Radar System Engineering

SEC.9.15] SCI HEIGHT FINDER 301
In actual fact, the feed horn moves, not linearly, but circularly. The
rotating feature is achieved by folding and bending the trapezoidal plates,
keeping them parallel with a minimum of stretching, in such a way that
the feed base is rolled into a complete annular aperture while the flared
base is held straight. Figure 9.30 illustrates how this can be done with a
single trapezoidal sheet. Various additional mechanical and electrical
complications appear when a two-sheet metal horn is made in this shape,
but the convoluted parallel-plate region has substantially the same optical
Trapezoidal parallel plates
Moving feed horn
\
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-----
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_--?%
10.5” scan
ii-”
~=>
1
Horizon
Feed base
1.2° Beamwidth
‘ ;D F
\
Flared base
(
Vertex
Reflector
FKci.9.29.—SimplitiedSCI height finder, side view.
properties as the trapezoidal plates before rolling. When the rotating
guide feeds into the annular aperture of the horn, the beam describes a
linear “fro” scan in elevation. The feed position marked O in Fig. 9.30
gives a beam on axis. When the feed is in the neighborhood of the crossover
point (about ~ of the time required for one complete scan) the signal
is ambiguous because there are two beams.
The parallel-plate assembly consists of two fabricated aluminum
surfaces spaced ~ in. apart, each consisting of a cast and machined center
portion which supports a ribbed sheet metal “wing” section above and
below. Specially designed steel posts are used to help maintain the
spailng between these surfaces without ill effect on the electric waves.
The rotary waveguide feed was made radial for compactness, resulting in
the arrangement of the horn, chokes, and annular aperture shown in
Fig. 9.31. The absorbing septum shown in this figure was introduced to
reduce the amount of ambiguity caused by a double beam and to improve