31-05-2012, 11:09 AM
Antenna Theory and Micro strip Antennas
[attachment=23630]
Basic Concepts of Antennas
Introduction
For wireless systems, the antenna is one of the critical components. A good design of
the antenna can relax system requirements and improve overall system performance. The
wireless systems include a large variety of different kinds, such as radar, navigation, landing
systems, direct broadcast TV, satellite communications, mobile communications and so on.
An antenna could be as large as 100m by 100m for radio telescope or as small as the order of
centimeters in built-in handsets. All of them play an important role in science and daily life.
Today we enjoy much benefit from wireless, and the significant contributions of antennas
should not be underestimated.
An antenna is an electromagnetic transducer, used to convert, in the transmitting mode,
guided waves within transmission lines to radiated free-space waves, or to convert, in the
receiving mode, free-space waves to guided waves.
In 1886, Hertz demonstrated the first wireless electromagnetic system. In 1901, Marconi
succeeded in sending signals over large distance from England to Newfoundland. Since
Marconi’s invention, through the 1940s antenna technology was primarily focused on wire
related radiation elements and their operation frequencies up to about UHF. It was not
until World War II that modern antenna technology was born and new elements, such
as waveguide aperture, horns, reflectors, lenses, etc. were first introduced. The first use of
phased array was reported in 1937. Most of the major advances in the theory of phased array
antennas and their implementation occurred in 1960s. This kind of antenna can accomplish
functions which the conventional one cannot do. Because the antenna beam in phased arrays
can be steered to a new direction in microseconds and it may be widened or narrowed in
microseconds as well, it provides much agility. Prior to 1950s, antennas with broadband
patterns and impedance characteristics had bandwidths not much greater than about 2:1.
In the 1950s, a breakthrough in antenna development occurred extending the maximum
bandwidth to as great as 40:1 or more by using equiangular spiral or logarithmically periodic
structures. Because the geometries of these antennas are specified by angle instead of linear
dimensions, they have theoretically an infinite bandwidth.
Arrays and Array Synthesis
In Chapter 1, the linear antenna shown in Figure 1.3 may be considered as formed by
continuously distributed infinitesimal electric dipoles. This is actually an example of continuous
linear array. The rectangular aperture in Figure 1.6 may be considered as a continuous
planar array. The radiation pattern of both examples may be obtained by (1.3.34), where
the element factor F1 is the radiation pattern of differential element and the space factor F2
is called the array factor AF.
In the above examples, all the elements forming the array are identical and have the same
orientation. Therefore, in computing the far field E, the element factor F1 may be taken out
of the integral. Consequently, the total radiation pattern F is equal to the product of the
element factor F1 and the space (array) factor F2, as given in (1.3.34). This is referred to as
pattern multiplication for both the continuously distributed and the discretely distributed
sources. It is seen that F2 is actually the radiation pattern of a point-source array formed by
omnidirectional elements. The array factor is a function of the elements, their geometrical
arrangement, their relative magnitudes, their relative phases, and their spacing. Through
knowledge of all these controlled parameters, it is possible to obtain a required radiation
pattern of the antenna.
Microstrip Patch Antennas
The microstrip antennas and arrays have been widely used in recent years because of
their good characteristics; they are electrically thin, lightweight, low cost, conformable and
so on. However, the electrical performance of the basic microstrip antenna or array suffers
from a number of serious drawbacks, including very narrow bandwidth, high feed network
losses, high cross polarization, and low power handling capacity. With progress in both
theory and technology, some of these drawbacks have been overcome, or at least alleviated
to some extent. The rapidly developing markets, especially in personal communication
systems (PCS), mobile satellite communications, direct broadcast (DBS), wireless local area
networks(WLAN) and intelligent vehicle highway systems (IVHS), suggest that the demand
for microstrip antennas and arrays will increase even further. In the meantime, the increasing
demand calls for the further development of them.
A microstrip patch antenna element is very useful and also is the basic element of an
array. In this chapter, we will focus on the description of the principle of the microstrip
patch antenna. The approximate analysis introduced in this chapter is useful in two cases,
one is to work out the design method which is good enough in some engineering application;
the other is to derive the coarse models which are necessary in optimization through space
mapping technique. The full wave analysis will be briefly introduced. All of them are useful
for a good understanding of the recent development and innovative design of microstrip
patch antennas.
