_______________________________________________________________________ Antennas
seen as a line. The RR1 axis, now seen as a line, is perpendicular to the PP1
axis as well as to the QQ1 axis. The end-fire array is directional in this plane
also, although not quite as sharply. The reason for the greater broadness of
the lobes can be seen by following the path of energy radiating from the
midpoint of element B toward point S in view B. This energy passes the A
element at one end after traveling slightly more than the perpendicular
distance between the dipoles. Energy, therefore, does not combine in exact
phase toward point S. Although maximum radiation cannot take place in this
direction, energy from the two sources combines closely enough in phase to
produce considerable reinforcement. A similar situation exists for wavefronts
traveling toward T. However; the wider angle from Q to T produces a greater
phase difference and results in a decrease in the strength of the combined
wave.
4-135. Directivity occurs from either one or both ends of the end-fire array,
along the axis of the array, as shown by the broken arrows in figure 4-29,
view A; hence, the term end-fire is used.
4-136. The major lobe or lobes occur along the axis of the array. The pattern
is sharper in the plane that is at right angles to the plane containing the
elements (figure 4-30, view A). If the elements are not exact half-wave
dipoles, operation is not significantly affected. However, because of the
required balance of phase relationships and critical feeding, the array must
be symmetrical. Folded dipoles, such as the one shown in figure 4-21, view A,
are used frequently because the impedance at their terminals is higher. This
is an effective way of avoiding excessive antenna losses. Another expedient to
reduce losses is the use of tubular elements of wide diameter.
4-137. Gain and directivity. In end-fire arrays, directivity increases with
the addition of more elements and with spacings approaching the optimum.
The directive pattern for a two-element, bidirectional system is illustrated in
figure 4-30. View A shows radiation along the array axis in a plane
perpendicular to the dipoles, and view B shows radiation along the array axis
in the plane of the elements. These patterns were developed with a 180-
degree phase difference between the elements. Additional elements introduce
small, minor lobes.
4-138. With a 90-degree phase difference in the energy fed to a pair of end-
fire elements spaced approximately one-quarter wavelength apart,
unidirectional radiation can be obtained. The pattern perpendicular to the
plane of the two elements is shown in figure 4-31, view A. The pattern shown
in view B, taken in the same plane, is for a six-element array with 90-degree
phasing between adjacent elements. Because both patterns show relative
gain only, the increase in gain produced by the six-element array is not
evident. End-fire arrays are the only unidirectional arrays wholly made up of
driven elements.
4-41
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