Radar System Engineering

SEC. 11.5] D,C”PLEXING AND TR SWITCHES 407
tions at low frequencies.
The essential reason for this is that such low--
frequency circuits do not provide a correspondingly large volume for the
storage of energy.
To make a cavity useful it is necessary to provide some means of
introducing and removing energy, or in other words to couple it to the
external circuit. This may be done by an electron stream, by a coupling
loop to a coaxial line, or by an iris (hole) leading into a waveguide.
Examples of these are cited in the discussion of klystrons (Sec. 11.7) and
of TR switches (Sec. 11.5).
11.6. Duplexing and TR Switches.-As was explained in Sec. 1.3,
the use of a common antenna for
transmitting and receiving requires
fast-acting switchesl to disconnect
the receiving apparatus from the
antenna during the transmitted tron
pulse, and to disconnect the magnetron
during the period when
echoes are being received. These
two switches are called the TR
(transmit-receive) switch and the
W
anti-TR or ATR switch, respeclhQ.
11.14 .—Duplexing system on tw~wire
tively. The duplexer is that por- transmission line.
tion of the microwave circuit, near
the T-junction of the receiving branch and the magnetron-antenna line,
where the TR and ATR switches are located.
The great disparity in transmitted and received powers immediately
suggests that a spark gap or gas-discharge tube can be connected in th-e
circuit in such a way as to perform the necessary switching operations.
These gas-discharge tubes are referred to as TR or ATR tubes. A rudimentary
system using a two-wire transmission line is shown in Fig. 11.14.
The high-power pulse from the magnetron breaks down the gap in the
ATR tube and the power flows out toward the antenna. The gap in the
TR tube in the receiving branch likewise breaks down, and if it is designed
so that the discharge takes negligible power to maintain, puts a short
circuit across the line to the receiver. The delicate input circuits of the
receiver are thereby protected. Since the short circuit is a quarter wavelength
from the T-junction, the impedance put in parallel with the
antenna line at the junction is very high and does not affect the wave
traveling toward the antenna. At the end of the transmitted pulse, the
discharge across the gaps goes out and the system is ready to receive echo
signals. The impedance at the T-junction looking toward the magnetron
is infinite because there is an open circuit half a wavelength away.
1 Microwave Dupb%rs, Vol. 14.