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1.6 THEORY OF OPERATION
The theory of operation for the antenna contains theory of operation for microwave and electronic
circuits and provides detailed circuit descriptions explaining circuit arrangements in the order of
operational or signal sequence. Functional operation of mechanical assemblies is provided as needed
and as related to electronic circuits and system operation.
1.6.1 Dual Feed Assembly
The dual feed assembly allows remote selection of linear or circular polarization and provides two
secondary antenna patterns displaced approximately 3.5 degrees in the elevation plane. (Secondary
patterns are those produced by the reflector, using primary patterns produced by the feedhorns.)
Under normal operating conditions linear polarization of the radiated signal will be used since it
provides higher level target returns. Under inclement weather conditions circular polarization can be
selected to reduce the radar sensitivity to clouds and precipitation.
Both feeds in this assembly are identical in construction. The normal or active feed (Low Beam) is
located above the passive feed (High Beam) to produce a secondary active beam directed more
toward the ground than the passive beam. The transmitted signal is radiated by this lower beam.
During the high ground clutter return period, the first 10 or 15 nautical miles, signals from the upper
beam (lower feed) are coupled to the receiver by means of a beam select switch located in the
receiver. This arrangement reduces the ground level returns without reducing returns from targets at
higher elevation angles.
1.6.2 Reflector
The reflector focuses RF energy to provide beams approximately 1.5 degrees in azimuth and shaped
in elevation to produce coverage up to approximately 32 degrees. The reflector surface has a circular
opening at the center for mounting a test antenna. The top of the reflector has provision for
supporting an LVA SSR antenna.
1.6.3 Detailed Circuit Analysis
1.6.3.1 Antenna Unit
The antenna, rotating at a rate of 12.5 r/min, scans an area with a radius of 55 nautical miles, using
two beams approximately 1.5 degrees in azimuth with shaped elevation patterns to provide coverage
up to 32 degrees. By selecting returns from the higher beam during the first part of the receive
period, ground clutter is reduced while good sensitivity is maintained for the rest of the coverage
volume. Each subassembly is discussed in the following paragraphs.
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