TC 9-64 _________________________________________________________________________
for the collinear element, you will find that greater lengths are often used.
Effective arrays of this type have been constructed in which the elements are
0.7 and even 0.8 wavelength long. This type of array provides efficient
operation at more than one frequency or over a wider frequency range.
Whatever length is decided upon, all of the elements in a particular array
should closely adhere to that length. If elements of different lengths are
combined, current phasing and distribution are changed, throwing the
system out of balance and seriously affecting the radiation pattern.
4-121. Spacing. The lower relative efficiency of collinear arrays of many
elements, compared with other multi-element arrays, relates directly to
spacing and mutual impedance effects. Mutual impedance is an important
factor to be considered when any two elements are parallel and are spaced so
that considerable coupling is between them. There is very little mutual
impedance between collinear sections. Where impedance does exist, it is
caused by the coupling between the ends of adjacent elements. Placing the
ends of elements close together is frequently necessary because of
construction problems, especially where long lengths of wire are involved.
4-122. The effects of spacing and the advantages of proper spacing can be
demonstrated by some practical examples. A collinear array consisting of two
half-wave elements with one-half-wavelength spacing between centers has a
gain of 1.8 decibels (dB). If the ends of these same dipoles are separated so
that the distance from center to center is three-quarters of a wavelength and
they are driven from the same source, the gain increases to approximately 2.9
4-123. A three-dipole array with negligible spacing between elements gives a
gain of 3.3 dB. In other words, when two elements are used with wider
spacing, the gain obtained is approximately equal to the gain obtainable from
three elements with close spacing. The spacing of this array permits simpler
construction, because only two dipoles are used. It also allows the antenna to
occupy less space. Construction problems usually dictate small-array spacing.
4-124. A broadside array is shown in figure 4-27, view A. Physically, it looks
somewhat like a ladder. When the array and the elements in it are polarized
horizontally, it looks like an upright ladder. When the array is polarized
vertically, it looks like a ladder lying on one side (view B). View C is an
illustration of the radiation pattern of a broadside array. Horizontally
polarized arrays using more than two elements are not common. This is
because the requirement that the bottom of the array be a significant
distance above the earth presents construction problems. Compared with
collinear arrays, broadside arrays tune sharply, but lose efficiency rapidly
when not operated on the frequencies for which they are designed.