February / March1932 Short Wave Craft
Wax nostalgic about and learn from the history of early electronics. See articles
from Short Wave Craft,
published 1930 - 1936. All copyrights hereby acknowledged.
In 2015 we would hardly think of electromagnetic radiation in the
5 cm wavelength realm as being 'quasi-optical' as far as circuit-based
manipulation is concerned. Optical wavelengths begin at around 6,300 Å
for red light, which is 6.3x10-5 cm, or 630 nm.
The 5 cm wavelength used an example in a 1932 article in
Short Wave Craft magazine is equivalent to 6 GHz.
6 GHz was an extraordinarily high frequency to be using for
communications back then, and the author did not intend to liken
it to anywhere near visible light. Instead, his terming its properties
as 'quasi-optical' referred to how the waves interacted with physical
objects; e.g., reflection, refraction, absorption, and scattering. Barkhausen
oscillations were a popular subject of the era, as I pointed out
recently in the article "The
Spook - Another Weird Effect to Haunt TV."
Quasi-Optical Short Waves - Electron Oscillations
By C. H. W. Nason
What is the wavelength of a '99 tube? What is the basic action
occurring in an electron oscillator? What conditions or factors
govern the frequency of such an oscillator? How are the signals
from such an oscillator received?
Fig. 1 shows path of an electron between filament
negatively charged plate in an electron oscillator.
Fig. 2 - Hook-up of simple "electron oscillator."
The waves from 10 meters down to 5 centimeters are often referred
to as the "Quasi-Optical range" (the inference may be gleaned from
the dictionary they are "near-light" waves). These waves may be
treated in many cases as light waves - particularly in the shorter
ranges, where they may be reflected from metallic bodies having
either plane surfaces or specific curvatures for directive transmission
or reception. The waves have been acted upon by lenses made from
dielectric materials (Bakelite for example) in such a manner as
to demonstrate the fact that they closely resemble in behavior light
waves, more usually thought of under the classification of "Optical."
By means of special circuits, devised for the utmost of simplicity
and efficiency, the normal vacuum-tube oscillator may be employed
with good effect at wavelengths as low as 1 meter! Below this the
stray tube and circuit capacitances are much in evidence, and the
classical circuits are rather hopeless. In the lower range - speaking
now in terms of wavelengths rather than frequencies, to simplify
matters - the "electron oscillations" of Barkhausen and Kurz, and
of Gill and Morrell, are most effective. Although these oscillations
are known to the physicist, very little may be found regarding them
in standard works on radio. It is necessary, therefore, that we
first consider the mechanics of these oscillations, before entering
into a more detailed exposition.
The Mechanism of the Electron Oscillation
Before going further, it is well to state that the most satisfactory
tubes for use in these oscillators are those having concentric elements
- cylindrical plates, etc. These are the '99, the '27, the '52 (more
ambitious, of course) and certain of the tubes provided the government
by various organizations during the war. Karplus in the General
Radio Experimenter for May, 1931, indicates success with the G.E.
"CG-1162" which is available from many radio salvage organizations.
Electron oscillations may be obtained with other tubes specially
developed for the service but, quite naturally, our interest rests
with those tubes available for experiment.
Fig. 3 - How to construct ultra-short wavemeter.
Fig. 4 - Schematic diagram of an electron oscillator.
With the electron oscillators, the determination of frequency
no longer rests upon the inductance and capacitance values of a
tuned circuit, but rather on the electrostatic forces acting upon
the individual electrons given off by the filament, and the resultant
time element. Fig. 1 indicates the inter-electrode spacing of a
triode (3-element tube) of the usual character. Note that the grid
is positive and the plate negative, with respect to the filament.
