September 1932 Radio News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Magnetrons are fairly ubiquitous in society these days for use in
heating, radar, and even lighting. They were probably the first
useful means of producing high power microwave signals. The concept
was first brought to fruition in the early 1920s as a laboratory
curiosity and rapidly developed into a practical type of device
with many applications and spin-off products like the klystron,
the traveling wave tube, and the cross-field amplifier. This article
from a 1932 edition of Radio News reports on the state
of the art a decade after the magnetron's inception.
How to Construct a 56 Megacycle Magnetron Transmitter
Here it is, something new in radio! A new oscillator principle
for the ultra-short waves employing the gridless double plate tube
in a manner comparable in many respects to the Gill-Morill method.
This article, a feature of the Radio News series on opening up the
ultra-short wave field, will be invaluable to the technician and
earnest experimental amateur
By James Millen
The High C Tuner
Figure 10. This is the tuned circuit for resonating the
new magnetron oscillator tube as explained in the text.
The American amateur was the first to make practical use of the
200-meter band and the first to develop the utility of still shorter
and vastly more important communication channels. A new field of
research, the ultra-short waves, now challenges his ingenuity, and
we are sincere in our belief that here again the "ham" will make
a genuine contribution to ultra-high-frequency technique. The magnetron,
in particular, offers fertile possibilities, and its application
to commercial enterprise may be, to our way of thinking, materially
accelerated by its exploitation and development in the amateur ranks.
This article finds dual justification in the effort to stimulate
such experimentation, and in presenting a practical 56-megacycle
Figure 1. The tube which with its field coil offers new
fields of experimentation on the ultra-short waves.
As we pointed out in the original article of this series, there
exist a variety of methods whereby ultra-short-wave energy can be
set in motion. However, the necessity for efficiency (reasonably
high-power output for practical input powers) and stability places
a definite limitation on the systems serviceable for useful communication.
The magnetron, today, offers the most economical method for generating
quasi-optical power. As it is an electronic device, its functioning
is perhaps best understood by indicating its similarity to more
conventional systems. It is not particularly difficult to design
the usual sort of tube oscillators for wavelengths between 5 and
10 meters, and by the utilization of their harmonics to extend this
range to a still lower minimum. However, as might be imagined, the
stability of such systems leaves much to be desired, and the power
output is generally inadequate. Also, as may be readily understood,
maximum frequency limitations are necessarily imposed by considerations
of the capacity and inductance by which resonance is determined.
An additional complication, the fact that as the frequency is still
further raised the period approaches the times required for the
electrons to complete their inter-electrode cycle, imposes further
limitations - at the same time offering a solution to the problem.
It was found that, under proper conditions, oscillations could be
sustained the frequency of which was dependent on the geometry of
the tube or on the potentials applied to the elements rather than
upon the LC characteristics of the circuit. Such systems have been
described categorically as Barkhausen-Kurz circuits in deference
to their two most-prominent investigators. It is logical and true
that such arrangements are capable of delivering higher powers,
at very short wavelengths, than those with which we have become
familiar on the conventional short waves. It was also discovered
that the power output could be increased by resonating the circuit
to the natural electronic frequency, and such transmitters have
come to be known as Gill-Morill circuits and are comparable in many
respects with the magnetron system.
the New Transmitter for 1 1/2 Meters
Figure 2. This illustration shows the essentials
of the magnetron oscillator system. The field coil, rear center,
has been removed from around the tube for clarity. The particular
arrangement shown is for a frequency range corresponding to approximately
1 1/2 meters. The plate supply circuit is not shown.
The magnetron is not a new tube. It has been with us for well
over a decade, and was originally designed as a tube in which electron
control was effected by magnetic influence rather than electrostatically.
In other words, the grid of the tube was a magnetic coil, and, peculiarly
enough, the magnetron was first used as a radio-frequency amplifier
and oscillator in the neighborhood of 8000 meters!
