4-36
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.
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.
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.
The directivity pattern resulting from the action of parasitic elements depends on two factors. These
are (1) the tuning, determined by the length of the parasitic element; and (2) the spacing between the
parasitic and driven elements. To a lesser degree, it also depends on the diameter of the parasitic element,
since diameter has an effect on tuning.
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.
In the parasitic array in figure 4-31, view A, the parasitic and driven elements are spaced 1/4
wavelength apart. The radiated signal coming from the driven element strikes the parasitic element after
1/4 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 1/4 cycle later because the spacing between the elements
is 1/4 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 4-31, view A. Since 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.
