The transmission line transfers the RF power from the
transmitter to the antenna. There are two general types of
transmission lines: resonant (tuned) and nonresonant (untuned).
(a) Resonant feeder line is the same length as the antenna. It
is rarely used in tactical applications.
(b) A nonresonant transmission line is one that has an SWR of
less than 1.5. In order to achieve this, the impedance of the
antenna and the transmission line must match. An antenna tuning unit
is used in some applications to match the transmitter to the line and
antenna.
(1) Single-wire feed. A single wire can be used as a
nonresonant feed line. Because the impedance of a single-wire feed
is 500 to 600 ohms, a point on the half-wave antenna must be selected
that will match the impedance of the line. The antenna impedance
varies from 2500 at the end to about 73 ohms in the center. A point
14 percent from the center of the antenna will provide the 500 to 600
ohms required (A of figure 31). To reduce radiation or coupling make
sure the single-wire feed is at right angles to the antenna. A good
electrical ground connection is also required to provide a return
path to the transmitter.
(2) Twisted-pair feed. WD-1 can be used in an emergency to
provide a feed line from the transmitter to the antenna (B of figure
31). The impedance requirement of a twisted pair is 70 to 80 ohms.
The center of the half wave antenna provides that impedance. This
type of feed should be used only as a last resort because of the very
high power loss.
(3) Coaxial line feed. A coaxial feed provides a two conductor
line which offers the least line loss of all practical field feed
systems.
An antenna radiates energy in a particular pattern in free
space. It is useful to examine these radiation patterns. It is
possible to design an antenna system to provide us with the best
possible communication.
(1) Radiation types and patterns.
(a) An example of a source that radiates in all directions is the
sun. This type of radiator is called an isotropic radiator. If we
could measure the sun's radiation as we move around it in a circle,
we would find it was the same all along the circle.
(b) Another type of radiator is called anisotropic. An example
is a flashlight. The light beam radiates only a small portion of the
total space around the flashlight. If we move in a circle around the
flashlight, we find the level goes from zero to maximum then back to
zero again.
(2) Dipole antenna radiation.
(a) The terms dipole and doublet are used interchangeably.
Both are used to indicate a basic half-wave antenna.
(b) Radiation pattern of a doublet. The doublet is the
simplest form of an antenna. The radiation pattern is similar to the
flashlight. There is a vertical as well as a
horizontal radiation pattern. As you can see, the pattern is in the
form of a doughnut. Whether it is seen from the side or from the
top, the pattern is full.
(c) By looking you can see that the antenna can
be mounted either vertically or horizontally. The radiation patterns
are similar. The difference is that a horizontal antenna radiates
horizontally in two directions, while a vertical antenna radiates
horizontally in all directions. Figure 36 indicates the beam width
and relative power patterns.
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