CHAPTER 5

Lighthouse and Acorn tubes suffered greatly from extreme frequency drift As continued use heated their metal parts, the rigidly fixed grids and plates moved to the point where their operating characteristics were significandy changed. Furthermore, not much SHF power could be derived from these tubes, whose size was the limiting factor. While representing the very first vacuum tubes capable of generating the SHF energies, neither Lighthouse nor Acorn tubes could carry sufficient energy for the needs of an efficient RADAR system. These little tubes were excellent in experimental demonstration of UHF and SHF energies, and retained their use in low power transmitters. The UHF and SHF tubes of Russel Ohl (1928) are still used in radiosondes and other such small SHF communications applications. Capable of transmitting low power superhigh frequency radiowaves, many of these little tubes combine tube and transmitter in one piece.

Some of these tubes were fitted with an extension lead or pointed metallic caps, SHF energy being removed through the little wire antennae. Placed in copper pipe or at the focus of polished metal concaves, these first SHF transmitters proved to give surprising line-of-sight signalling capabilities. Mounted on tripods, these simple first transmitters bore remarkable resemblance to modern microwave comlink systems. But another means for generating SHF energy had to be found which employed some new and more potent alternation phenomenon. These means were sought in the development of Klystron tubes and Magnetrons, the two other SHF classes which were developed during the preceding decade. When developed, none of these tubes were intended for wartime use. They were therefore not powerful embodiments, and the war demands for superhigh radio frequency power demanded substantial new upgrades which could not yet have been anticipated by engineers of the day. Klystron tubes took their name from a Greek term which means “clustre” or “bunch”, the electronic action on which their operation depended.

The first Klystron tubes were relatively short affairs, not unlike Lighthouse tubes. Electrons were directed through two annular rings, each externally connected through a resonant circuit made to alternate with fixed superhigh frequency. Streams of thermionic electrons approached the first ring and, while passing through it, induced an electrical wave pulse in the external circuit This pulse sped through the external circuit to the opposed ring. Electrons which reached the ring before this pulse were pushed into the anode. But those electrons caught between the rings were “bunched” or “clustred” back into a fixed volume. The coordinated alternations provided by the external circuit induced bunching throughout the normally steady electron streams. Each clustre arrived in the ring spaces at just the right time to induce very powerful SHF alternations.

As successful as this simple American made device became, it yet could not provide the sheer power required by the British engineers. Even after Klystron tube improvements had lengthened the electron path, while also internalizing and replacing the resonant circuit with a resonant copper cylinder, these tubes could not provide explosive SHF power. The chief advantage of Klystrons was and is their continuous operating characteristics which, provided moderately high power in continuous supply. Klystron tube improvement has now permitted the use of geometrically large tube geometries. Klystron tubes are miniature linear accelerators, often standing several feet in height. They are the transmitting tubes of choice in microwave arts today, using phase velocity bunching to produce successive electron pulses of great power. High current super short period successions are induced in their electron streams. Stable coordination between the alternations and the clustering effect occurs, the result being a steady pulsations of very powerful fixed SHF output.

Farnsworth “Multipactor” tubes were super powerful embodiments of UHF tubes, a rarity even in their time (1937). Invented by Dr. Philo Farnsworth to serve the transmission needs of his original and first television system, Multipactor Tube operation, depended on the photoelectric effect. All the more curious because they were electrical oscillators, the cold cathode tubes relied on the cascade release of photoelectrons produced by bombardment. Two large surface concave electrodes, coated with radioactive combinations of caesium, thorium, or even radium, produced incredible amounts of UHF power per unit volume tube space. Indeed, these devices steadily increased their efficiencies with continued use, becoming problematic to radio engineers who could never account for their virtually lossless conversion efficiencies. Multipactors used magnetic fields to restrained photoelectrons within the action space, one through which photoelectrons grew in density, while drifting through several successive cylindrical geometries. Strong UHF and SHF oscillations naturally began appearing when the external leads of these cylinders were connected with various lumped resonance circuits.

MAGNETRON RADAR

These yet unusual and highly efficient electron tubes could easily have been adapted to the production of SHF radio beacons, but remained unnoticed and unrecognized. The least likely tube for British consideration was the lowly “Magnetron” tubes, simple constructions which utilized axially applied magnetic fields in conjunction with a cylindrical anode (Hull, Hennelly 1921). The use of magnetic fields was predicated on an observation made when developing discharge switching tubes early in the century. It had been observed that magnetic fields frustrated and actually inhibited the approach of electrons to target anodes. By using a tangentially applied magnetic field, it was possible to actually “load” the space between cathode and anode with ever concentrated thermionic electrons. Electrons experienced side-thrusting because of the strong magnetic field applied along the tube axis. A simple adjustment in the voltage bias between cathode and anode balanced the forces on electrons in their space. Electrons then orbited the cylindrical anode but could be made never to reach the anode. The combinations of magnetic side thrust and the forward kinetic energy of electrons could produce such a high density “space charge” that the tubes began acting as capacitors.

In their initial embodiments, these tubes were useful as switching devices. They also found use as amplifiers for a variety of radio applications. Alexanderson of General Electric purloined the design for use in shortwave applications. Magnetron tubes were yet little more than curiosities, a means for providing high current alternations at shortwave broadcasting frequencies. With the experimental innovation of a split anode cylinder, new and wonderful phenomena were observed. Split along its axis, the normally solid cylindrical anode became the unexpected site where powerful SHF alternations were suddenly being generated. When pulsed with a powerful and sudden voltage application, their SHF output was significant. Furthermore such tubes proved stable over continued periods of time. The Magnetron Tube, once a mere curiosity of electrical science, suddenly revealed itself as the probable solution to the RADAR problem. Sidestepping the Farnsworth Mulfipactor, the Magnetron suddenly became the prime device on which British engineers focussed all of their attentions. Magnetron development was taken into the halls of Birmingham University, a top secret project headed by Robert Watson-Watt.