4-31
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.
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 1/4-wavelength spacing between
centers has a gain of 1.8 dB. If the ends of these same dipoles are separated so that the distance from
center to center is 3/4 wavelengths and they are driven from the same source, the gain increases to
approximately 2.9 dB.
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, since only two dipoles are used. It also allows the antenna to occupy less space.
Construction problems usually dictate small-array spacing.
Broadside Arrays
A broadside array is shown in figure 4-26, 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.
Figure 4-26.—Typical broadside array.
RADIATION PATTERN.—Figure 4-27 shows an end view of two parallel half-wave antennas (A
and B) operating in the same phase and located 1/2 wavelength apart. At a point (P) far removed from the
antennas, the antennas appear as a single point. Energy radiating toward P from antenna A starts out in
phase with the energy radiating from antenna B in the same direction. Propagation from each antenna
travels over the same distance to point P, arriving there in phase. The antennas reinforce each other in this
direction, making a strong signal available at P. Field strength measured at P is greater than it would be if
the total power supplied to both antennas had been fed to a single dipole. Radiation toward point P1 is
built up in the same manner.
