MICROWAVE RECEIVER
A typical microwave receiver is shown in figure
2-12. Though not shown, sensing and alarm functions
are integral to all microwave communications
equipment.
During system operation, the signal from the an-
tenna passes through a waveguide preselector that
eliminates interference from adjacent rf channels. The
signal then enters a waveguide filter tuned to its fre-
quency, which rejects all other unwanted frequencies.
Next, the signal passes through an isolator that minim-
izes intermodulation noise and holds the VSWR below
1.2:1. The signal is then mixed with the local oscillator
(LO) output to produce the standard 70-MHz inter-
mediate frequency (IF). The IF output is amplitude-
limited and applied to an automatic frequency control
(afc) discriminator, which controls the frequency of
the LO. The signal is also applied to an IF discrimina-
tor, a de-emphasis circuit, and a squelch circuit that
disconnects the baseband amplifier and demultiplex-
ing equipment if noise increases above a preset level.
After the squelch circuit, the signal passes through a
baseband amplifier and then to the demultiplexing
equipment, where the original intelligence is retrieved.
Microwave communications systems operating
in the shf portion of the frequency spectrum use the
principle that propagation approaches an optical
straight-line path. Propagation takes place in the lower
atmosphere and is affected by meteorological factors.
Communications in this medium are usually either
line-of-sight or tropospheric scatter.
LINE-OF-SIGHT (LOS)
A line-of sight microwave system consists of one
or more point-to-point hops. Each hop is designed to be
integrated into a worldwide communications network.
Los system characteristics are as follows:
Propagation—Free space as affected by the tro-
posphere.
Communications Capacity/Bandwidth—Up to
600-4kHz voice channels; wideband, can accept TV.
Range—Usually 50 to 150 km (31 to 95 statute
miles). This depends upon antenna height, earth curva-
ture, and intervening terrain.
RF Power—Usually less than 10 watts.
Antennas—Both transmitting and receiving
antennas are horn-driven paraboloids, providing high
gain and narrow beam widths. In some applications,
plane reflectors are used with the paraboloids.
Figure 2-12.—Typical microwave receiver.
2-12
