. The transmission line model is the simplest. It gives a good physical insight but is less
accurate. The cavity model, which is used in this work, is quite complex but gives good physical insight and is more
accurate. The full wave model is the most complex. It is very accurate in the design of finite and infinite arrays or
stacked structures.
CIRCULAR PATCH AND FIELD CONFIGURATION
The mode supported by the circular patch antenna can be found by treating the patch, ground plane and the material
between the two as a circular cavity. The radius of the patch is the only degree of freedom to control the modes of
the antenna (Balanis, 1982). The antenna can be conveniently analyzed using the cavity model (Richards, 1988;
Gonca, 2005). The cavity is composed of two electric conductors at the top and the bottom to represent the patch
and the ground plane and by a cylindrical perfect magnetic conductor around the circular periphery of the cavity.
The dielectric material of the substrate is assumed to be truncated beyond the extent of the patch (Richards, 1988).
The field configuration within the cavity can be found using the vector potential.
DISCUSSION OF RESULTS
From the results obtained from both the simulation and the manual computation as presented on Tables 1 to 4, it was
clear that the outputs of both were very close. The slight differences could be traced to approximations made in
manual computations. Table 5 showed that GaAs with a patch radius of 0.2374 cm, has the smallest radius compared
to other chosen substrates, hence is best in miniaturization. It also showed a high input resistant (impedance) of
1552.20, which means that loading effect will be minimal. However, it has the least radiation conductance, which
implies that its radiation is least when compared to the other substrates. It also showed the least directivity of
3.3187. The reduced radiation can be traced to the fact that it showed the highest losses due to radiation conductance
(Qrad), which is the dominant loss for thin substrates. This makes it a prime choice for Bluetooth application. Indium
Phosphide is next to Galium Arsenide when reduced size and less loading effect are the major requirements. Duroid
has the best conductance (0.002227 Siemens), which means if used to construct an antenna will give a high radiation
when compared to the other chosen substrates. The high radiation is due to the high fringing observed. It also
showed the best directivity and the least losses due to radiation conductance (Qrad). However, these advantages are at
the expense of an enlarged patch radius and a reduced input resistance (increase in loading effect). Therefore, when
directivity is apriori, Duroid is the best option. This makes it useful in high frequency mobile telecom. The stability
feature of Silicon makes it suitable for patch antenna construction. Patch radius decreases as the required resonant
frequency increases.
[attachment=24402]
INTRODUCTION
Microstrip antennas basically consist of a radiating patch on one side of a dielectric substrate, which has a ground plane on the other side. The patch is generally made of conducting material such as copper and gold (Wikipedia, 2010). The patch is very thin (t<<λo where λo is free space wavelength) and is placed a small fraction of a wavelength (h<< λo usually 0.003 λo ≤h ≤ 0.05 λo ) above the ground plane. The microstrip patch is designed so its pattern maximum is normal to the patch (broadside radiator). This is accomplished by properly choosing the mode (field configuration) of excitation beneath the patch. There are numerous substrates that can be used for the design of microstrip patch antennas and their dielectric constants are usually in the range of 2.2 ≤εr≤ 12. Those desirable for antenna performance are thick substrates whose dielectric constant are in the lower end of the range due to better efficiency, larger bandwidth, and loosely bound fields for radiation into space but at the expense of larger element size. Microstrip patch antennas radiate primarily because of the fringing fields between the patch edge and the ground plane. The radiation increases with frequency, thicker substrates, lower permittivity, and originates mostly at discontinuities (Lewin, 1960)
METHODS OF ANALYSIS
There are three popular models for the analysis of microstrip antennas - viz transmission line model, cavity model,
and full wave model. The transmission line model is the simplest. It gives a good physical insight but is less
accurate. The cavity model, which is used in this work, is quite complex but gives good physical insight and is more
accurate. The full wave model is the most complex. It is very accurate in the design of finite and infinite arrays or
stacked structures.
CIRCULAR PATCH AND FIELD CONFIGURATION
The mode supported by the circular patch antenna can be found by treating the patch, ground plane and the material
between the two as a circular cavity. The radius of the patch is the only degree of freedom to control the modes of
the antenna (Balanis, 1982). The antenna can be conveniently analyzed using the cavity model (Richards, 1988;
Gonca, 2005). The cavity is composed of two electric conductors at the top and the bottom to represent the patch
and the ground plane and by a cylindrical perfect magnetic conductor around the circular periphery of the cavity.
The dielectric material of the substrate is assumed to be truncated beyond the extent of the patch (Richards, 1988).
The field configuration within the cavity can be found using the vector potential.
DISCUSSION OF RESULTS
From the results obtained from both the simulation and the manual computation as presented on Tables 1 to 4, it was
clear that the outputs of both were very close. The slight differences could be traced to approximations made in
manual computations. Table 5 showed that GaAs with a patch radius of 0.2374 cm, has the smallest radius compared
to other chosen substrates, hence is best in miniaturization. It also showed a high input resistant (impedance) of
1552.20, which means that loading effect will be minimal. However, it has the least radiation conductance, which
implies that its radiation is least when compared to the other substrates. It also showed the least directivity of
3.3187. The reduced radiation can be traced to the fact that it showed the highest losses due to radiation conductance
(Qrad), which is the dominant loss for thin substrates. This makes it a prime choice for Bluetooth application. Indium
Phosphide is next to Galium Arsenide when reduced size and less loading effect are the major requirements. Duroid
has the best conductance (0.002227 Siemens), which means if used to construct an antenna will give a high radiation
when compared to the other chosen substrates. The high radiation is due to the high fringing observed. It also
showed the best directivity and the least losses due to radiation conductance (Qrad). However, these advantages are at
the expense of an enlarged patch radius and a reduced input resistance (increase in loading effect). Therefore, when
directivity is apriori, Duroid is the best option. This makes it useful in high frequency mobile telecom. The stability
feature of Silicon makes it suitable for patch antenna construction. Patch radius decreases as the required resonant
frequency increases.