This type of array is combined by two or more monopole
antennas where a single vertical quarter-wave monopole antenna has an omnidirectional
radiation pattern in the horizontal plane. However, when it is applied in
AM broadcast stations, it must have a directional pattern to avoid interference
with other stations. It can operate in both of end-fire or broadside form
with operating frequency of 2 - 30 MHz. Figure below show both typical form.
A hybrid unit feeds the monopole elements. The hybrid feed
location determines the broadside/end-fire radiation pattern. When the antennas
are fed 180° out of phase, the radiation will cancel broadside to the
array and will be at a maximum off the ends. Radiation pattern of Monopole
Phased Array can be determined by varying the number, spacing, and phase angle
of the elements. Figure 2.2 shows the maximum radiation pattern where antennas
are fed in phase and are one-half wavelength apart. On the other hand, figure
2.3 show the radiation pattern by out of phase.
A broadside array is shown in Figure 2.4, it looks somewhat
like a ladder. It is still in same phase with crossed line connection between
the feedline. The 180 degrees phase shift is cancelled by crossed line connection
between the feedline. 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.
Broadside arrays tune sharply, but lose efficiency rapidly when not operated
on the frequencies for which they are designed.
An end-fire array looks similar to a broadside array. Figure
4.6 shows a construction of an end-fire array. 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 radiation pattern for a pair of parallel half-wave elements fed 180 degrees
out of phase. The 180 degrees out of phase is caused by feedline between elements
is not crossed and cancelled by the 1/2 wavelength physical separation between
them. In end-fire arrays, directivity increases with the addition of more
elements and with spacings approaching the optimum.
Figure 2.1 - Monopole phased array
Figure 2.2 - Same
Figure 2.3 - Out
Figure 2.4 - Geometric of broadside array
Figure 2.5 - Geometric of end-fire array