The magnetron used in ultra-short-wave work varies from its prototype
in several essentials. The magnetic coil no longer functions as
a grid (in the control sense), but its effect is similar to that
of a "bias." By increasing the magnetization current, the space
current can be cut off - corresponding to an increase in negative
bias in a conventional triode circuit. At optimum magnetic bias,
the space current is reduced while space charges build up within
the tube. Connected with a suitable circuit, a negative-resistance
characteristic permits these charges to dissipate and re-accumulate,
producing a cycle the time constant of which is partly determined
by the spacing of the elements, the intensity of the magnetic field
and the potentials applied. Subsequent experiments have shown that,
as in the Gill-Morill circuits, the efficiency of the magnetron
is greatly increased when the circuit resonance approaches the natural
The Magnetron Circuit
Figure 3. The data supplied in this article was obtained
from a transmitter using this simple circuit. The top diagram
shows field coil hook-up
Figure 4. Curves showing increased stability by using
high C circuit
Choosing Proper Current
Figure 5. Showing relation of field-current ripple and
frequency variation with the high C circuit
Obtaining Minimum Variation
Figure 6. This is a field current, plate-voltage curve
for getting the least frequency variation
Figure 7. Linear relation of plate voltage-plate current
adapts the magnetron readily to modulation.
Low C Coil
Figure 9. This is the tapped four turn low C coil used
in the original experiments.
The magnetron used in the experiments and transmitter to be described
is the GE-239, and is shown in Figure 1. The anode is split into
two semi-cylindrical sections mounted coaxially with the heavy tungsten
filament. It is an air-cooled tube, used principally as an oscillator.
The lowest operable wavelength is 0.75 meters, and at 1 meter, with
a plate potential of 1500 volts, has a plate impedance of 5000 ohms.
The maximum operating anode potentials are 1500 volts d.c. and 2000
volts r.m.s. a.c. The maximum d.c. anode current is .075 ampere,
and the maximum plate dissipation 60 watts. The inter-electrode
capacities are anode-to-anode (filament grounded) .5 mmfd.; anode-to-filament
(other anode grounded) .7 mmfd. The tungsten filament draws 5 amperes
at 5 volts.
The mechanical characteristics of the tube are indicated in the
photographs. The anodes are connected from the top, and the base
plugs into a standard 50-watt socket. The overall length is 10 inches;
diameter 2 1/4 inches.
The output of this tube, in both power and stability (when properly
operated) is definitely superior to other arrangements, its output
at .75 meter being vastly better than that available with the B-K
Figure 2 shows the experimental layout from which the various
data accompanying this article were secured. The circuit, as may
be observed from Figure 3, is simplicity itself.
It consists of the tube between the anodes of which is connected
the center-tapped coil. (As will be described, this coil was changed
in the course of the experiments, and an exterior capacity shunted
across it. The simple inductor is shown in Figure 9 and the coil
plus the tuning condenser, are shown in Figure 10.) The field coil
has been removed from its place around the magnetron for the sake
The oscillating frequency is largely determined by the inductance
and the capacity of the circuit. For the highest frequency (400
megacycles), the inductance of the anode leads and the capacity
between them is sufficient. In this case the external anode leads
are short-circuited with a copper bar or ribbon) about one inch
from the glass, and the high voltage is connected to a tap at the
center of the bar.
The Field Coil
The field coil is wound with 70 pounds of number 14 enameled
wire, on a form having an inside diameter of 5 inches. The length
of winding is 5 inches, 74 turns to the layer, with a total number
of 2665 turns. Each layer is separated with paper insulation .015-inch
thick. The coil is excited with a potential of 100 volts through
a suitable resistor, providing a maximum current of 5.8 amperes
and a maximum continuous field strength of 830 gauss with a dissipation
of 580 watts. The outside diameter of the coil is 11 inches. The
variable series resistor should be capable of carrying the maximum
excitation current and should have a resistance of at least 17 ohms.
The effect of the magnetic field on the operating characteristics
of the magnetron is, as we have already suggested, somewhat similar
to that of a bias of the grid of a triode oscillator. At low values
of field, below 20 amperes, corresponding to a small bias, the plates
heat excessively; the plate current is high and the efficiency is
low. As the field is strengthened, the circuit becomes more stable,
operating with greatly improved efficiency and output. The current
in this region varies from 3 to 5 amperes. Plate-current "cut-off"
can be obtained with an excitation current of about 7 amperes.
