Andrew 174100, Installation And Maintenance Manual

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1.6.3.6 Reflector Mounting and Tilt Adjustment
The reflector is mounted directly on the reflector support which is attached to the pedestal by two
hinged joints and two elevation tilt screws. The dual feed assembly with its associated waveguide
and related hardware is supported on a feed support which is mounted to the antenna base, causing
both the reflector and the dual feed assembly to tilt as necessary to achieve the desired elevation tilt
angle for the rotating beam. Adjusting the tilt screw is a manual operation, requiring only a few
minutes at the antenna platform. Antenna tilt is adjustable so that the peak of the main beam can be
set at any angle between 3.0 degrees below and 5.0 degrees above the horizontal. A pointer and
scale, marked in increments of 0.1 degree, indicate the elevation angle of the beam peak to an
accuracy of 0.2 degrees at the center of the frequency band. If an accuracy better than 0.2 degrees is
required, the use of an inclinometer is recommended for a more accurate reading of antenna tilt
adjustment. A tilt instruction plate mounted on the reflector support near the tilt screw provides
antenna tilt correction data as a function of frequency.
1.6.4 Dual Feed Assembly
Dual feed assembly (Figure 3-4) consists of two identical polarizer assemblies, transducers,
transitions to type >N=, two feedhorns, two radomes, and associated supporting hardware. Both
feeds are held in place by a mounting frame. Feed positioning is established during pattern testing
by Andrew. After positioning, locating pins are installed to permit proper realignment if field
disassembly/reassembly of the feed assembly is required. The dual feed assembly is attached to the
feed support arm of the reflector support by three large diameter threaded rods. All but the top three
nuts, which are self-locking, are pinned after antenna test to ensure proper location of the dual feed
assembly. Removal of the top three nuts is required to remove the dual feed assembly.
1.6.4.1 Feed Assembly
Figure 3-4 shows the relative positions of the subassemblies in one feed assembly. These are a dual-
mode transducer, a phase-shift section, a feedhorn and a transition.
1.6.4.2. Circular Polarization Generation
To obtain a circularly polarized signal the linearly polarized input signal is split into two equal,
orthogonally polarized (polarized perpendicular to each other) components. One of the signals is
then shifted in time 90 degrees relative to the other. The phase-shift section (polarizer) provides the
differential 90-degree phase shift for circular polarization and 0-degree phase shift for linear
polarization. Phase-shifter orientation with respect to the transducer (approximately 45 degrees)
determines relative amplitude of orthogonally polarized signals.