October 1944 Radio-Craft
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles
So much time has passed since an average home garage mechanic could
service his car or truck with standard tools - combination wrenches,
screwdrivers, socket sets, timing light, and a multimeter - that
asking "remember when?" is passé. That era pretty much ended in
the late 1980s as computerized cars were becoming the industry norm.
A good percentage of people nowadays have never and will never service
their own vehicles. In the mid 1940s, the electronics world was
lamenting a similar situation with diminishing ability to build
and modify electronic components like coils and stacked plate capacitors
because of the increasingly higher frequencies being used in communications
(way up into the UHF band!). This
article introduces the klystron tube, having been around for less
than a decade at the time, as being one of the culprits that was
enabling the disturbing trend ;-) . A layman's introduction to the
physics behind its operation is provided. Those guys thought they
had it bad, but knew nothing of the reduction of serviceability
of electronics circuits and components compared with our present
day multi-layered printed circuit boards stuffed to the gills with
condiment-size surface mount components.
Klystron: Tube of the Future
The Klystron Will Fill Many Important Post-War U.H.F. Jobs
By Capt. Eugene E. Skinner Hq. A.A.F. Training Aids Division
Fig. 1 - Evolution of a resonant-cavity circuit from
the coil-condenser combination may be traced with the help
of this diagram.
Cut-away photo of a typical Klystron.
All Illustrations Courtesy Sperry
The new fields of radio that have been, developed in recent years
have so changed some radio components that it is now impossible
for the amateur to wind a coil, change the number and size of the
plates on a condenser, and follow the latest developments. Instead,
as frequencies become higher, special, critical components are required.
Examples of these are found in the resonant circuits and tubes.
For the ordinary range of frequencies with which radio technicians
and amateurs were concerned a few years ago, a resonant circuit
could be designed to fit any frequency or band of frequencies by
changing the number of turns on a coil and the size of the condensers,
or by changing crystals. With an increase in frequency, fewer turns
in the coils and fewer condenser plates are used. As a very high
range of frequencies is reached, a single small turn of wire connecting
two condenser plates must be used and crystals become so thin as
to be useless. Any further variation along conventional lines is
very difficult. New methods must be adopted.
In addition to the problems offered by these lumped circuit constants,
as inductance of turns of coils and capacitance of plates or condensers
are called, the problems of the vacuum tubes themselves become evident.
These problems, caused by characteristics which were not bothersome
in ordinary tubes at lower frequencies, become of major importance
in the ultra-high-frequencies. In these ranges, such small quantities
of capacitance and inductance are needed that the capacitance between
the electrodes of the tubes and the inductance of the loops formed
by the electrical circuits through which the electrons flow in the
tubes are of relatively large value. In addition to this, the electrodes
are of such size distance apart, and relative position, that the
time required for the electrons to travel through the tube becomes
an appreciable part of a cycle. Finally a frequency is reached for
each type of tube at which the starting and maintenance of steady
oscillations becomes impossible. This feature is compensated for
by the construction of smaller tubes of the "acorn" and other types,
but the difficulties still exist above the ultra-high-frequencies,
for eventually the point is reached at which the tubes are necessarily
so small and possess such little power capacity as to be impractical.
One method of solving both these problems at the same time was
devised by Russell and Sigurd Varian and W. W. Hansen at Stanford
University in 1937, by the invention of "Klystron"* tubes. This
type of tube embodies the principle of modulating the velocity of
the electrons as they flow through it and which has at least a portion
and possibly all of the oscillating circuit components included
as integral parts of the tube.
The most important component of the Klystron is the resonant
cavity. It is an outgrowth of the application of the lumped circuit
constants in resonant circuits. When the point is reached at which
a single small loop and a pair of condenser plates will no longer
serve for the extremely high frequency desired, a method must be
devised to apply the same principles in a different form. To decrease
the inductance beyond that of a single loop, the principle that
loops in parallel have less inductance than one loop is applied.
Therefore, coil loops are placed between the same pair of condenser
plates, until an infinite number of them have been added. See Fig.
1. The result is a doughnut-shaped ring slotted within the hole
at the shortest diameter, with two plates placed parallel to the
axis of the doughnut, one at each edge of the slot, so that it appears
to a casual observer to be only a solid doughnut shaped object with
a short plug in the center. If laid flat and cut a one would cut
a pie, the cut edge would be like the outline of a dumbbell.
As the frequency is increased, this doughnut becomes smaller.
In most Klystron tubes two of these resonant cavities are used,
one as the "buncher" and one as the "catcher." It is obvious that
an electron stream cannot pass through the solid plates 'in-the
centers of these cavities. These plates are therefore replaced by
meshes or grids. In actual applications, the cavities are not always
tubular doughnut in shape but more often have distorted variations.
The action of the tube may be understood by referring to Fig. 2.
A stream of electrons is beamed by a cathode through focusing electrodes
and an accelerator grid toward the pair of bundler grids through
which the stream must pass. The cathode and focusing electrodes
have applied to them a high negative voltage with respect to the
rest of the tube, which is grounded on the positive side. Voltage
must necessarily be well regulated as fluctuations will affect the
frequency of the tube. The buncher grids and their associated resonant
cavity are excited by a radio frequency source in such a manner
that as one is positively charged, the other is negatively charged
as in coil-and-condenser action. These grids have between them,
therefore, an electrostatic field which is parallel to the flow
of electrons. The strength of this field is such that it appreciably
changes the velocity of the electrons, but does not do so to the
point that it will stop the flow completely at any time. As an electron
comes into the field, assume that it enters such a portion of the
cycle as to cause it to speed up. As the radio frequency excitation
passes through the zero point of the cycle, the speed of the electron
is not affected. Then, as the second half of the cycle is applied,
or the charges are reversed, the electrostatic field opposes the
flow of the electron, slowing it down. Now it can be seen that as
the excitation passes through several cycles, those electrons which
have been slowed down during a cycle are overtaken by those whose
speeds were not affected, and both groups are overtaken by those
electrons which were sped up, causing a bunching of the electrons,
at a point past the "buncher" grids.
