The fixst major development of Super High Frequency tubes came with the needs of World War II England. The heavy bombing raids on English soil, the early attempt by the Luftwaffe of Adolf Hitler to crush that nation, was resisted through an initially meagre scientific resource. Only the indomitable fortitude of brave souls fighting impossible odds proved the turning of a tide, in both revolutionary science and in the eradication of tyranny. The two seemed closely associated in that the rise of science spelled defeat for fascism. The only available technology for the “early warning” of Nazi air raids were acoustic listening systems, technology adapted fi'om the scientific amusement pages of Victorian Epoch periodicals. Though developed into a wonderful state of perfection, these large multiple concave cup systems mounted on swivels, could not provide the fast warning capabilities so desperately sought The only other tools for early warning were the and fog penetrating arclamps, yet another science which had been perfected before the prior Century’s turn. The challenge stood before those whose tasks were now doubled, fighting the foe while simultaneously producing new technology.
Those who at last had begun enjoying peace were not concerned with the outlandish threat of a Second World War. Nevertheless as guided missiles became the vengeful expressions of lunacy, the absolute need for readiness, for new technology to defeat and utterly obliterate the Nazi foe, commenced. An enemy unseen is a frightful prospect to defeat An enemy seen is not threatening. Soaring at supersonic speeds across the English Channel, V-1 “buzz bombs”, and later V-2 missiles, required an alert ready system. Having an abihty to reach targets miles out over the sea, long before they reached the coast. A new radiobeam technology was desperately sought Those who recalled the UHF “radio searchlight” of Christian Hulsmeyer attempted duplication and adaptation of the same for their more contemporary and immediate needs. Science again was coming to the needs of an assailed people.
By August 1938, North Sea and Channel coasdines were watched by a chain of “metric” (UHF) radar stations. After observing the initial experimental results and military effectiveness of the first simple “longwave” RADAR systems, engineers were encouraged to pursue their objectives with neither delay nor diminished effort. What began with UHF beams soon shifted the emphasis toward the development of a SHF beam system for long distance ranging and detection. The essential problem facing these engineers was that of power. The effectiveness of the system depended upon the intensity of reflected energies. The extensive sea-searching beams now required both higher resolution as well as extremely high power.
SHF beams were relatively more “optical” than the UFIF metric waves, being required precisely because of their high resolving properties. Bounced fi-om metal surfaces, these echoes contained inherent detail which could be discriminated by appropriate scanning detectors, at first a simple oscilloscope. The engineering problems demanded the generation of these super high fi-e-quencies. Additionally, these SHF signals had to be enormous in strength. The extensive sea-searching beacons, required by a coastal watch along both the North Sea and the Channel, demanded power levels of a megawatt per system. But those UHDF and SHF vacuum tubes of the 1920’s, considered novelties of the electronic tube trade, were not made to produce powerfid signals at these frequency levels. Electron currents had to release enormous amounts of SHF energies, and supply this amount of power in a continuous reserve. The operation characteristics of any potential RADAR power source were already theoretically severe.
Superhigh frequency vacuum tubes of this era may be classified into three succinct categories. Notable in this first expansion of frequencies upward were the early “lighthouse” and “acorn” tubes, which attempted the generation of SHF across the smallest possible vacuum gaps. These simple and straightforward designs, ncuned for their actual appearance, were analogues of triodes. These tiny embodiments sought the achievement of UHF and SHF energies in an analogous manner as triodes, which used resonant circuits to build up powerful self-regenerating alternations. The chief importance of these early tubes was that they permitted the study of UHF and SHF energies, radio frequencies so very high that they could only be measured in terms of wavelength! The problem with SHF alternations was their small resonant product values. There were geometric reasons why certain radio frequencies became the most powerful expressions of the art, reasons which successfully produced the most powerful communication embodiments. These tubes could employ neither the large capacitor surfaces nor inductors which, in triodes, produced the most powerful high frequency alternations.
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 significantly 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 electriccil 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.
Multipactor Oscillator and Amplifier
World History