An electron, leaving the filament or cathode, is accelerated toward
the grid by virtue of the grid's positive potential. The majority
of these accelerated electrons will pass through the grid's mesh
and, by virtue of their momentum, will travel onward toward the
plate until they reach a point where the negative charge on the
plate is sufficiently effective to halt their flight: they will
then assume a backward path, toward the grid. They rejoin then the
other electrons passing toward the grid, following a path somewhat
as indicated in the figure. The length of the path taken and the
initial acceleration are the criteria for determining the frequency
of the cycle. The actual A.C. voltages, making up the oscillatory
energy-cycle, are induced by changes in the grid and plate charges
created by the moving electrons.
The original formula of Barkhausen covering the wavelength of
the oscillations - barring factors too complex for inclusion in
our discussion - is as follows:
where "d" is the distance between electrodes and "E" the voltage.
(The equation is for the original two-element tube of Barkhausen
and Kurz, and not for a triode.)
The Barkhausen oscillations are independent of the circuit constants,
to all intents and purposes.
Circuits for Producing Electron Oscillations
Fig. 2 shows the circuit arrangement of an oscillator for producing
the Barkhausen effect. The Gill-Morrell oscillations are true electron
oscillations, but their frequency is determined by the distance
"d" between the elements and the short-circuiting condenser "C."
The change between the two types of oscillations may be effected
at will by altering the circuit conditions. In the Gill-Morrell
effect, the oscillation is due to the timing of the electron's orbits
by the oscillating circuit formed by the Lecher wires. The Gill-Morrell
oscillations are much stronger and are preferable to the simple
Barkhausen type. The transition may readily be obtained by setting
the distance equal to one-half the desired wavelength, and adjusting
the voltages for the maximum oscillation. The Lecher wires should
be calibrated directly in centimeters to check the wavelength -
remembering of course that the accuracy is not great. Fig. 3 shows
a "trombone" wavemeter for rough measurement of the emitted wave;
this is useful in determining the transition point between the two
effects. By replacing the milliammeter with a crystal detector and
phones, the device may be used to monitor modulated signals.
Fig. 5 - Shows antenna placed in focus of a parabolic
Fig, 6 - Arrangement of directive aerials with
In Fig. 4 there is illustrated a more complex arrangement of
the original figure, showing the oscillator circuit. To this, it
will be seen, there has been added an antenna; this should be positioned
exactly one-fourth of a wavelength away from the bridging condenser.
This places the antenna approximately in the center of the Lecher
wire, where the Gill-Morrell oscillations are used, and at an indeterminate
distance, depending upon the wavelength of the oscillations generated.
The antenna is formed by two copper or brass rods clamped to the
Lecher wires by a movable slide.. They should be 1/4-wavelength
long, and might be "tromboned" for ready variation.
Fig. 7 - Receivers for laboratory work may be
quite simple, as shown in diagram above.
The grid-current meter should be of the order of 0-100 milliamperes
while a 0-1-ma. meter may be used in the plate circuit. The. oscillatory
current may be measured by a 100-ma. thermal milliammeter; the condensers
are 0.001-mf. mica units. The R.F. chokes are simply wound from
annunciator wire on a broom handle, and slid off. They are somewhat
like the pretty curlicues that we used to employ to connect up buzzers
and what not, before we had "wireless" to play with. In tuning the
transmitter, the maximum oscillation is indicated by a maximum reading
of the plate milliammeter.
The antenna may be backed by a parabolic reflector of the type shown
in Fig. 5, with the antenna situated at the focal point. (See the
preceding issue of Short Wave Craft - page 254, Dec.-Jan., for details,
of a parabolic curve.) Other types of directive antennas may be
employed by the "ham" desirous of going deeply into the operation
of the system. A reference to the article by Yagi, in the June,
1928, I. R. E. Proceedings will yield much data on the use of directive
antennas at such short wavelengths. The directive effect of the
reflector may be greatly increased by employing an arrangement such
as that indicated in Fig. 6, at both transmitter and receiver; the
rods used in the director chain are 1/2-wavelength long, and spaced
1/4-wavelength apart. The metallic reflector may be replaced by
a system of five 1/2-wave length rods arranged in a parabola, at
the focus of which the antenna is placed.
