Radar System Engineering

410 RF COMPONENTS [SEC.11.5
that would be coupled from loop to loop with a solid metal post in the
center of the cavity, the annular space around it being a waveguide
beyond cutofi.
To insure rapid breakdown at
the beginning of each pulse, a SUP
ply of ions in the gap is maintained
by a continuous auxiliary discharge
inside one of the cones.
This requires an extra electrode,
known as the ‘‘ keepalive” electrode,
which draws about 150 pa
from an 800-volt supply. A ballast
resistor drops the voltage to
about 400 volts at the electrode
itself. Despite this precaution,
the leakage power through a TR
tube shows an initial spike (Fig.
11. 16) which precedes the “flat”
region of constant leakage power.
For rapid deionization of the
gap one constituent of the gas must
have an electron affinity. After
the discharge is over, in the absence
!
!%
~
J
Less than
0.01 p sec
“Flat”
t-+
FIG. 11.16.—TR leakage power during
a 1-P59cpulse.
FIG. 11.17 .—1B24 TR tube: 3-cm band.
of a strong field, electrons are quickly removed by attachment to molecules.
Molecular ions have so much inertia that at the frequencies involved
they cause negligible attenuation. Water vapor is the constituent
usually introduced to hurry the electron cleanup. A typical filling would
be hydrogen at a pressure of 10 mm of mercury, and water vapor likewise
at lo-mm pressure. The hydrogen gives protection if the water vapor is
frozen out, but deionization will then be slow. Argon is used in some
tubes instead of hydrogen.
The loss to the received signal in passing through an unfired TR tube
is from 1.0 to 1.5 db, occurring mainly in the walls of the cavity.