4-38
this element is made about 5 percent longer than 1/2 wavelength, it will act as a reflector when the
spacing is 15 percent of a wavelength.
Changing the spacing and length can change the radiation pattern so that maximum radiation is on
the same side of the driven element as the parasitic element. In this instance the parasitic element is called
a director.
Combining a reflector and a director with the driven element causes a decrease in back radiation and
an increase in directivity. This combination results in the two main advantages of a parasitic array—
unidirectivity and increased gain. If the parasitic array is rotated, it can pick up or transmit in different
directions because of the reduction of transmitted energy in all but the desired direction. An antenna of
this type is called a ROTARY ARRAY. Size for size, both the gain and directivity of parasitic arrays are
greater than those of driven arrays. The disadvantage of parasitic arrays is that their adjustment is critical
and they do not operate over a wide frequency range.
GAIN AND DIRECTIVITY.—Changing the spacing between either the director or the reflector
and the driven element results in a change in the radiation pattern. More gain and directivity are obtained
by changing the length of the parasitic elements.
The FRONT-TO-BACK RATIO of an array is the proportion of energy radiated in the principal
direction of radiation to the energy radiated in the opposite direction. A high front-to-back ratio is
desirable because this means that a minimum amount of energy is radiated in the undesired direction.
Since completely suppressing all such radiation is impossible, an infinite ratio cannot be achieved. In
actual practice, however, rather high values can be attained. Usually the length and spacing of the
parasitic elements are adjusted so that a maximum front-to-back ratio is obtained, rather than maximum
gain in the desired direction.
Q38. What two factors determine the directivity pattern of the parasitic array?
Q39. What two main advantages of a parasitic array can be obtained by combining a reflector and a
director with the driven element?
Q40. The parasitic array can be rotated to receive or transmit in different directions. What is the name
given to such an antenna?
Q41. What are the disadvantages of the parasitic array?
Multielement Parasitic Array
A MULTIELEMENT PARASITIC array is one that contains two or more parasitic elements with the
driven element. If the array contains two parasitic elements (a reflector and a director) in addition to the
driven element, it is usually known as a THREE-ELEMENT ARRAY. If three parasitic elements are
used, the array is known as a FOUR-ELEMENT ARRAY, and so on. Generally speaking, if more
parasitic elements are added to a three-element array, each added element is a director. The field behind a
reflector is so small that additional reflectors would have little effect on the overall radiation pattern. In
radar, from one to five directors are used.
CONSTRUCTION.—The parasitic elements of a multi-element parasitic array usually are
positioned as shown in figure 4-32, views A and B. Proper spacings and lengths are determined
experimentally. A folded dipole (view B) is often used as the driven element to obtain greater values of
radiation resistance.
