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. Included in Part II of "Microwave - Generation of Microwaves"
is a good, brief explanation of the operation of both B-K and magnetron
circuits. Part I appeared
earlier in the July issue of Radio-Craft magazine.
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 work.
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.
Fig. 4 - Magnetron oscillator, basic circuit.
Fig. 5 - Electron paths in a magnetron tube.
Fig. 6 - A magnetron of the split-anode type.
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.
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
Posted September 24, 2014