Velocity Modulation of Electron Beams
May 1939 QST Article

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 engineer brothers 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