_______________________________________________________________________ Antennas
the flashlight. Suppose an antenna system concentrates the radio waves so
that at a particular point the field strength is 10 times more than it would be
at the same distance from a half-wave antenna. The antenna system is then
said to have a gain of 10.
4-141. Parasitic arrays represent another method of achieving high antenna
gains. A parasitic array consists of one or more parasitic elements placed in
parallel with each other and, in most cases, at the same line-of-sight level.
The parasitic element is fed inductively by radiated energy coming from the
driven element connected to the transmitter. It is in no way connected
directly to the driven element.
4-142. When the parasitic element is placed so that it radiates away from
the driven element, the element is a director. When the parasitic element is
placed so that it radiates toward the driven element, the parasitic element is
a reflector.
4-143. The directivity pattern resulting from the action of parasitic elements
depends on two factors:
The tuning determined by the length of the parasitic element.
The spacing between the parasitic and driven elements.
To a lesser degree, the directivity pattern also depends on the diameter of the
parasitic element, because diameter has an effect on tuning.
4-144. Operation. When a parasitic element is placed a fraction of a
wavelength away from the driven element and is of approximately resonant
length, it will re-radiate the energy it intercepts. The parasitic element is
effectively a tuned circuit coupled to the driven element, much as the two
windings of a transformer are coupled together. The radiated energy from the
driven element causes a voltage to be developed in the parasitic element,
which, in turn, sets up a magnetic field. This magnetic field extends over to
the driven element, which then has a voltage induced in it. The magnitude
and phase of the induced voltage depend on the length of the parasitic
element and the spacing between the elements. In actual practice the length
and spacing are arranged so that the phase and magnitude of the induced
voltage cause a unidirectional, horizontal-radiation pattern and an increase
in gain.
4-145. In the parasitic array in figure 4-32, view A, the parasitic and driven
elements are spaced one-quarter wavelength apart. The radiated signal
coming from the driven element strikes the parasitic element after one-
quarter cycle. The voltage developed in the parasitic element is 180 degrees
out of phase with that of the driven element. This is because of the distance
traveled (90 degrees) and because the induced current lags the inducing flux
by 90 degrees (90 + 90 = 180 degrees). The magnetic field set up by the
parasitic element induces a voltage in the driven element one-quarter cycle
later because the spacing between the elements is one-quarter wavelength.
This induced voltage is in phase with that in the driven element and causes
an increase in radiation in the direction indicated in figure 4-32, view A.
Because the direction of the radiated energy is stronger in the direction away
from the parasitic element (toward the driven element), the parasitic element
is called a reflector. The radiation pattern as it would appear if you were
looking down on the antenna is shown in view B. The pattern as it would look
if viewed from the ends of the elements is shown in view C.
4-43
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