Andrew 174100, Installation And Maintenance Manual

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1.6.4.5 Transmitting and Receiving Linearly Polarized Signals
To produce linear polarization, signals from the transmitter enter the dual-mode transducer vertical
port and the phase-shift section. In this case, however, the dielectric vanes are positioned
approximately 0.2 inches from the waveguide wall. Both orthogonal signal components receive
exactly the same phase shift. Since no differential phase shift exists between the orthogonal
components, the two signal components exit from the phase-shift section linearly polarized. The
signals pass through the feedhorn, illuminating the reflector and resulting in a linear vertically
polarized beam. As in circular polarization, desired returns pass from the reflector through the
feedhorn to the dual-mode transducer. Vertically polarized signal returns pass unchanged through the
phase-shift section and the waveguide run to the receiver. The antenna actually has a linear signal
with a small circular component (11db linear mode axial ratio). This is achieved by introducing a
small differential phase shift.
1.6.4.6 Feedhorn Assembly
The feedhorn attached to the active phase-shift assembly properly distributes the outgoing signal over
the antenna reflector for all polarizations. Both the Lowbeam and Highbeam horns collect signals
from targets. A radome over the mouth of the feedhorn provides an airtight seal without degradation
of signal strength.
1.6.4.7 Polarizer/Feed Assembly Mechanical Operation - Refer to Figures 1-3, 4-1 and 4-2
Two dielectric vanes inserted from one wall of the square waveguide section produce the phase shift
required for circular polarization. These vanes are moved to a position near the waveguide wall to
restore linear polarization. Proper limits of travel between maximum and minimum penetration are
set during calibration at the manufacturer=s test range and should not be disturbed elsewhere. Each
dielectric vane is supported by two dielectric rods that pass through matching clearance holes in the
square waveguide sidewall and are anchored in a single movable mounting plate. A button mounted
on an adjustable extension from the movable mounting plate serves as a bearing for the motor-driven
vane cam. This cam drives the movable mounting plate between its limits. Maximum depth of vane
insertion (circular polarization) is determined by the height of the button. The highest point of the
cam is a constant radius section to ensure proper vane penetration even if the cam does not stop in
exactly the same position every time. Minimum depth of vane insertion (linear polarization) is
determined by two adjustable screws that serve as stops for the movable mounting plate. In linear
position the button completely disengages from the cam and the mounting plate is spring loaded
against the linear stops. A second cam on the shaft operates four micro switches that control motor
operation and readback information.