_______________________________________________________________________ Antennas
made electrically inductive before it will act as a reflector. If this parasitic
element is made about 5 percent longer than one-half wavelength, it will act
as a reflector when the spacing is 15 percent of a wavelength.
4-148. 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.
4-149. 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.
4-150. 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.
4-151. 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.
Because 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.
Multi-Element Parasitic Array
4-152. A multi-element 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.
4-153. Construction. The parasitic elements of a multi-element parasitic
array usually are positioned as shown in figure 4-33, 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
4-45
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