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June 1955 Radio-Electronics
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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When I read about Du Mont's
Iconumerator, the first thing that came to mind was the video
of the Rockwell Retro Encabulator and General Electric's
Turboencabulator.
As it turns out, the Du Mont device is real. This article from a 1955 issue
of Radio Electronics discusses a new type of oscillator-amplifier that
works on the principle of microwave amplification by stimulated emission of radiation
(maser). It used ammonia as a masing medium. Masers were quickly applied to commercial
broadcast systems, to military communications systems, and in laboratories. The
state of the art has of course advanced far beyond the relatively crude apparatus
shown here, but it is always good to have a working knowledge of the technology's
history.
Revolutionary New Oscillator-Amplifier
By Fred Shunaman, Managing Editor
Path of molecules through maser, showing separation of active
and inactive ones.
Two of the new glass trimmer capacitors.
New RCA tricolor Vidicon camera tube.
Du Mont's Iconumerator, a device which can count objects of any
size and shape in its field of vision. Here its subjects are similar to colonies
of bacteria counted in medical laboratories, etc. The most startling feature of
the in-strument is that the size of the object (as long as it can be contained in
the field) or its shape make no difference in the accuracy of the numeration.
The first experimental maser, with Professor
Charles. H. Townes (left) and
J. P. Gordon.
Ellensburg, Washington, a town of about 9,000, is located 100
air miles and a range of mountains east of Seattle. Every attempt to receive a picture
from Seattle met with failure. In a final attempt, the Jerrold-Northwest Community
Co. hit upon a plan for the installation of 16 stacked Yagis. Half of the antennas
are designed for channel 4, the other half for channel 5. The project was successful,
and the people of Ellensburg are now receiving nearly perfect pictures. The antennas
used are eight-element Baline Yagis, made by JFD.
The instrument shown on our cover represents a completely new method of producing
electronic oscillations or amplifying radio signals.
Called "Microwave Amplification by Stimulated Emission of Radiation" (maser)
it uses ammonia molecules as a source of energy and as an oscillating medium.
The maser was developed at the Columbia Radiation Laboratory, which is jointly
sponsored by the Armed Services and Columbia University. It was conceived by Prof.
Charles H. Townes, executive officer of the Physics Department of the university.
The work on it was carried out by him and his assistants: Dr. H. J. Zeiger, Carbide
and Chemicals Co., post-doctoral fellow in physics at Columbia, and J. P. Gordon,
a graduate student. The device was conceived as an aid to microwave spectroscopy,
but its applications will cover a far wider field.
Oscillations produced by the maser are so steady in frequency that they can be
used in an "atomic clock" 20 to 50 times more accurate than any now in use or in
a simplified navigation aid similar to Loran but dispensing with some of the stations
necessary for that system. As an amplifier, it has a fantastic signal-noise ratio.
The noise level is practically at the theoretical minimum level of zero. Thus, while
the output of the amplifier is very low (about 10-9 watt), it can be
used to amplify signals far below the noise threshold of the best vacuum tubes,
bringing them up to a level where they can be amplified by more conventional methods.
In spite of its large size, the maser is a microwave oscillator, operating at 23,870
mc, the resonant frequency of the ammonia molecule. The brass box is simply a container
into which ammonia gas at low pressure (10-6 atmosphere) may be injected.
It contains the active part of the equipment - four cylindrical electrodes which
form an electrostatic field, and a resonant cavity about 1 centimeter in diameter
and 3 inches long. It is also fitted with airtight seals through which pass waveguides,
control adjustments and pipes for admitting and removing gas, pumping the chamber
to a low pressure and circulating coolants.
How It Works
Operation of the maser is remarkably simple, though different from anything in
the history of electronics. A stream of ammonia molecules is injected into the brass
chamber and directed down the center of the field formed by the four copper cylinders.
These molecules may be in one of two states - a low-energy state that may absorb
energy and a high-energy one that can radiate energy. As they drift through the
electrostatic field - formed by keeping two of the copper cylinders at a potential
of 6 to 20 kilovolts and the two diagonally opposite grounded - the low-energy "dead"
molecules are diverted and scattered while the high-energy ones are focused into
a sort of beam which enters the resonant cavity just beyond the field. The process
may be compared with the refining of uranium, in which the inert U-237 is removed
and only the active U-235 left in a more or less pure state.
Once inside the cavity, which is dimensioned to resonate at the frequency of
the ammonia molecule, some of the molecules radiate - give up some of their energy.
These tiny quanta of energy trigger other molecules, building up a chain reaction
which may again be compared to that of purified uranium. In a short time the molecules
have produced a vigorous oscillation in the cavity. The microwave energy is piped
out of the cavity in an ordinary waveguide.
This type of oscillation is unique - it is like nothing previously experienced
in controlled electronic reactions. There is a local electric source - to maintain
the high-voltage electrostatic field - but the energy which produces the oscillations
does not come from that supply. It comes from the ammonia gas itself. Approximately
1014 molecules per second must be admitted into the chamber to maintain
oscillation.
The Maser as Amplifier
If the number of ammonia molecules admitted is not great enough to sustain oscillation,
the maser can act as an amplifier. An external signal may be introduced and increased
in amplitude by energy picked up from the radiating molecules. The output of approximately
one-billionth watt is high enough to apply to the grids of standard vacuum tubes,
making the maser a preamplifier with a signal-to-noise ratio essentially equal to
the limit set by fundamental thermal noise. The maser amplifier may in addition
be cooled so that the fundamental thermal noise is further reduced.
Coupling the signal to be amplified to the maser is simple. Signals have been
transmitted down the waveguide which enters the resonant cavity at the top and the
amplified signals reflected back through the same guide. Should it prove more convenient,
the signals could be admitted from a guide on one side of the cavity, passed straight
through and removed by a guide on the other side. The amount of amplification is
linked with the stability of the device. Operated to give very high gain, the system
may break into oscillation. An amplification factor of 100 can probably be obtained
without serious instability.
Two masers were in existence when this was written. It was necessary to build
the second one so that it could check the first. No other instrument is accurate
enough to test a maser.
During the tests the oscillation frequencies were compared with an accuracy of
one part in 100 billion. Professor Townes states that this is probably the most
accurate comparison or measurement of any two physical quantities that has ever
yet been made.
While Professor Townes and his associates were chiefly interested in microwave
spectroscopy, one of the most important immediate applications of the maser is as
a frequency standard. It is at least 30 times as stable as the best systems using
crystal oscillators. Thus it would make an excellent atomic clock. Incidentally,
it operates on exactly the opposite principle from that of the atomic clock described
in this magazine in the March, 1949, issue. In that clock, ammonia gas acted as
a wave-trap at the resonant frequency, absorbing energy produced by a crystal oscillator.
The maser produces its own power and does not have to depend on auxiliary equipment
for its oscillations.
It is expected to increase the resolution or detail which can be seen by microwave
spectroscopy about 10 times, improving and extending our knowledge of the structure
of molecules, atoms and nuclei.
Many other applications - such as the navigation aid mentioned before - become
apparent. The maser will find applications in radio astronomy and in many uses where
a radio frequency must be measured with greater accuracy than is possible with present
apparatus.
But it is in directions unknown at present that the maser may find its greatest
usefulness. Simply because it can act beyond our known horizons, it is difficult
to assign its fields of future application. Who can say, for example, what uses
there may not be for an amplifier which can amplify signals in a whole region which
is now unexplored territory and whose very existence we may not suspect at present?
Posted October 5, 2021
(updated from original post on 5/22/2013)
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