In general, the requirements of the oscillatory circuit, in respect
to efficiency, stability, etc., are similar to those of the dynatron
oscillator, although, of course, the frequency range is much greater.
As the filament is tungsten, it is possible to operate it over
a wide range of voltages without encountering the trouble experienced
with thoriated filaments. It was found, however, that for best stability
the filament should be operated near its rated voltage - though
a 10 or 15 percent. variation has a relatively small effect.
The first tests, while altogether satisfactory in respect to
output, left a great deal to be desired from the standpoint of stability.
Curve A, of Figure 4, illustrates the manner in which frequency
varied as the plate voltage was changed. In this original experiment
the tuned circuit consisted of 4 turns of 1/4-inch copper tubing,
2 inches in diameter, mounted directly on the anode leads, the tuning
capacity being the inter-electrode capacity of the tube (about .5
mmfd.) and the distributed capacity of the coil and leads.
The frequency was approximately 56 mc. The circuit and operating
conditions under which this test was made probably represented the
most unfavorable, with respect to frequency stability, that would
be encountered in practice.
The total change in frequency, i.e., about 200 kc., seems rather
appalling to one familiar with the operation of low-frequency apparatus.
As a matter of fact it is not much worse than that encountered in
a self-excited push-pull oscillator operating on the same ultra-high
frequency. Such as oscillator, however, cannot be considered satisfactory.
An investigation (with the idea of improving stability) was therefore
begun, and while it is not fully completed, it represents definite
steps in the right direction.
Stabilizing the Circuits
Each portion of the magnetron circuit has a relationship to frequency
stability. Variations in the magnetic field were very bothersome,
so it was deemed advisable first to investigate this. It was discovered
that when a certain magnetic field strength was employed, variations
in the field current had a minimum effect on frequency change. The
optimum field current, from this point of view, was about 4 amperes.
This is important since the field-supply filtering is something
of a problem, due to the high current required. In these tests a
motor-generator furnished the field power and when the correct adjustment
was attained, a monitoring test disclosed the fact that such disturbances
as commutator ripple and minor line-voltage variations were no longer
of major importance. The magnetron was not operating quite at its
maximum efficiency at this field setting, but the drop in efficiency
was small enough to be considered important in the light of the
improved stability obtained.
The frequency variation, with change in plate voltage, was still
the same, however, so the "low C" circuit was abandoned in the hope
that a certain amount of tuning capacity would be beneficial. The
magnetron operates most efficiently into an oscillatory circuit
of high impedance, making a "high C" circuit theoretically undesirable.
Nevertheless a tuned circuit was constructed employing a coil of
3 turns of 1/4-inch copper tubing 1 1/2 inches in diameter, tuned
with a condenser of about 20 mmfd. capacity. Since the inter-electrode
capacity of the tube is only about .5 mmfd., this additional capacity
effected a considerable improvement in stability, without serious
loss in efficiency.
The operation of the circuit, as a whole, was quite different
from the original tests. Changes in field current, plate and filament
voltages caused only a relatively small variation in frequency.
The curves in Figure 4 show the contrast between the "low" and "high
A definite value of field current which minimized the effects
of commutator ripple, etc., was found as before, but was considerably
lower in value, being approximately 3 amperes instead of 4 amperes
(Figure 5). It is interesting to note that the optimum operating
value of field current, with regard to frequency instability caused
by field variation, is approximately the same at all plate voltages,
as is shown in Figure 6. As before, better output was obtained with
the field current reduced slightly from that value giving most stable
operation. The choice of field current will therefore depend somewhat
on operating conditions. If a well-filtered supply is available,
efficiency and output are the deciding factors. Where the field
supply is subject to variation, stability is the predominant consideration
The value of R is not critical as far as efficiency or frequency
vs. plate-voltage variations are concerned. However, it was found
that by the correct adjustment of R, frequency variations due to
field strength changes could be still further reduced materially.
