August 1945 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
The Barkhausen-Kurz (B-K) oscillator is credited as being the first
high power microwave generator that exploited the electron transit
time effect. It was developed in 1920 by German physicists
Heinrich Georg Barkhausen and Karl Kurz. As this article's author
points out, the vacuum tube and supporting circuits were difficult
to produce and were not very well understood theoretically. Shortly
thereafter, the magnetron and klystron tubes came along and dominated
the high power microwave generation realm. This is a good, brief
explanation of the operation of both B-K and magnetron circuits.
Microwaves - Part II
Part II - Generation of Microwaves
By Captain Eugene Skinner*
Fig. 1 - Electron paths in a B-K oscillator.
Fig. 2-a, left - Electron path, grid going positive.
2-b - Same, grid going negative.
Fig. 3 - Complete Barkhausen-Kurz circuit.
Fundamentals governing the use of microwaves, and operation and
applications of the Klystron have been presented in previous articles.
In addition to Klystron tubes, there are other types of tubes and
circuits which are used at microwave frequencies. Two of the most
important are the Barkhausen-Kurz circuits and the magnetron circuits.
While the Barkhausen-Kurz oscillator is largely an experimental
one and is not widely used in actual microwave applications, it
is as basic a circuit for microwaves as the Hartley oscillator is
for ordinary frequencies, and an understanding of how it works will
give the amateur and the experimenter a better background for their
Barkhausen and Kurz discovered a new type of oscillator in 1920.
It is also known as the B-K, retarding field, or positive-grid oscillator.
This type of oscillator has been used to generate ultra-high frequencies
and microwaves up to a few centimeters in length, and works on principles
which are relatively simple when considered qualitatively. Exact
mathematical treatment is very difficult and does not lend itself
to a better understanding of the operation of the tube, so will
not be touched on here.
Basically the tube itself consists of a single straight wire
filament surrounded by a cylindrical grill and plate. The grid may
be of parallel wires, or it may be of a number of wires twisted
into a helical shape. The plate itself is merely a cylindrical tube.
This tube is a triode with the elements specially arranged.
For producing oscillations in this type of tube, the grid is
positive instead of negative. The plate, instead of being positive,
is usually slightly more negative than the filament, but may be
at the same potential. Electrons from the filament are accelerated
toward the grid by its positive potential, most of them passing
through the meshes, and entering the field between the grid and
the plate, where they, being negative, are repelled by the negative
or relatively negative plate. They stop, reverse direction, and
then accelerate back toward the positive grid, which attracts them.
Again, most of them pass through the grid, enter the field between
the filament and grid, where they are again repelled, this time
by the filament itself. They stop, and together with the new electrons
which are leaving the filament at that instant, start toward, then
through the grid again. Each time that the electrons pass through
the grid, some of them are lost to it. Those which continue to oscillate
back and forth return to the grid each successive time with lower
energy, and move a shorter distance away from it. Eventually the
electron strikes the grid and is lost. This grid operates at a high
temperature, and necessarily has to withstand high power dissipation.
Grid failure is the most common cause of failure of this type of
tube. The path of the average electron is shown in Fig. 1.
If an A.C. voltage that has a period approximately equal to the
electron transit time from the cathode to the plate is superimposed
on the positive grid source, it is possible to either extract energy
from the D.C. source, or give energy to it. Figs. 2-a and 2-b show
typical paths of electrons for two conditions: Fig. 2-a shows an
average path when the electrons start from the filament at an instant
that the applied A.C. on the grid is going positive, therefore making
the grid more positive than it normally would be, and Fig. 2-b shows
an average path when the electrons start from the filament at which
this applied voltage is going negative.
If the grid is more positive than normal, the electron is sped
up. As the electron approaches the plate, the A.C. voltage on the
grid reverses, and the grid is less negative, causing the electron
to slow down less on its return trip. In a trip like this, it is
possible that the electron will strike the plate, but if it does
not, it returns to the grid or cathode. As the electron has been
sped up during its entire trip, it returns to the cathode with an
appreciably increased velocity, and the energy with which it strikes
the cathode must have been obtained from the A.C. source applied
to the grid. If the electron starts a trip when the A.C. voltage
is decreasing, the electron is constantly slowed down rather than
sped up, and after making several decreasing oscillations, it comes
to rest on the grid. In this case, energy is given up to the A.C.
source rather than taken from it. Electrons will be leaving the
filament during every instant of the cycle of the A.C. voltage,
but as the energy taken from the A.C. source during one-half cycle
is approximately equal to the energy given up to the A.C. source
during the first "trip" of the electrons during the second-half
of the cycle, and these latter electrons make several trips, giving
up energy during each, there is a net gain of energy by the A.C.
source on the grid.
