_______________________________________________________________________ Antennas
however, tends to throw off the phase relationship between the original
current in one element and the current induced in it by the other element.
The result is that, although the major lobes are sharpened, minor lobes are
introduced, even with two elements. These minor lobes, however, are not
large enough to be of concern.
4-129. If you add the same number of elements to both a broadside array
and a collinear array, the gain of the broadside array will be greater. Reduced
radiation resistance resulting from the efficient coupling between dipoles
accounts for most of this gain. However, certain practical factors limit the
number of elements that may be used. The construction problem increases
with the number of elements, especially when they are polarized horizontally.
End-Fire Arrays
4-130. An end-fire array looks similar to a broadside array. The ladder-like
appearance is characteristic of both (figure 4-29, view A). The currents in the
elements of the end-fire array, however, are usually 180 degrees out of phase
with each other as indicated by the arrows. The construction of the end-fire
array is like that of a ladder lying on its side (elements horizontal). The
dipoles in an end-fire array are closer together (one-eighth wavelength to one-
quarter wavelength spacing) than they are for a broadside array.
Figure 4-29. Typical End-Fire Array
4-131. Closer spacing between elements permits compactness of
construction. For this reason, an end-fire array is preferred to other arrays
when high gain or sharp directivity is desired in a confined space. However,
the close coupling creates certain disadvantages. Radiation resistance is
extremely low, sometimes as low as 10 ohms, making antenna losses greater.
The end-fire array is confined to a single frequency. With changes in climatic
or atmospheric conditions, the danger of detuning exists.
4-132. Radiation pattern. The radiation pattern for a pair of parallel half-
wave elements fed 180 degrees out of phase is shown in figure 4-30, view A.
The elements shown are spaced one-half wavelength apart. In practice,
smaller spacings are used. Radiation from elements L and M traveling
toward point P begins 180 degrees out of phase. Moving the same distance
over approximately parallel paths, the respective wavefronts from these
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