January 1930 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.
2.1 GHz (5.6-inch, or 14 cm wavelength)
radio waves were an almost totally unexplored realm in 1930,
with it and higher frequencies being the domain of theoretical
research laboratories. Signals generators capable of producing
much more than a few hundred megahertz were rare even in commercial
applications. As reported here, centimeter-length electromagnetic
waves were "according to the theories of Barkhausen and Kurz,
[the] result of purely electronic vibrations, whose frequency
was determined only by the operative data of the tube and was
not dependent on any internal or external oscillation circuit."
A half-wave receiving antenna picked up the transmitted signal
with a simple diode detector to enable, after a couple stages
of amplification, and audible signal. These are some of the
earliest experiments at RF with parabolic metallic reflectors
and polarization with a diffraction grid that could be rotated
to set the polarization direction.
Six-Inch Radio Waves
They are produced by a "tuned-tube" and picked up by a crystal
detector. Parabolic mirrors are used. Experiments are described.
By Ernst Gerhard
At a meeting of the German "Heinrich Hertz-Gesellschaft"
(Heinrich Hertz Society), which took place a short time ago,
Dr. K. Kohl, of the University of Erlangen, showed some very
interesting experiments with undamped "monochromatic" waves
14 centimeters in length. The well-known Hertz experiments were
presented with the aid of a modern auxiliary, the electron tube,
together with many newer experiments.
Fig. 1 - The tube is emitting ultra-short
waves which are reflected, just like light, through the lens
in the center, to a glass of water, and thence to the little
Some years ago Barkhausen and Kurz, while making investigations
of the vacuum in electron tubes, discovered the presence of
ultra-short waves of less than one meter wavelength. It appeared
that, to produce these vibrations, the grid of the tube must
be given a high positive voltage and the anode (plate) a relatively
slight negative voltage. According to the theories of Barkhausen
and Kurz, it was the result of purely electronic vibrations,
whose frequency was determined only by the operative data of
the tube and was not dependent on any internal or external oscillation
circuit. Dr. Kohl, however, was able to demonstrate by new researches
that, to excite these oscillations, there "must always be present
an oscillatory circuit to determine the frequency. Especially
by proper reduction of the elements determining the frequency,
Kohl was successful, under normal operating conditions, in producing
undamped waves with a fundamental length as short as 8 centimeters
(3.2 inches) and to demonstrate their radiation into free space!
In the experiments described below, the transmitter was a
tube constructed by the firm of Tekade (Nuremberg, Germany)
according to Dr. Kohl's directions, and containing in the glass
bulb as the oscillatory clement a small spiral grid, which is
excited at its natural frequency and radiates a constant wave
14 centimeters (5.6 inches) in length. A specially-made receiver,
in the form of a rod half as long as the wavelength, was used;
the detecting element (crystal) being set in the middle. (See
Fig. 5.) The oscillation was modulated at audio frequency in
the transmitter by a special process; so that the reception
could be heard in the loud speaker, after two stages of audio-frequency
Fig. 2 - Here the radio waves focused by
the mirror are reflected by the metal disc at the right. The
result is an interference pattern of R.F. signal voltages.
The experiments described below correspond to well-known
experiments with "monochromatic" (single-wavelength) light.
The practical demonstration of such optical experiments becomes
possible if the wavelength of the electric waves is comparable
with the linear dimensions of the experimental apparatus. With
a 14-centimeter wavelength this requirement is completely satisfied.
First, presence of free radiation was proved directly with
the detector in the vicinity of the sending tube. It was shown
that the radiation was polarized. in a plane and in this case,
the direction of the electric field was horizontal. If the axis
of the receiver rod was turned until it was horizontal and parallel
to the grid spiral of the transmitting tube, the sound received
was a maximum. Turning the detector 90 degrees, in a horizontal
or vertical plane, caused almost complete disappearance of reception.
The influence of a straight "resonator" on the transmitter
or receiver was shown by the following experiment:
A small copper rod half as long as the wavelength (i.e., 7 centimeters
- 2.8 inches) was placed behind the transmitter and parallel
to the electric field, about a wavelength away. The rod was
itself at the same time excited to its natural oscillations
at the same frequency, and thus became a secondary radiator.
In this way the intensity of reception could be noticeably increased.
A similar experiment could be demonstrated at the receiving
To produce a directed radiation there was placed behind the
tube a parabolic mirror about 50 centimeters (19.7 inches) across,
the tube being located at the focus of the mirror. In this way
it was possible to produce an almost parallel "pencil" of rays.
If a metal screen was placed in this, in such a way that the
pencil fell diagonally on the screen, the rays were reflected
in accordance with the laws of optical reflection. Their course
could be followed exactly at the same time with the detector.
(See Fig. 3.)
Fig. 3 - The experiment of Fig. 2 is shown
here schematically. The metal of the mirror and the screen reflects
the 14-cm. waves sharply.
The phenomena of diffraction, which are well-known in optics,
could also be demonstrated with this radiation of waves. A round
metal disc, 50 centimeters in diameter, was placed in the path
of the rays. If the receiver was placed close behind this screen,
no reception could be obtained; that is, the receiver was completely
in the "electrical shadow" of the screen. At a distance of about
a meter (39.37 inches) from the metal screen, however, the wave
could be received again; that is to say, the waves curved around
the edge of the screen. By moving the receiver still further
away, it was possible to find in turn "maxima" and "minima"!