It has been shown theoretically how D.C. energy can be converted
into A.C. energy. Since the requirement for sustaining oscillations
is that more energy be given to the tuned circuit than is taken
from it, a tuned circuit may be connected to this triode between
the grid and plate. Oscillations down to about ten centimeters wave
length may be obtained, but generally the efficiency is very low,
and the maximum power output is about 10 watts. Figure 3 shows a
circuit of the type described. Similar oscillator circuits may be
obtained by connecting the tuned circuit between the grid and cathode
or the plate and cathode. In constructing this circuit the external
circuit should be a Lecher-wire system plus the other components
shown in the circuit diagram. This makes it very simple for the
experimenter, as the only component that he needs that he cannot
easily construct is the tube. In fact, the B-K circuits are the
only ones he can work with at present. Fairly high frequencies can
be obtained with certain types of standard triodes having cylindrical
grids and plates, in purely experimental circuits where power output
is not a consideration. Other microwave circuit depend on special
tubes which will not be obtainable by the civilian experimenter
for some time.
Fig. 4 - Magnetron oscillator, basic circuit.
Fig. 5 - Electron paths in a magnetron tube.
Fig. 6 - A magnetron of the split-anode type.
As the condenser shown in Figure 3 is moved along the two parallel
wires from the tube, the wave length of the oscillations will slowly
increase, suddenly drop, then increase again. This is known as the
Gill-Morell effect, and shows that the external circuit obviously
influences the oscillations inside the tube.
Probably the most important type of tube in present-day microwave
applications is the magnetron. Basically, the magnetron consists
of a plate in the form or a cylinder, a filament that runs axially
through the cylindrical plate, and the poles or a strong magnet
so placed that the lines of magnetic force also run axially through
the cylinder, as shown in Figure 4. With no magnetic field, the
electrons travel from the filament to the plate without interference,
but when a magnetic field is applied, these paths become curved,
increasing in curvature with the increasing magnetic strength, until
a cutoff point is reached. At this point, the electrons just graze
the cylinder, and return to the cathode. With a still greater increase
in magnetic field, the electrons travel a much shorter path, and
miss the cathode completely.
At the cutoff point, the plate current drops to practically zero,
and past cutoff point, it does become zero. Typical electron paths
are shown in Fig. 5. Most often in practical applications the plate
is split into two or more segments as shown in Fig. 6. In this type
of circuit the tuned circuit between the two magnetron sections
interacts on the electrons in such a manner that they move spirally,
as shown in Fig. 5-e. There are several methods of producing oscillations
with the magnetrons, but since the field is so large, only the transit-time
method will be considered here. It is the method most nearly like
that previously described for the Barkhausen-Kurz oscillator. Assume
that we have a split-anode magnetron, as in Fig. 6, with an A.C.
voltage applied between the segments.
Those electrons which leave the cathode and strike the plate
give up energy to the A.C. source applied to it. Those which return
to the cathode give up energy to it which was extracted from the
A.C. plate source. In order that the oscillations be sustained,
it is necessary that more energy be given up to the A.C. source
than taken from it, as this A.C. source is actually the tuned circuit.
Electron velocities are increased and decreased by the A.C. source
in the magnetron in the same manner as in the B-K circuit, and the
energy extractions and deliveries are the same.
Therefore, it may be seen that the ideal situation is for the
electron to make several oscillations and eventually land on the
plate. This is accomplished in two manners. The first is to tilt
the magnetic field at an angle not exceeding 10 degrees. This gives
the electron a helical path, ending on the plate. The other method
is to put end plates on the cylinder, so that the effect is the
Magnetrons have many applications in the microwave field, and
the developments have gone much further than security regulations
will permit discussion of. They are used to produce the shortest
sustained oscillations yet attained, with wave lengths down to less
than 1 centimeter in length.
End of Part II
*Hq. AAF. Office Asst. Chief of Air Staff,
Training Aids Division.
Posted September 24, 2014