Receivers for Electron Oscillations
Because of the relatively low frequency stability obtained, little
success will be achieved with electron oscillators for communications
where straight C.W. is employed; although with A.C. on the filament,
the hum modulation will be so great as to alter these conditions
by creating an interrupted continuous-wave effect. Receivers for
intramural (laboratory reception) work may be quite simple - as
shown in the two circuits shown in Fig. 7. It is also possible to
achieve high sensitivity in the receiver system by means of the
"super-regenerator." Such a circuit arrangement is shown in Fig.
Fig. 8-A - How apparatus is connected to make
a super-regenerator "receiver" for electron oscillations.
The most practical arrangement is that of employing another electron
oscillator, almost identical with that employed as it transmitter.
Indeed, a "changeover," of such type that a single oscillator may
be used for both transmission and reception, may readily be effected.
The most logical arrangement is that shown in Fig. 8-b, where
the circuit constants are clearly indicated. The antenna is approximately
1/4-wavelength in dimension, and attached directly to the plate
of the tube. Here it might be mentioned that any good tube socket
may be used, and that "de-basing" of the tubes (as usual when extremely
short waves are desired from the more usual oscillators) is unnecessary.
The output of the receiver may be taken by means of a pair of phones,
or by a transformer feeding a standard A.F. amplifier in the plate
circuit of the receiver, as shown.
Fig. 8-B - Receiving circuit for electron oscillator
signals (wave lengths such as 15 inches).
The best tube to be used in the receiver is perhaps the '99,
because of the extreme portability obtainable.
Phone Modulation With the Electron Oscillator
Telephonic modulation of the electron transmitter is achieved
in the plate circuit, by substituting a modulation or microphone
transformer for the phones shown in the receiver schematic. It is
not necessary to provide a speech amplifier, although it is best
to do so where long-range operation is desired. It should be remembered
that a variation in the plate voltage of the oscillator effects
a frequency change rather than - or as well as - an amplitude change.
The modulation achieved is not so perfect, therefore, as in the
case of the usual oscillators. Telephone communication has been
achieved up to distances of about twenty miles with "electron transmitters,"
and telegraphic communication is possible over much greater distances.
When we consider the fact that all possible forms of modulation
involve a frequency shift, it is surprising that good quality can
be obtained. Nevertheless, the quality of speech is quite good.
Tubes and Voltages to Be Used
The following table gives some idea of the voltages to be applied
for various tubes and the oscillation wavelength to be expected.
All tubes of a given class do not function as electron oscillators
and many tubes must be operated with trick filament potentials.
With the '27 the filament voltage should be rather low; whereas,
in the case of the '99, best results seem to be obtained when the
filament is completely deactivated and operated at a high voltage.
The grid current is high in all cases, and the frequency range may
be limited by the voltage which can be applied without melting the
grid - or by the effects of grid emission.
45-50 cms. (17.7 to 19.6 inches)
40 to 75 cms. (15.7 to 29.5 inches)
40 to 75 cms. (15.7 to 29.5 inches)
The whole outfit may be laid out on a breadboard, with Lecher
wires about one meter (39.37 inches) long, made from heavy copper
rod, mounted on G.R. stand-off insulators. Copper clamps spaced
with bakelite strips may be used to provide riders for the short-circuiting
meter, or condensers, so that they may be readily slid along the
Lecher wires. The plate and grid voltages should be made continuously
variable by the use of potentiometers, and the necessary meters
should be provided for taking readings.
All work with electron oscillations is of a highly experimental
nature, and no specific data can be provided with a sure-fire operation
guaranteed. The experimenter undertaking this work should have had
considerable experience with radio equipment, if any hope of success
is to be held out to him; it is no game for the tyro.
Posted July 12, 2015