Fig. 2 - Schematic view of the Klystron.
Fig. 3 - How the tube is tuned. Coarse adjustments are
made with the three large struts, fine tuning by varying
one against the other two.
A second pair of grids, known as the "catcher" grids, is placed
in such a position that the bunched electrons pass through them
at the rate of one bunch per cycle. It is assumed that the first
of the grids is in the negative half cycle of an oscillation. As
the electrons approach the first of these two grids, which are part
of a tuned circuit, the negative charges in the electrons induce
a positive charge on the first grid in a condenser-like action,
then pass on toward the second grid. The original negative charge
on the first grid has a slowing-down effect on the electrons, and
as the energy of motion cannot be lost by the electrons without
being gained elsewhere, this loss of electrons induces energy in
the tuned circuit.
The change of charge supports the oscillation in the tuned circuit,
and by the time the bunch of electrons has reached the second catcher
grid, the oscillations have changed the charge on it so that it
is then negative. This negative charge, again in opposition to the
charge of the electrons still further decreases the velocity of
the electrons, and, as before, this loss of energy by the electrons
induces energy in this tuned circuit. After the electrons have passed
the catcher grids, they are picked up on a collector plate.
It can readily be seen that the energy considerations are very
important. It has been shown how energy is taken from the catcher
grids while none is supplied from sources other than the electron
stream. In , the buncher, it is necessary to supply a source of
energy to provide for the bunching, but as some of the electrons
are speeded up, taking up energy from the buncher, so are some of
them slowed down, giving up an almost equal amount of energy to
the buncher. The overall result is that there is a very little additional
energy needed. A short coaxial cable feedback from the catcher to
the buncher provides the necessary source of radio frequency oscillations
and energy. The excess energy from the catcher is the output of
the tube and is also taken from the resonant cavity by means of
a coaxial cable. Output and the feedback is accomplished by means
of one-turn loops which are formed by the central conductor of the
coaxial cables, which enter the hollow "doughnut," form a loop,
and are grounded at the end. A part of the magnetic flux of the
resonant cavities which exists within the hollow doughnut cavities,
also passes through this loop, giving an inductive coupling which
is the equivalent of an air-cored transformer.
The Klystron tubes are very versatile and have many applications.
In addition to their electrical capabilities, they are reasonably
rugged, lending great value under far-from-ideal combat conditions.
As the frequencies increase, the tubes become smaller, the wave
lengths being the controlling factor of practically all the physical
dimensions. The general design is such as to provide satisfactorily
for the necessary dissipation of heat.
Tuning the Klystrons may be accomplished by several different
methods, depending upon their physical construction. If a tube is
so constructed that the cavity is sealed within it, provision may
be made whereby tuning can be accomplished by changing the spacing
between the grids. Most Sperry Klystrons utilize a screw-type arrangement
by which the spacing between the grids can be varied, made possible
by having the grids mounted on a diaphragm which can be flexed by
the screw control. See Fig. 3. This spacing is the most critical
dimension as far as frequency is concerned.
If the cavity is outside the tube, tuning may be accomplished
by screwing plugs into the cavity, effectively changing its volume.
Several other methods of varying the dimensions of the resonant
cavity or the flux inside it may be used, as well as variation of
the applied voltage.
When no method of feedback is used with the Klystron tube, but
a separate oscillation is used in the buncher, the tube is being
applied in its simplest form as an amplifier. Since the beam of
electrons gives off a great deal more energy than it absorbs, it
is therefore a power amplifier (equivalent of a Class C type) and
may also be used as a voltage amplifier for small radio-frequency
All that is necessary to convert such an amplifier into an oscillator
is a method of feedback so as to sustain oscillations, and this
is accomplished by means of the coaxial feedback coupling previously
described. The Klystron is generally as efficient as any other type
of ultra-high-frequency oscillator. While theory indicated that
efficiencies of 58% are possible, in actual practice they are considerably
It is very improbable that electrons would be bunched in such
a manner as to produce a pure sine wave. The wave produced is actually
composed of a large number of harmonics. This being the case, it
is only natural that the Klystron tube should find application as
a frequency multiplier. In order to accomplish this, it is necessary
that the relative phase of the two resonant cavities be such that
bunches of electrons hit the catcher at a time which will cause
the oscillations to build up, and the catcher cavity must be designed
for the desired harmonic frequency.
Unfortunately a great many of the most important applications
of the Klystron tubes cannot be discussed. In the meantime, more
and more applications are becoming apparent daily in the fields
opened by the discovery of this important electronic device.
One thing that the war, or rather, the peace which follows it,
should be able to do for radio is to clean up and set in order the
wavebands assigned to broadcasting. You remember the pre-war semi-chaos
which conferences, plans and much hard work on the part of the U.I.R.
(Radio International Union) had failed to straighten out? The end
of the war will furnish a glorious opportunity of setting all this
to rights and of forming a governing body for broadcasting in Europe
as enlightened and as powerful as the Federal Communications Commission
in the United States. One thing that most of us would like to see
is a return to the original 10-kilocycle separation between stations
- the 9-Kc. separation in operation when the war broke out was not
sufficient to ensure decent quality or to prevent heterodynes.-(Wireless
World, July 1944)
*Trade Mark Registered by Sperry Gyroscope
Posted November 12, 2014