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.
Figure 1 - The tube which with its field coil offers new
fields of experimentation on the ultra-short waves.
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 magnetron transmitter.
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
Set-up of 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
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
Figure 4 - Curves showing increased stability by using high
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 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 electronic
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 G-M circuits.
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
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
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 C" circuits.
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
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 conventional.
The radiating system was a simple vertical, half-wave, copper-rod antenna
approximately 8 feet long, and fed by means of a balanced transmission line.
Complete 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
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 local interference.
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 modu-lation might
be undesirable due to the ex-cessive 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
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
* The National Co.
Posted October 21, 2014