4-25
Q24. What is the difference in the amount of impedance between a three-wire dipole and a simple
center-fed dipole?
Q25. Which has a wider frequency range, a simple dipole or a folded dipole?
ARRAY ANTENNAS
An array antenna is a special arrangement of basic antenna components involving new factors and
concepts. Before you begin studying about arrays, you need to study some new terminology.
DEFINITION OF TERMS
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.
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.
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.
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.
MULTIELEMENT 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.
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.
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.
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.
Sometimes a system uses the characteristics of more than one of the three types mentioned. For
instance, some of the elements may be collinear while others may be parallel. Such an arrangement is
