Radar System Engineering

SEC. 1714] MICROWAVE SYSTEM FOR POINT-TO-POINT SERVICE 723
Two miniature double diodes are included in the transmitter for
monitoring purposes. One of these tubes is connected as a discriminator
to provide a video-output test point at the antenna-line connector. During
all testing, this video signal gives an accurate over-all check of the
r-f and video sections of the transmitter. One half of the remaining
double diode is used as an r-f detector to provide a relative power indication
on a panel meter which is also used, by means of a rotary selector
switch, to measure the grid currents of the several doublers. This diode
is also connected to a test point to allow a scope to be used when over-all
alignment and bandwidth measurements are being made. Because of
the stability of this transmitter, it was found unnecessary to use a bidirectional
coupler with it.
The receiver has one tuned r-f stage, a mixer, a local oscillator, seven
i-f stages, two limiters, a discriminator, a video amplifier, and three
tubes connected in parallel as a cathode-follower output stage. All the
tubes in this section of the receiver are 6AK5 miniature pentodes, except
the one used in the dk.criminator which is a 6AL5 miniature double diode.
The r-f, i-f, and video sections, as well as the discriminator, are standard
in design, an 8-Me/see bandwidth in the i-f amplifier being obtained
by double staggering of alternate stages. A two-stage limiter is used to
insure constant input-signal voltage at the discriminator. Both limiter
tubes operate at reduced plate and screen voltages to reduce the grid
swing necessary to cause plate limiting. A fast time constant in the grid
circuit of the first limiter was chosen to discriminate against impulse
noise by producing the Ylmiting bias quickly. Longer time constants are
possible in the second limiter since the signal variations at thk tube are
neither large nor of short duration.
This equipment has been given extensive airborne tests in conjunction
with two different types of synchronization equipment, and has also been
operated between fixed ground stations.
17.14. Microwave System for Point -to-point Service.-The two
equipments described in Sees. 17.12 and 17”13 were developed primarily
for air-to-surface work involving the use of omnidirectional antennas; an
upper limit was therefore set to the possible radio frequency. The present
section will describe equipment designed for use between fixed ground
stations which permit the use of directional antennas.
Safe margins of power are easily attainable in such applications since
the maximum range is usually sharply limited by the horizon or by roughness
of the terrain. The received signal was specified to be 40 db above
thermal noise at maximum range in order to provide a safe operating
margin. In the application of this criterion to the selection of frequency
and antenna sizes, the following table is illuminating. Paraboloid
antennas are assumed at both stations.