TM 11-5895-846-14
(4) The transmit and receive paths, as noted,
(1) The LNA consists of a single-stage
undergo circular polarization processing.
This is
parametric amplifier (paramp) driving a single-stage FET
necessary to avoid signal losses or dropouts that could
" amplifier. Because of its remote location, ac power,
occur when using linearly polarized signals.
For
control, and monitoring are provided by the LNA
instance, if a vertically polarized signal (electric field
control/translator unit within the shelter.
vertical) reaches an antenna that is horizontally polarized
(2) The LNA is shown in the block diagram of
(will accept signals with horizontal electric fields), very
little, or none, of the signal will be accepted by the
parametric amplifier stage to amplify the signal with a
receiving antenna. A circularly polarized signal will be
high signal-to-noise ratio. Its output drives an SHF field-
accepted by the antenna with about a 3-dB loss. As long
effect transistor stage to amplify the signal further, for a
as one antenna is circularly polarized, signals passing
combined stage gain of about 39 dB at the FET output.
between two antennas directed toward each other should
A bandpass filter, to reduce out-of-band noise, and an
be received at about the same intensity even if one
output isolator follow. The unit is maintained at a
antenna is rotated about the signal path. The antennas
constant temperature of + 75 C by a dual heater system.
used in the satellite are also circularly polarized doubly
A normal heat unit of 130 watts is on and thermostatically
ensuring that signal fading due to signal polarization will
controlled whenever the LNA is operating. In low-
be kept to a minimum.
temperature areas an additional 300-watt unit can be
b. Low Noise Amplifier. The low-noise amplifier
used in a high heat mode to bring the LNA up to
operating temperature more quickly. All dc voltages are
(LNA) is contained in a temperature controlled,
provided by an internal power supply. Fault circuitry is
weatherproof case mounted at the rear of the antenna
connected to a FAULT lamp on the LNA control to
reflector. It is an SHF preamplifier which provides an
indicate failure, low limit in dc bias levels, or high or low
excellent noise figure and increases the received signal
temperature.
power to a level sufficient to provide an adequate input to
the downconverters in the shelter.
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