The actual value varies between 100 and 400 ohms and depends on
the plate voltage - higher resistance being required for higher
potentials. Some energy is necessarily dissipated in this resistor,
but this loss is relatively unimportant in view of the main goal
of a stable circuit. Also, some compensation of this loss is achieved
through an improved anode circuit efficiency.
A d.c. Filament Source Preferred
After a fair degree of stability had been attained, as indicated
in curve B, Figure 4, another source of trouble became evident.
This was vibration of the filament structure of the tube, due to
the filament current, which was alternating current, reacting in
a "motor" effect with the strong d.c. field. When the filament was
light by d.c., this trouble was eliminated.
Modulating the Circuit
Referring to the curves in Figure 7, it is immediately seen that
the relation between the plate current and plate voltage is essentially
linear, and the circuit, therefore, lends itself admirably to modulation.
From the slope of the curve, the effective plate impedance is found
to be about 9000 ohms, which, while somewhat higher than that commonly
encountered in the usual transmitting tubes of similar power, should
present no serious difficulty from the standpoint of obtaining a
satisfactory impedance relationship in the modulator output circuit.
Photograph, Figure 8, shows the complete set-up of the magnetron,
with its attendant apparatus, including modulation and speech equipment.
The modulator consists of two type -50 tubes in push-pull, connected
through a suitable transformer to the oscillator plate circuit.
Since the magnetron was operating with an input of about 35 watts
at 650 volts, a fair percentage of modulation was obtained with
the two -50's operating at the same plate potential. In any case
the modulation should not exceed 70% if reasonably good fidelity
is desired. The speech equipment and amplifier circuits are quite
The radiating system was a simple vertical, half-wave, copper-rod
antenna approximately 8 feet long, and fed by means of a balanced
5 Meter Amateur Phone Transmitter
Figure 8. Robert McCoy of the Jackson Research
Laboratories shown operating the station. The high C tuned circuit
may be seen just above the field coil which is in correct position
surrounding the coil. Speech amplifier equipment is at the right
energized by a storage battery and B batteries. The motor generator
supplies the field coil energy
Field tests were made up to distances of about 5 miles, the signal
being checked in comparison with that of a conventional modulated
oscillator. Two types of receivers were used, one being a super-regenerator,
and the other the latest type of ultra-high-frequency superheterodyne
described in Radio News for August. It was, of course, impossible
to check the effects of frequency modulation of the super-regenerator
since the receiver itself is subject to a degree of frequency modulation
from the suppressor frequency that is comparable with that of the
worst transmitter. Due to this fact, only slight differences between
the transmitters were apparent on the first receiver (the signal
from the magnetron was appreciably sharper, but the tone quality
was not noticeably better.)
On the other hand, when using the super-heterodyne, the magnetron
was found to give a clear-cut signal of good quality, in contrast
to a decidedly broad and wobbly signal of low intelligibility from
the modulated oscillator.
The tests definitely checked the various circuit requirements
that laboratory experiments had indicated to be desirable. Of the
two transmitters, each employing similar modulating equipment, the
magnetron gave a much stronger signal due, of course, to the higher
degree of efficiency. Increased power at these frequencies does
not result in a corresponding extension of range, since the behavior
of the signal tends to comply with optical laws. However, the stronger
signal was found to be much more effective in location subject to
The possibility of modulating the r.f. output of the circuit
by supplying the modulating power to the field coil will doubtless
occur to many experimenters. However, aside from the difficulty
of controlling the high excitation current with any depth of audio-frequency
variation, field circuit modulation might be undesirable due to
the excessive change in radio-frequency with field current change.
Also, the r.f. output does not vary uniformly with the field, except
over rather small limits and when using a somewhat restricted range
of plate voltages.
Higher Frequency Possibilities
The tests described above were confined to the neighborhood of
56 megacycles. A vast amount of experimentation still remains before
the possibilities of the magnetron are fully realized on frequencies
above 300 megacycles (below 1 meter). Work in this region, which
is not being used at present for any practical purpose, affords
a highly interesting and unusually fertile field for the experimenter,
and the magnetron, at the present time, is the tube best suited
for this phase of exploration.
* The National Co.
Posted October 21, 2014