This phenomenon corresponds to the well-known Arago experiment
in optics; which shows the bending of visible light around the
edge of a circular disc.
The refraction of the wave-radiation was demonstrated by
letting the parallel pencil of rays from the transmitter fall
on a glass lens bout 30 centimeters (a foot) in diameter. By
using the detector behind the lens, it was possible to demonstrate
clearly the course of the radiation and in particular the place
of greatest intensity, the focus of the lens.
Absorption of Waves by a Conductor
With this arrangement, the absorption of the waves by various
substances was very beautifully shown. If a sheet of cardboard,
hard rubber, dry wood, or a glass vessel of paraffin oil, was
placed between the lens and the detector, there was no weakening
of the reception. But, if a glass vessel of distilled water
was placed in the path of the rays, the radiation was almost
Fig. 1 shows this experiment: the rays emitted by the tube
are made parallel by the parabolic mirror. The parallel rays
strike the glass lens and are concentrated by this into a focus;
the detector is set up at this point. The glass vessel, between
the lens and the detector, contains the liquid which is to be
tested as to its absorption.
Fig. 4 - When this frame of parallel wires,
at right angles to the wave front, is per perpendicular to the
electric field, it allows the wave, to pass freely.
Fig. 4a - If, on the other hand, the wires
of the Hertz "polarization grid" are turned until they parallel
the electric field, they shield against it.
One of the most interesting experiments is the production
of vertical electric waves. For this purpose the parallel rays
are reflected perpendicularly from the concave mirror upon a
metal screen, as shown in Fig. 2. The parallel rays strike perpendicularly
on the circular metal disk and are thereby reflected. The approaching
wave and the reflected wave coincide, between the concave mirror
and the metal disc, and cause a vertical wave. If the detector
is at one of the places marked with white in the illustration,
the maximum strength of reception is secured. Between these
places one gets tone minima. These maxima and minima correspond
to the crests and nodes of the vertical wave.
The distinctive polarization of the waves was shown by means
of the Hertz polarization grid. (See Fig. 4 and 4a.) This grid
consists of a row of parallel copper wires, stretched 1 centimeter
(0.4-inch) apart on a wooden frame 40 centimeters (16 inches)
square. When this grid was placed in the path of radiation between
the transmitter and the detector, with the grid wires perpendicular
to the electric field, there was no influence on the reception.
But when the frame was revolved 90 degrees, so that the wires
lay parallel to the electric field, the radiation was almost
entirely shut off.
Fig. 5 - A close-up of the antenna and detector
of the 14-cm. radio receiver.
A further interesting experiment was the rotation of the
plane of polarization. For this there was used a wire shape
made up of three copper rods at right angles to each other,
each 7 centimeters long (that is, half the wavelength). If the
axis of the receiver rod was placed perpendicular to the electric
field f there was no longer any reception. But, if
the wire shape was placed before the receiving rod in such a
way that one of the end-wires was parallel to the rod, reception
could be clearly heard. In this case, in fact, the first length
of wire lay parallel to the movement of the electric field and
thus, by coupling, conveyed the primary oscillations into a
direction perpendicular to the primary field; so that there
resulted a secondary electric field f1 in this direction.
(See Fig. 6.) This may be regarded as a model experiment for
the rotation of the plane of polarization by the flow of electrons.
A Tube Receives Its Own Waves
By two experiments, for the conclusion, there was shown the
possibility of tube-reception. For this purpose, the radiation
sent from the tube was reflected back upon the tube. In one
case the metal screen above described was used to do this; in
the second case a small linear resonator, 7 centimeters long,
was sufficient. It was shown that, according to the position
of the resonator (i.e., the phase of the reflected radiation)
the plate current of the tube can be modified. This experiment
proves the possibility of reception of a wave by the very tube
which sends it out and, at the same time, constitutes an actual,
visible indicator of the operation of the tube.
Lately, Dr. Kohl and his co-workers have even succeeded in
making directional telephone experiments over an experimental
distance of about 1,500 meters. The tubes used for receiving
and transmitting in these experiments were absolutely alike;
so that there was a possibility of using them alternately for
the purposes of conversation.
There is a great field of possibilities for the use of these
short waves. Thus, in the technique of communication, they will
be serviceable wherever the communication is to take place within
the range of vision, but, above all, in cases where definite
directional radiation is desirable for signal purposes (e.g.,
for coastal shipping and aviation).
In medicine there seem to be great possibilities of their
use in ultra-short-wave diathermy (i.e., the application of
internal heat by short waves). Very recently an intensive investigation
has been made of the physiological effect of short waves, about
three meters in length, which has led already to surprising
results at this frequency. Possibly shorter waves will prove
still more effective. It should be recalled that an absorption
maximum of water is found just at the frequency of the 14-centimeter
Fig. 6. - The heavy line indicates the short
coupling rod, which would pick up a signal in one of its sections;
and, by the flow of current, transfer the signal to the detector.
Finally, let us call attention to the possibility of the
spectroscopic investigation of matter with these four-inch spectral
lines. Along this line we may expect probably many discoveries
regarding the inner structure of matter.
Posted November 22, 2015