Figure 155. Front Feed System.
(1) Tapered horn.
The ordinary front feed system is nothing more
than a tapered horn several wavelengths long.
It acts as a tapered
impedance matching section that minimizes the mismatch between the impedance
of space and that of the waveguide. It also allows the small waveguide, the
dimensions of which are fixed by the necessity of avoiding operation in
undesired modes, to feed a rather large two-dimensional source. A tapered
horn feeding a paraboloid of revolution is shown in Part A of Figure 156 and
the horn itself is shown in Part B of Figure 156. The system shown in the
illustration is arranged to radiate horizontally polarized signals, as may
be seen from the fact that the narrow dimension of the waveguide is
horizontal.
The open end of the horn, which acts as a two-dimensional
source, is located at the focal point of the paraboloid. The dimensions of
the open end of the horn are chosen to obtain the desired radiation pattern,
which will distribute energy uniformly over the reflector with a minimum
loss of power.
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