ANTENNA CHARACTERISTICS
As you will learn in this section, all antennas exhibit
common characteristics. The study of antennas
involves the following terms with which you must
become familiar:
Antenna Reciprocity
The ability of an antenna to both transmit and
receive electromagnetic energy is known as its
reciprocity. Antenna reciprocity is possible because
antenna characteristics are essentially the same for
sending and receiving electromagnetic energy.
Even though an antenna can be used to transmit or
receive, it cannot be used for both functions at the same
time. The antenna must be connected to either a
transmitter or a receiver.
Antenna Feed Point
Feed point is the point on an antenna where the RF
cable is attached. If the RF transmission line is attached
to the base of an antenna, the antenna is end-fed. If the
RF transmission line is connected at the center of an
antenna, the antenna is mid-fed or center-fed.
Directivity
The directivity of an antenna refers to the width of
the radiation beam pattern. A directional antenna
concentrates its radiation in a relatively narrow beam. If
the beam is narrow in either the horizontal or vertical
plane, the antenna will have a high degree of directivity
in that plane. An antenna can be highly directive in one
plane only or in both planes, depending upon its use.
In general, we use three terms to describe the type of
directional qualities associated with an antenna:
omnidirectional, bidirectional, and unidirectional.
Omnidirectional antennas radiate and receive equally
well in all directions, except off the ends. Bidirectional
antennas radiate or receive efficiently in only two
directions. Unidirectional antennas radiate or receive
efficiently in only one direction.
Most antennas used in naval communications are
either omnidirectional or unidirectional. Bidirectional
antennas are rarely used. Omnidirectional antennas are
used to transmit fleet broadcasts and are used aboard
ship for medium-to-high frequencies. A parabolic, or
dish, antenna (figure 2-14) is an example of a
unidirectional antenna. As you can see in the figure, an
Figure 2-14.—Principle of parabolic reflection.
antenna (normally a half wave) is placed at the “focal”
point and radiates the signal back into a large reflecting
surface (the dish). The effect is to transmit a very
narrow beam of energy that is essentially unidirectional.
Figure 2-15 shows a large, unidirectional parabolic
antenna. Directional antennas are commonly used at
shore installations.
Wave Polarization
Polarization of a radio wave is a major
consideration in the efficient transmission and
reception of radio signals. If a single-wire antenna is
Figure 2-15.—Unidirectional parabolic antenna.
2-15
