July 1945 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
In 1945 when this series was
published in Radio-Craft magazine, microwaves were pretty much the realm of
corporate and university research laboratories and - often coincidentally -
secret Department of War projects. Radar was the primary application, although
some work was being done on high bandwidth communications by the aforementioned
entities, in some cases following in the footsteps of amateur radio operator's
discoveries about how the higher frequencies were affected by the ionized layers
of the Earth's atmosphere. Immediately following the end of World War II,
the government began declassifying a lot of information learned about microwave,
and magazines were fast to pick up on it. In fact, there were instances where
editors saw fit to write columns asserting their right to do so when criticisms
were heaped upon them by readers accusing them of divulging critical security-related
data.* Captain Skinner, of the U.S. Army Air Force's Training Aids Division,
contributed this information, so obviously it was approved for release by the
focusing on signal generation, appeared a month later in the August issue of
vs. Radio Progress," by Hugo Gernsback
Microwaves Part I - Introductory and General
Sperry 410R Klystron, showing coaxial feedback cable and
frequency adjustment knob.
By Capt. Eugene F. Skinner*
One of the newest fields being opened to the radio world, and one which the
amateur should be able to enter soon, is that of Microwaves. The subject of
microwaves should be approached with the idea that there is nothing mysterious
about them. In using microwaves the experimenter is merely making another move
up the radio spectrum, similar to his previous move from long waves to short
waves and later from short waves to the ultra-high-frequencies.
Radio waves whose wave lengths are less than one meter have been designated
as microwaves. Actually, the division between microwaves and ultra-high-frequencies
is not as precise as the difference in names might imply. Very little difference
exists between the higher of the ultra-high-frequencies and the lower frequency
microwaves. They are in fact identical where they run into each other, and there
are numerous component parts and applications which apply equally well to both
As radio frequencies get higher and higher, they begin to assume many of
the characteristics of light. In the microwave field these effects are very
noticeable. This is an aid to handling them, but has disadvantages also. At
these frequencies, it is a very simple matter to focus and beam the energy so
a very accurate determination of . where the energy is going is possible. At
lower frequencies, one just has to pour in the power and wonder where it is
being radiated. Even if he knows, he can do relatively little to control it.
At lower frequencies, the local conditions affect the coverage seriously and.
cannot be adequately controlled due to the tremendous size of the antenna arrays.
For microwaves, the elements are small, and reflectors, especially of the parabolic
type, so focus and beam the radiation that the local conditions have little
effect. The antenna arrays will fit nicely into the space limitations that most
amateurs have. There is no need for large bulky arrays when a quarter or half-wavelength
is only a few inches. Microwaves seem to thrive on bad weather.
Radar and other wartime applications have brought microwave
techniques into the practical field. Postwar applications will reach into many
branches of electronics. Radio-Craft is therefore starting a series covering
these important waves.
The line-of-sight usable distance naturally limits the uses of microwaves
considerably, and permits them to be used only for special installations. Also,
"Shadows" are caused by most solid objects larger than the wave length being
used, and there is negligible bending. Sky waves are unknown to date, but in
any case in which they might be noticed, they would be rare and could not be
depended upon for any practical use. This will discourage DX work on the part
of amateurs. However, many other characteristics make their use extremely desirable,
and repeater stations strategically placed could conceivably increase their
range. It is probable that a "mother" set and several repeater stations might
cost no more than one complete station for lower frequencies.
In the wave length band between one centimeter and one meter, there is a
range of 300,000 to 30,000,000 kilocycles, making available almost 3,000,000
channels for amplitude modulation and 150,000 for frequency modulation. These
numbers can be amplified to an almost infinite number when the distance limitations
that exist are remembered, and that high directivity will permit several stations
on the same frequency in the immediate vicinity of one another.
Fig. 1 - Wave guides may have various shapes.
The high directivity permitted by beaming the output makes it possible to
use extremely low power outputs, even down to a fraction of a watt. For most
purposes the output of a single tube is many times more powerful than is required.
To date, frequency controls for the transmitter necessitate careful control
of temperature and power, and are somewhat troublesome at times. However, dependable
methods of locking the frequency of the receiver to that of the transmitter
exists. Crystal control is not possible to a satisfactory degree asset, because
a crystal cannot be cut thin enough for these frequencies, and any frequency
multiplier system merely multiplies the errors in the crystals. Greater accuracy
and frequency stability is being developed in crystals, and it is possible that
in the future crystal control may become practical.
