ANTENNAS
An antenna can be defined as an electrical conductor or system of conductors used
either for radiating electromagnetic energy or for collecting electromagnetic
energy. For transmission of a signal, radio-frequency electrical energy from the
transmitter is converted into electromagnetic energy by the antenna and radiated
into the surrounding environment (atmosphere, space, water). For reception of a
signal, electromagnetic energy impinging on the antenna is converted into radiofrequency
electrical energy and fed into the receiver.
In two-way communication, the same antenna can be and often is used for
both transmission and reception. This is possible because any antenna transfers
energy from the surrounding environment to its input receiver terminals with the
same efficiency that it transfers energy from the output transmitter terminals into
the surrounding environment, assuming that the same frequency is used in both
directions. Put another way, antenna characteristics are essentially the same whether
an antenna is sending or receiving electromagnetic energy.
Radiation Patterns
An antenna will radiate power in all directions but, typically, does not perform
equally well in all directions. A common way to characterize the performance of an
antenna is the radiation pattern, which is a graphical representation of the radiation
properties of an antenna as a function of space coordinates. The simplest pattern is
produced by an idealized antenna known as the isotropic antenna. An isotropic
antenna is a point in space that radiates power in all directions equally. The actual
radiation pattern for the isotropic antenna is a sphere with the antenna at the center.
However, radiation patterns are almost always depicted as a two-dimensional
cross section of the three-dimensional pattern. The pattern for the isotropic antenna
is shown in Figure 5.la. The distance from the antenna to each point on the radiation
pattern is proportional to the power radiated from the antenna in that direction.
Figure 5.lb shows the radiation pattern of another idealized antenna. This is a
directional antenna in which the preferred direction of radiation is along one axis.
The actual size of a radiation pattern is arbitrary. What is important is the relative
distance from the antenna position in each direction. The relative distance
determines the relative power. To determine the relative power in a given direction,
a line is drawn from the antenna position at the appropriate angle, and the point of
intercept with the radiation pattern is determined. Figure 5.1 shows a comparison of
two transmission angles, A and B, drawn on the two radiation patterns. The isotropic
antenna produces an omnidirectional radiation pattern of equal strength in all
directions, so the A and B vectors are of equal length. For the antenna pattern of
Figure 5.lb, the B vector is longer than the A vector, indicating that more power is
radiated in the B direction than in the A direction, and the relative lengths of the
two vectors are proportional to the amount of power radiated in the two directions.
The radiation pattern provides a convenient means of determining the beam
width of an antenna, which is a common measure of the directivity of an antenna.
The beam width, also referred to as the half-power beam width, is the angle within
which the power radiated by the antenna is at least half of what it is in the most
preferred direction.
When an antenna is used for reception, the radiation pattern becomes
a reception pattern. The longest section of the pattern indicates the best direction
for reception.
An antenna can be defined as an electrical conductor or system of conductors used
either for radiating electromagnetic energy or for collecting electromagnetic
energy. For transmission of a signal, radio-frequency electrical energy from the
transmitter is converted into electromagnetic energy by the antenna and radiated
into the surrounding environment (atmosphere, space, water). For reception of a
signal, electromagnetic energy impinging on the antenna is converted into radiofrequency
electrical energy and fed into the receiver.
In two-way communication, the same antenna can be and often is used for
both transmission and reception. This is possible because any antenna transfers
energy from the surrounding environment to its input receiver terminals with the
same efficiency that it transfers energy from the output transmitter terminals into
the surrounding environment, assuming that the same frequency is used in both
directions. Put another way, antenna characteristics are essentially the same whether
an antenna is sending or receiving electromagnetic energy.
Radiation Patterns
An antenna will radiate power in all directions but, typically, does not perform
equally well in all directions. A common way to characterize the performance of an
antenna is the radiation pattern, which is a graphical representation of the radiation
properties of an antenna as a function of space coordinates. The simplest pattern is
produced by an idealized antenna known as the isotropic antenna. An isotropic
antenna is a point in space that radiates power in all directions equally. The actual
radiation pattern for the isotropic antenna is a sphere with the antenna at the center.
However, radiation patterns are almost always depicted as a two-dimensional
cross section of the three-dimensional pattern. The pattern for the isotropic antenna
is shown in Figure 5.la. The distance from the antenna to each point on the radiation
pattern is proportional to the power radiated from the antenna in that direction.
Figure 5.lb shows the radiation pattern of another idealized antenna. This is a
directional antenna in which the preferred direction of radiation is along one axis.
The actual size of a radiation pattern is arbitrary. What is important is the relative
distance from the antenna position in each direction. The relative distance
determines the relative power. To determine the relative power in a given direction,
a line is drawn from the antenna position at the appropriate angle, and the point of
intercept with the radiation pattern is determined. Figure 5.1 shows a comparison of
two transmission angles, A and B, drawn on the two radiation patterns. The isotropic
antenna produces an omnidirectional radiation pattern of equal strength in all
directions, so the A and B vectors are of equal length. For the antenna pattern of
Figure 5.lb, the B vector is longer than the A vector, indicating that more power is
radiated in the B direction than in the A direction, and the relative lengths of the
two vectors are proportional to the amount of power radiated in the two directions.
The radiation pattern provides a convenient means of determining the beam
width of an antenna, which is a common measure of the directivity of an antenna.
The beam width, also referred to as the half-power beam width, is the angle within
which the power radiated by the antenna is at least half of what it is in the most
preferred direction.
When an antenna is used for reception, the radiation pattern becomes
a reception pattern. The longest section of the pattern indicates the best direction
for reception.
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