(4) RF system.
The RF system consists of transmission lines that
carry the RF energy to the antenna with a minimum loss of power.
The
transmission lines also carry the echo from the antenna to the receiver.
Waveguides are usually used in preference to coaxial cables because of their
greater efficiency and lower losses.
(5) Duplexer. The duplexer, or TR switch, makes it possible for the
radar to use the same antenna for both transmitting and receiving. When the
modulator pulses the magnetron, the TR switch connects the antenna to the
transmitter and disconnects the receiver.
This protects the receiver
because it prevents the full power of the magnetron from reaching the
receiver. Then, as soon as the transmitter goes off, the TR switch connects
the antenna to the receiver and disconnects the transmitter.
This allows
the echoes reflected from targets to go only to the receiver. The TR switch
is a high speed switch that works automatically.
(6) Antenna. The antenna takes the RF energy from the transmission
line and sends it into space. The antenna also picks up the return echoes.
The radar antenna is made very directional by using reflectors to
concentrate the RF energy into the desired beam pattern. Various types of
antennas are used in radar and the type used depends upon the function of
the radar system.
Learning Event 2:
HIGH-LEVEL MODULATION
1.
General.
a. Most radars today use a pulse modulation system in which a
magnetron, under the control of the modulator, generates short, regularly-
spaced bursts (pulses) of RF energy. The modulator controls the magnetron
with high-voltage, rectangular DC pulses. The accuracy of the radar depends
on the shape of the pulse developed by the modulator. You learned that the
modulating pulse must have a constant amplitude to keep the magnetron
oscillating at its correct frequency, and it must also have steep sides to
ensure accurate range determination.
b. You also learned that there are two ways to form the modulating
pulse: either by low-level modulation or high-level modulation.
Although
some older radar sets still use low-level modulation, modern radars use the
more efficient
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