Radar System Engineering

SEC. 11.7] THE LOCAL OSCILLATOR 415
just beyond the grids there is a strong retarding field produced by a
reflector electrode maidained about 100 volts negative with respect to
the cathode. The trajectory of the reference electron in this space is
similar to that of a ball thrown into the air. It will return to the grids
after a time proportional to its initial velocity and inversely proportional
tothe retarding field. Anelectron that leaves thegrids earlier than the
reference electron will have been accelerated by the r-f voltage across
the cavity and, because of its higher velocity, will spend a greater time in
the reflection space. Byproperadjustment of the retarding field, the delay
may be made to compensate for its +40
earlier departure, and it may be
made to arrive back at the grids at
the same time as the reference
electron. Similarly, an electron
leaving later than the reference
electron catches up by spending less
g
~ +20 wavelength=10.7cm
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S’ -20
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Cathdde-anode voltage=290 v
.-40 Ei=Ea
Ii;=
-30 -40 -50 -60
Reflector voltage
time in the retarding field as a result
of its lowered velocity. The
net effect is that the electrons gather
in a bunch. I At certain reflector
voltages the bunch will pass through
the cavity grids in such a phase that
F1~.11.23.—Reflectorcharacteristics
707A.
of the
the r-f field retards the electrons. The electrons then give energy to the
cavity and thereby sustain the oscillations. Oscillation is observed for
more than one reflector voltage because drift times differing by a whole
r-f cycle still produce satisfactory bunching.
The net energy given to the electrons during their first passage through
the cavity is negligible when averaged over a whole cycl~, being balanced
between acceleration and deceleration. On the return passage, however,
most of the electrons go through in a bunch at the most favorable phase
to aid the oscillation. Half a cycle later, when returning electrons
would absorb energy in being accelerated, very few electrons are passing
through. Useful power is delivered to an external load through a coaxial
line, loop-coupled to the cavity. The efficiency is rather low, in the
neighborhood of 1 per cent. Power outputs of 20 to 50 mw are typical.
Local oscillators are tuned by mechanically changing the size of the
cavity. A limited amount of electrical tuning is possible through variations
of the reflector voltage. Figure 11.23 shows the frequency and
power output of a 10-cm reflex klystron as a function of reflector voltage.
When the reflector is made more negative, the bunch arrives at the cavity
I In the older two-cavity klystrons, where the bunching takes place in a field-free
space, the bunch forms about the electron that passesthrough the first cavity as the
field is changing from deceleration to acceleration.