_______________________________________________________________________ Antennas
difference, the folded dipole can be used over a much wider frequency range
than is possible with a simple dipole.
4-93. An array antenna is a special arrangement of basic antenna
components. The discussion of array antennas includes the following new
terms that you should study before moving on to the section on phasing.
4-94. An array antenna is made up of more than one element, but the basic
element is generally the dipole. Sometimes the basic element is made longer
or shorter than a half-wave, but the deviation usually is not great.
4-95. A driven element is similar to the dipole you have been studying and is
connected directly to the transmission line. It obtains its power directly from
the transmitter or, as a receiving antenna; it delivers the received energy
directly to the receiver. A parasitic element is located near the driven
element from which it gets its power. It is placed close enough to the driven
element to permit coupling.
4-96. A parasitic element is sometimes placed so it will produce maximum
radiation (during transmission) from its associated driver. When it operates
to reinforce energy coming from the driver toward itself, the parasitic
element is referred to as a director. If a parasitic element is placed so it
causes maximum energy radiation in a direction away from itself and toward
the driven element, that parasitic element is called a reflector.
4-97. If all of the elements in an array are driven, the array is referred to as
a driven array (sometimes as a connected array). If one or more elements are
parasitic, the entire system usually is considered to be a parasitic array.
4-98. Multi-element arrays frequently are classified according to their
directivity. A bidirectional array radiates in opposite directions along the line
of maximum radiation. A unidirectional array radiates in only one general
direction.
4-99. Arrays can be described with respect to their radiation patterns and the
types of elements of which they are made. However, you will find it useful to
identify them by the physical placement of the elements and the direction of
radiation with respect to these elements. Generally speaking, the term
broadside array designates an array in which the direction of maximum
radiation is perpendicular to the plane containing these elements. In actual
practice, this term is confined to those arrays in which the elements
themselves are also broadside, or parallel, with respect to each other.
4-100. A collinear array is one in which all the elements lie in a straight line
with no radiation at the ends of the array. The direction of maximum
radiation is perpendicular to the axis of the elements.
4-101. An end-fire array is one in which the principal direction of radiation
is along the plane of the array and perpendicular to the elements. Radiation
is from the end of the array, which is the reason this arrangement is referred
to as an end-fire array.
4-102. Sometimes a system uses the characteristics of more than one of the
three types mentioned. For instance, some of the elements may be collinear
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