Andrew 174100, Installation And Maintenance Manual

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1.6.43 Transmitting Circularly Polarized Signals
A linear vertically polarized signal enters the transducer vertical port. The signal then passes through
an impedance matching section in the transducer and enters the phase shift section. The phase shifter
is rotated approximately 45 degrees about the longitudinal axis of the waveguide run. At this point
the signal can be considered to consist of two orthogonally polarized signals, each polarized parallel
to the walls of the square waveguide. As the dielectric vanes are moved in the phase-shift section,
the phase of the E-field component perpendicular to the plane of the vanes is unchanged; however,
the orthogonal component is changed in phase. By positioning the dielectric vanes approximately 0.9
inches from the waveguide wall, a 90-degree phase differential is created between the two signals,
producing circular polarization. The circularly polarized signal passes through the feedhorn,
illuminating the reflector which forms the desired radiation pattern.
1.6.4.4 Receiving Circularly Polarized Signals
Returns resulting from precipitation are reverse-sense circularly polarized. They enter the polarizer,
pass through the phase-shift section, and enter the dual-mode transducer as horizontally polarized
signals. These signals are coupled through the dual-mode transducer horizontal port via a transition
to type >N= and a coaxial cable to a Beamswitch. The Beamswitch allows either the low beam or
high beam precipitation return to be lead to the weather channel of the antenna. Precipitation returns
are thus diverted from the regular return path. Desired returns are circularly polarized in the same
sense as the transmitted signal. They are converted to vertical polarization by the phase-shift section
and enter the dual-mode transducer vertical port. These signals then pass through connecting
waveguide to the receiver.
The above applies when the antenna is operated in the circular polarization mode. One of the features
of this antenna is that in the linear polarization mode (see Section 1.6.4.5) a small circular component
is also transmitted together with the vertically polarized signal. This small circularly polarized signal
is approximately 11 dB below the linearly polarized signal. It is used to provide weather channel
output (attenuated by 11dB) for linear mode operations.