c. Linear array.
A linear array is required to feed a parabolic
cylinder reflector.
(1) Probe-fed dipoles. The usual array consists of a set of probe-
fed dipoles. Each dipole is, in effect, a center-fed, half-wave antenna as
shown in Figure 163.
Figure 163.
Probe-Fed Dipoles.
One half of each dipole is fed by the probe, which extends into
the waveguide parallel to the electric field. The other half of the dipole
is fed by transfer of energy across the quarter-wave, two-wire transmission
line which supports the two halves of the dipole.
Successive pairs of
dipoles are spaced along the waveguide at specific intervals. At the input
end of the linear array shown in Figure 164, the energy level in the
waveguide is relatively high and the probes extend only a short distance
into the waveguide.
Near the other end of the array the energy level is
lower, because energy has been withdrawn by the earlier dipole probes and
the probes extend farther into the waveguide. To avoid any reflection, the
waveguide is terminated in its characteristic impedance by a dissipative
termination.
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