Little trouble is anticipated from interference on the microwaves. Static,
both man-made and natural, is practically non-existent. In addition to getting
rid of bothersome noise, this makes lower power requirements possible, due to
the noise-to-signal ratio. Harmless diathermy effects may be noticed, but there
is no need to worry about any damaging results from them.
In using microwaves, the mechanical and electrical considerations are of
about equal importance. Parabolic reflectors may be used with the antennas,
and the physical or mechanical perfection of these reflectors and the dimensions
of the radiator itself determine the sharpness of focus of the beam, in the
same manner that a reflector in a flashlight and the dimension of the bulb determines
the characteristics of the light beam. "High-Frequency-Plumbing," or wave guides
are used to transfer the energy from one point to another instead of coaxial
cables. Coaxial cables may be used, but they are generally unsatisfactory except
when used for very short distances. These wave guides may be tubular like pipe,
or of rectangular cross-section area, and may be of a variety of styles (Fig.
Circuit layouts must be of such a nature that there is a minimum of inductance
and capacitance involved. Such minute quantities are required that the tubes
are so designed as to include the quantities required for resonance within the
tube. From this, it can be seen that such things as interelectrode capacitance
are of great importance. Naturally, with such physical characteristics involved,
the overall size of a system is automatically much smaller. A mechanical consideration
in the design of the tubes is the transit time of the electrons, which becomes
of great importance. The wave lengths are so very short that if the tube is
of appreciable size, the time required for an electron to travel from one element
to another might readily be a considerable portion of a cycle, or even several
cycles, as in the Klystron.
Experiments have been made with satisfactory results for using microwaves
in conjunction with moving railroad trains for control and conversation. Considerable
success has been attained in such work, and certain railroads now experimenting
with lower frequencies intend to move into the microwave region as soon as it
becomes possible to get equipment. Many of the applications are shrouded in
wartime secrecy, and will undoubtedly be revealed at a later date. Aircraft-to-ground
and aircraft-to-air-craft communications should adapt themselves readily to
this means, especially where a large degree of privacy is desirable. The privacy
of beamed microwaves is so great that it is comparable to talking over the telephone.
A few of the tubes have already been partially released, and include the Sperry
Klystron and the General Electric Lighthouse tubes. Two articles on the Klystron
by the author have previously been published in Radio-Craft.
Fig. 2 - Simplicity of microwave hookups is typified in
this basic Klystron reflex circuit.
Where the Ham Comes In
Naturally, the amateur wants to know just where he fits into the picture.
In the past, "hams" have been a good market for radio parts, and have been responsible
for arousing a great deal of interest in radio among people who previously thought
of it only in terms of turning on a switch and listening to a program. He is
responsible to a large degree for the advancement of this science since its
very introduction. There are agencies looking out for the interests of the "hams"
in Washington, and sections of the microwave band will be made available for
amateur use after the present restrictions are removed. Soon the day should
come when the amateur will again be able to have his demands supplied, and at
costs that will be considerably lower than ever before. In addition to the fields
he is already familiar with, microwaves should be within reach of his pocketbook.
The radio industry has been greatly expanded as a result of the demands of war,
and it is reasonable to assume that these plants which have expanded or opened
will want to continue as much production as there is demand for. Considerable
competition and low prices should result. This will benefit the amateur, because
he gets no financial return on his radio investment, and is, therefore, limited
as to the amount he can afford to spend.
Microwave parts production is tremendous. A large postwar market is necessary,
or these plants must close. The "ham" will be able to buy those items he needs,
which will probably be designed with sufficient latitude to cover an entire
ham band, and by adept use of a hacksaw, pipe, sheet metal, and a few other
readily available and cheap items, can construct for himself the other items
he needs. Circuits are simple - note the Klystron hookup of Fig. 2. In addition
to reducing the expense, this will stir up more interest in ham, because most
of the kick is the "tinkering" and experimenting, and there is great satisfaction
of having produced something with one's own hands. If things don't work out
the first time, a challenge is presented that is far from insurmountable, and
a few inches of pipe or sheet metal are so inexpensive that the experimenter
is not deterred from beginning again. Microwaves furnish a wider field for the
amateur and experimenter than any of the others he has so successfully ex-plored
in the past.
* "Hq. AAF, Office, Asst. Chief of Air Staff, Training Aids Division.
Posted January 20, 2021