09-11-2012, 04:07 PM
MODERN ANTENNA DESIGN
[attachment=38787]
PROPERTIES OF ANTENNAS
One approach to an antenna book starts with a discussion of how antennas radiate.
Beginning with Maxwell’s equations, we derive electromagnetic waves. After that
lengthy discussion, which contains a lot of mathematics, we discuss how these waves
excite currents on conductors. The second half of the story is that currents radiate
and produce electromagnetic waves. You may already have studied that subject, or if
you wish to further your background, consult books on electromagnetics. The study of
electromagnetics gives insight into the mathematics describing antenna radiation and
provides the rigor to prevent mistakes. We skip the discussion of those equations and
move directly to practical aspects.
It is important to realize that antennas radiate from currents. Design consists of
controlling currents to produce the desired radiation distribution, called its pattern.
In many situations the problem is how to prevent radiation from currents, such as in
circuits. Whenever a current becomes separated in distance from its return current, it
radiates. Simply stated, we design to keep the two currents close together, to reduce
radiation. Some discussions will ignore the current distribution and instead, consider
derived quantities, such as fields in an aperture or magnetic currents in a slot or around
the edges of a microstrip patch. You will discover that we use any concept that provides
insight or simplifies the mathematics.
An antenna converts bound circuit fields into propagating electromagnetic waves
and, by reciprocity, collects power from passing electromagnetic waves. Maxwell’s
equations predict that any time-varying electric or magnetic field produces the opposite
field and forms an electromagnetic wave. The wave has its two fields oriented
orthogonally, and it propagates in the direction normal to the plane defined by the
perpendicular electric and magnetic fields. The electric field, the magnetic field, and
the direction of propagation form a right-handed coordinate system.
ANTENNA RADIATION
Antennas radiate spherical waves that propagate in the radial direction for a coordinate
system centered on the antenna. At large distances, spherical waves can be approximated
by plane waves. Plane waves are useful because they simplify the problem.
They are not physical, however, because they require infinite power.
[attachment=38787]
PROPERTIES OF ANTENNAS
One approach to an antenna book starts with a discussion of how antennas radiate.
Beginning with Maxwell’s equations, we derive electromagnetic waves. After that
lengthy discussion, which contains a lot of mathematics, we discuss how these waves
excite currents on conductors. The second half of the story is that currents radiate
and produce electromagnetic waves. You may already have studied that subject, or if
you wish to further your background, consult books on electromagnetics. The study of
electromagnetics gives insight into the mathematics describing antenna radiation and
provides the rigor to prevent mistakes. We skip the discussion of those equations and
move directly to practical aspects.
It is important to realize that antennas radiate from currents. Design consists of
controlling currents to produce the desired radiation distribution, called its pattern.
In many situations the problem is how to prevent radiation from currents, such as in
circuits. Whenever a current becomes separated in distance from its return current, it
radiates. Simply stated, we design to keep the two currents close together, to reduce
radiation. Some discussions will ignore the current distribution and instead, consider
derived quantities, such as fields in an aperture or magnetic currents in a slot or around
the edges of a microstrip patch. You will discover that we use any concept that provides
insight or simplifies the mathematics.
An antenna converts bound circuit fields into propagating electromagnetic waves
and, by reciprocity, collects power from passing electromagnetic waves. Maxwell’s
equations predict that any time-varying electric or magnetic field produces the opposite
field and forms an electromagnetic wave. The wave has its two fields oriented
orthogonally, and it propagates in the direction normal to the plane defined by the
perpendicular electric and magnetic fields. The electric field, the magnetic field, and
the direction of propagation form a right-handed coordinate system.
ANTENNA RADIATION
Antennas radiate spherical waves that propagate in the radial direction for a coordinate
system centered on the antenna. At large distances, spherical waves can be approximated
by plane waves. Plane waves are useful because they simplify the problem.
They are not physical, however, because they require infinite power.