May 1939 QST
Table of Contents
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
Varian is a company familiar to most
people involved in early development of radar and other higher power microwave systems. While the name
sounds like a moniker construed from a combination of technical terms, it is actually the surname of
Russell H. and Sigurd F. Varian. They are credited with building the first practical reflex
klystron tube, and its variant the rhumbatron, while at Stanford University. Velocity modulation
changes the speed of a stream of electrons flowing at a constant current rate, rather than modulating
the current. It was a big deal that eventually found application in CRT displays for adding another
dimension to information on monochrome presentations.
See also "TV Receiver Conversion for
Velocity Modulation," an April 1951 Radio & Television News Article
Velocity Modulation of Electron Beams
A New U.H.F. Development
The problems of operation at wavelengths of the order of centimeters now seem likely to be solved
by the development of tubes working on a new principle - modulation of velocity of the electron stream
as contrasted to modulation of the conductance of the plate-cathode space. The limitations on performance
of ordinary tubes imposed by transit-time effects at the ultra-high frequencies bid fair to be overcome
by tubes operating on this principle.1 Stated briefly - and very approximately - the grid
in such a tube changes the velocity of the electrons passing from cathode to plate in accordance with
input-signal potential variations but, because of its special structure, has practically no effect on
the number of electrons so passing. The changes in velocity can be converted into a conventional current
change at the plate by several different methods, involving different types of tube structure and different
modes of operation.
Experimental tubes constructed in this way show an input impedance of the order of 50,000 ohms at
wavelengths as short as 5 centimeters. They can be used as oscillators, amplifiers and detectors; the
operating frequency is to a considerable extent a function of the tube dimensions, but a frequency range
of about 5 to 1 is obtainable in a given tube by varying the electrode voltages. No commercial tubes
are available as yet, the development still being in the laboratory stage.
The velocity variation principle is also inherent in the "Klystron," a new electronic device which
recently had a great deal of publicity in the newspapers of the country. Technical data have not been
released on this development as yet, although some general information has been made available. We are
indebted to W2OQ for the following summary:
Rhumbatrons and Electrons
A dynamic group of researchers at Stanford University - brothers Russell H. and Sigurd F. Varian,
research associates, Assoc. Prof. William W. Hansen, and Prof. David L. Webster, head of the physics
department - have developed a new type ultra-high frequency generator and receiver working on principles
strikingly different from those of the ordinary vacuum tube.
The discovery is known as a Klystron, or Rhumbatron. It was first described before the January colloquium
of M.I.T.'s department of Electrical Engineering by Dr. Webster. It was announced to the public from
Palo Alto on January 29th, and to readers of the M.I.T. Technology Review in the February issue, as
well as in the Journal of Applied Physics.
In the Klystron, a beam of electrons representing a constant current is sent through two resonant
metal containers known as Rhumbatrons. In the first is an oscillating electric field, parallel to the
stream and of such strength as to change the speed of the electrons by appreciable fractions of their
initial speed, accelerating some and slowing others. After passing this Rhumbatron, the electrons with
increased speeds begin to overtake those with decreased speed which are ahead of them. This motion groups
the electrons into bunches separated by relatively empty spaces. A considerable fraction of the energy
of these groups can then be converted into power at high frequency by passage of the stream through
the second Rhumbatron, within which is an oscillating electric field so changing synchronously as to
take energy away from the electrons in the bunches.
If the first Rhumbatron (which is called the buncher) is driven by an external source of power, such
as an antenna receiving radiation, and the electrons are strong enough to give the second Rhumbatron
(which is called the catcher) more power than the antenna gives to the buncher, the Klystron is acting
as an amplifier.
If the buncher is driven by power received through a coupling loop or otherwise from the catcher,
the Klystron is acting as an oscillator.
And finally, if the buncher is driven by power from both of these sources at once, the Klystron is
acting as a regenerative amplifier.
Advantages of the Klystron principle are three-fold. It produces strong waves; they are at stable
frequencies; and they have strong amplification at the receiving end. The present working minimum wavelength
employed by airlines in radio work is about one meter, but the Klystron produces waves one-tenth that
length. Such waves, when emitted from a reflector one meter in diameter, would radiate within a narrow
angle of only six degrees. The Klystron inventors believe wavelengths considerably less than 10 centimeters
can be reached, thereby still further narrowing the angle of radiation.
- Newbold Wheelock, W2OQ/WLNS
1 W. C. Hahn and G. F. Metcalf, "Velocity-Modulated
Tubes," Proc. I.R.E., February, 1939.
Posted August 10, 2016