[attachment=23630]
Basic Concepts of Antennas
Introduction
For wireless systems, the antenna is one of the critical components. A good design of
the antenna can relax system requirements and improve overall system performance. The
wireless systems include a large variety of different kinds, such as radar, navigation, landing
systems, direct broadcast TV, satellite communications, mobile communications and so on.
An antenna could be as large as 100m by 100m for radio telescope or as small as the order of
centimeters in built-in handsets. All of them play an important role in science and daily life.
Today we enjoy much benefit from wireless, and the significant contributions of antennas
should not be underestimated.
An antenna is an electromagnetic transducer, used to convert, in the transmitting mode,
guided waves within transmission lines to radiated free-space waves, or to convert, in the
receiving mode, free-space waves to guided waves.
In 1886, Hertz demonstrated the first wireless electromagnetic system. In 1901, Marconi
succeeded in sending signals over large distance from England to Newfoundland. Since
Marconi’s invention, through the 1940s antenna technology was primarily focused on wire
related radiation elements and their operation frequencies up to about UHF. It was not
until World War II that modern antenna technology was born and new elements, such
as waveguide aperture, horns, reflectors, lenses, etc. were first introduced. The first use of
phased array was reported in 1937. Most of the major advances in the theory of phased array
antennas and their implementation occurred in 1960s. This kind of antenna can accomplish
functions which the conventional one cannot do. Because the antenna beam in phased arrays
can be steered to a new direction in microseconds and it may be widened or narrowed in
microseconds as well, it provides much agility. Prior to 1950s, antennas with broadband
patterns and impedance characteristics had bandwidths not much greater than about 2:1.
In the 1950s, a breakthrough in antenna development occurred extending the maximum
bandwidth to as great as 40:1 or more by using equiangular spiral or logarithmically periodic
structures. Because the geometries of these antennas are specified by angle instead of linear
dimensions, they have theoretically an infinite bandwidth.
Arrays and Array Synthesis
In Chapter 1, the linear antenna shown in Figure 1.3 may be considered as formed by
continuously distributed infinitesimal electric dipoles. This is actually an example of continuous
linear array. The rectangular aperture in Figure 1.6 may be considered as a continuous
planar array. The radiation pattern of both examples may be obtained by (1.3.34), where
the element factor F1 is the radiation pattern of differential element and the space factor F2
is called the array factor AF.
In the above examples, all the elements forming the array are identical and have the same
orientation. Therefore, in computing the far field E, the element factor F1 may be taken out
of the integral. Consequently, the total radiation pattern F is equal to the product of the
element factor F1 and the space (array) factor F2, as given in (1.3.34). This is referred to as
pattern multiplication for both the continuously distributed and the discretely distributed
sources. It is seen that F2 is actually the radiation pattern of a point-source array formed by
omnidirectional elements. The array factor is a function of the elements, their geometrical
arrangement, their relative magnitudes, their relative phases, and their spacing. Through
knowledge of all these controlled parameters, it is possible to obtain a required radiation
pattern of the antenna.
Microstrip Patch Antennas
The microstrip antennas and arrays have been widely used in recent years because of
their good characteristics; they are electrically thin, lightweight, low cost, conformable and
so on. However, the electrical performance of the basic microstrip antenna or array suffers
from a number of serious drawbacks, including very narrow bandwidth, high feed network
losses, high cross polarization, and low power handling capacity. With progress in both
theory and technology, some of these drawbacks have been overcome, or at least alleviated
to some extent. The rapidly developing markets, especially in personal communication
systems (PCS), mobile satellite communications, direct broadcast (DBS), wireless local area
networks(WLAN) and intelligent vehicle highway systems (IVHS), suggest that the demand
for microstrip antennas and arrays will increase even further. In the meantime, the increasing
demand calls for the further development of them.
A microstrip patch antenna element is very useful and also is the basic element of an
array. In this chapter, we will focus on the description of the principle of the microstrip
patch antenna. The approximate analysis introduced in this chapter is useful in two cases,
one is to work out the design method which is good enough in some engineering application;
the other is to derive the coarse models which are necessary in optimization through space
mapping technique. The full wave analysis will be briefly introduced. All of them are useful
for a good understanding of the recent development and innovative design of microstrip
patch antennas.