THEORY
of the antenna axis, there are dif-
20. Antenna Theory
ferent amounts of radiation at dif-
The characteristics of transmitting and
angles to the axis. There is little
receiving antennas are similar; a good
radiation in direction OA, greater
transmitting antenna is also a good re-
radiation in direction OB, and max-
ceiving antenna.
imum radiation in direction OC.
(2) T h a t part of the radiation field
a. Basic Half Wave Antenna. D u r i n g
which travels upward, strikes the
transmission, the rf current in the antenna
ionosphere, and is returned at a
wire produces a radiated field. During re-
c o n s i d e r a b l e distance from the
ception, a radiated field from a distant
radiating antenna is called a sky-
station antenna cuts the antenna wire and
wave. A horizontal half wave an-
causes a current flow in the wire. In a half
tenna is very broadly bidirectional
wave antenna, current is maximum at the
in the horizontal plane. In practice,
center and minimum at the ends, while
the antenna is oriented in the di-
voltage is minimum at the center and maxi-
r e c t i o n that provides maximum
mum at the ends (fig. 11); therefore, im-
signal strength.
pedance is minimum at the center and
maximum at the ends. The impedance at
21. Theory of Antenna Assembly
the center of a basic half wave antenna is
72 ohms. The current distribution in the
The antenna assembly is a horizontal
a n t e n n a is the same regardless of the
center-fed Hertz antenna (doublet). A cen-
amount of current flow; the amplitude
ter-fed Hertz is one-half wavelength long
of the current at any point on the antenna
at the operating frequency and is fed at its
varies directly with the amplitude of the
physical center. The antenna assembly is
signal voltage.
adjustable and can be used for operating
frequencies from 1.5 to 20 mc. It consists
b. Directional Pattern.
of two lengths of wire, each adjustable to
(1) Maximum radiation occurs at the
a little less than a quarter wavelength at
center of a half wave antenna, be-
the operating frequency. The transmission
c a u s e the current is greatest at
line is a low-impedance coaxial cable that
that point. Minimum radiation takes
connects to the low-impedance point at the
place at the ends, because the cur-
center of the antenna assembly. A certain
rent is least at the ends. Radiation
amount of mismatch exists between the
is at right angles to the plane of
rf cable and the antenna, but it is not
the antenna wire and completely
critical. Current and voltage distribution
e n c i r c l e s the antenna wire. The
is the same as that described for a simple
resulting antenna directional pat-
half wave antenna (para 20). The formula
tern can be compared to a dough-
for determining the actual total length
nut with the radiator passing
of the antenna assembly required for a
through its center. Figure 12 shows
particular operating frequency is:
a cross section of the doughnut
shaped pattern. Although there is
468
Length in feet =
no radiation along the direct line
Frequency in mc.
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