November 1935 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.
Crowded frequency bands have been a problem since the beginning
of radio because technology is constantly not only filling available
bandwidth, but also pushing the frontiers higher. The advantage
of going higher in frequency is that required bandwidths for existing
modulation schemes represent a smaller percentage of the center
frequency. For example, an 802.11b WiFi signal's 22 MHz bandwidth
represents roughly 1% of its 2.4 MHz center frequency. 802.11a
does 20 MHz at 5 GHz for 0.4%. Extend that center frequency
up to 50 GHz and the channel occupancy is a mere 0.04%. That
means for the same total band occupancy of 1% as with 802.11b, you
can fit in 25 equivalent slots. The problem with going higher in
frequency is that components are typically more costly and trickier
to implement, and the power falls off faster with distance according
to the familiar Friis equation.
Millimeter waves are the subject of a great many articles today
describing advances being made in the part of the spectrum above
30 GHz (Ka and EHF bands); however, when this piece was published
in 1935, it was decimeter waves that were the big to-do.
Decimeter Waves - The Future of Radio
By Eckard Klein
The present article deals with some of the possibilities of waves
only a fraction of a meter in length, the new "split magnetron"
tube is described, also the manner of guiding a boat by means of
1 - Set of decimeter receiving antennas. 2 -
Laboratory set up for the production of decimeter waves by means
of a split Magnetron generator. 3 - Split Magnetron installed between
the poles of an electromagnet. 4 - Pilot indicator instrument installed
upon the bridge of a steamer. 5 - Portable decimeter wave transmitter
with concentration mirror.
The great number of radio transmitters now in operation and the
daily increasing demand for additional traffic channels, puzzles
the radio commissioners of all nations, and has brought the present
radio channel system to a point of practical saturation. Only by
application of auxiliary tricks, for example the mutual use of one
and the same wavelength by two transmitters, which operate in more
or less large geographical areas, has it been possible to keep the
world radio system in fairly smooth operation. These methods have
or are actually hindering further progress in radio communication,
The only hope of solving this problem lies in the belief that
future research work in the wave range below 5 meters may unearth
some heretofore unknown facts, which would enable us to utilize
the great number of radio channels in this region for practical
Despite the well-known fact that a great many of radio channels
are unused in the decimeter* (one-tenth meter) range, no intensive
research work had been carried on in Germany until about one and
a half years ago. At that time the Radio Corporation of Germany
(The Telefunken Co.) started secretly some very interesting experiments
with decimeter waves of a length between 40 and 90 centimeters,
which have furnished a great number of new facts about the character
and the qualities of these very short waves. These very interesting
experiments have indicated some new possibilities of decimeter wave
utilization, which might in a short time to come be of incalculable
value in the progress of radio communication.
Since these very short waves can be bundled or concentrated like
a light beam, and since, further, these waves are only receivable
as far as the direct optical sight goes, it is possible to use them
for a directed beam by which many transmitters and receivers may
operate in parallel on the same wavelength without any mutual disturbance.
The stumbling block in the utilization of these very short waves
was until recently the enormous number of oscillations per time
unit, amounting to many millions and even billions of cycles per
second. It is easy to understand that currents of such a high frequency
put insulation materials under a specially high electrical strain.
Entirely new methods of handling these new high frequency electrical
problems had to be designed; also brand new transmitter circuits
never used before for the generation of these very short waves had
to be developed.
Schematic diagram of decimeter wave transmitter,
operating with a split Magnetron tube.
To produce these ultra high frequencies the so called "Haban-Roehre"
(Haban tube) was used in Germany. This tube invented by the German
radio engineer, Dr. Haban, is often called in England and America
the split magnetron.
As to the construction of such a magnetron tube let us note that
this tube has an "inside" system, consisting only of a single cathode
surrounded by an anode cylinder, but it possesses no grid. The anode
cylinder (often called plate cylinder) consists of two main parts.
Each of these main parts is further divided into two separate sectors,
which are arranged opposite each other in the tube system. The cathode
is therefore surrounded, as Fig. 1 shows, by a cylinder which actually
consists of four different parts. Each pair of the oppositely positioned
parts are electrically connected by means of small pieces of wire.
An electro-magnet arranged outside of the glass bulb of the magnetron
tube, produces a powerful electro-magnetic field which influences
the tube in the direction of the cathode axis.
In addition to the magnetrons very small triodes are used for
the reception of the decimeter waves. These tubes bring to mind
the American "Acorn" tubes and are of tiny dimensions. The system
of construction is greatly concentrated so as to make the time of
the electrons' transit practically zero. Another type of receiving
tube also used for decimeter wave reception is the so-called "diode"
type. These diode tubes, similar in their design to the diodes as
applied in ordinary "broadcast" receivers, are of much smaller dimensions
and are used as detectors. Experiments with these diodes have proved
that they are well fitted for the reception of waves down to 40
centimeter in length.
How decimeter waves guide boat along course.
Used for Guiding Ships in Fog
The practical application of the decimeter wave qualities for piloting
of ships into foggy harbors was then demonstrated upon a large lake
near Berlin, upon the so-called Mueggelsee. On the shore of this
lake a transmitter was kept in operation radiating a twin-beam of
decimeter waves. To demonstrate the useful effect of these waves,
a small steamer had been equipped with two decimeter wave receivers.
The steamer cruised about on the lake, until the two receivers installed
aboard picked up both beam signals with about equal strength, and
the ship then proceeded in a direction toward the transmitters on
shore; directed only by a pilot instrument installed in front of
the man at the wheel. This pilot instrument gave the wheelsman an
exact indication as to how far the steamer had shifted outside of
the "invisibly marked" lane. The accuracy of this piloting method
was so great that it showed a strong indication on the piloting
instrument when the steamer was only a few miles off the focal line
of the twin-beam transmitter. Even in case the deviation was about
0.1 degree only the indicating instrument marked not only the shift
but also the side toward which the ship has shifted.
This has been further demonstrated during the experiments upon
the Mueggelsee near Berlin. Two receivers operating on the same
wavelength, and installed pretty close to each other, could be separately
received without interference. The receiver could be turned in any
desired direction. By turning this receiver in one or the other
direction, either one or the other of the transmitters could be
received, without being in danger of the slightest trace of interference
from the transmitter not wanted.
*Decimeter waves are here considered as those falling between
roughly 0.1 and 1 meter.
Posted June 21, 2015