The Cavity Magnetron.
Copied from the UK Amateur Radio Newsgroup.
=================================== "Joe McElvenney" wrote Randall and Boot's cavity magnetron didn't really come onto the scene until about 1940. =================================== R & B's cavity magnetron was developed at Birmingham University in the midst of the air raids on that industrial city. We can imagine R & B having to hide under the workbench whenever a descending bomb was heard, culminating in a loud bang and broken glass. The ingenious device generated a peak pulse power of 50 kilowatts at 3000 MHz. Pulse repetition frequency 400 Hz. Pulse width 1 microsecond. So far as I can remember there were 6 or 8 cavities milled into the copper block. Alternate anodes surrounding the cathode, and close to it, were strapped together at their ends via copper bars. The block diameter was about 2" and about 1" thick. The magnet was a U-shape with pole-pieces which closely fitted the flat ends round the block such that the magnetic field was parallel to the cathode. Because the luftwaffer in 1940/1 had more bombers than the RAF, and in view of its potential as a war weapon, Churchill personally banned installation in RAF aircraft in case the top-secret device should be shot down over Germany and fall into the hands of German scientists and engineers. So Churchill handed the cavity magnetron to Roosevelt as a free gift in return for 50 rusty, old, WW1 destroyers. The manufacturing capacity of the US radio industry far exceeded that of the UK. Not to be outdone, the Americans soon produced a 10,000 MHz version. I first held one in my hands in 1945 by which time centimetric radar had been installed in RAF Catalina and Sunderland flying boats on convoy-escort duties in the Battle of the Atlantic. By 1945 German submarine crews were on suicide missions like kamikazi pilots, only 1 U-boat in 10 returned to base. There are more than 100,000 merchant ship and U-boat crew-members sharing Davy Jones locker at the silent bottom of the North Atlantic Ocean. Thus was the ferocity of the war. Once having detected a centimetric radar beam, and being accurately located themselves, submarine commanders preferred to remain on the surface, uncover the guns, and fight it out, day or night. During most of the war there had been little effect on German industrial production by RAF raids. Many bombs fell on open fields and sometimes killed cattle. But by 1944 RAF navigatigators had maps of rivers and cities laid before them. More than a 1000 heavy bombers, Lancasters, could be put into the air, night after night. With radar they couldn't miss whole cities and individual districts. Nevertheless on one occasion more than 100 bombers, complete with crews, failed to return to base. Such occurrences greatly exceeded the capacity of factories to produce them and to train aircrews. During the last 12 months of the war, radar equipped RAF bombers killed more German civilians than died in the concentration camps. Hiroshima and Nagasaki, which came shortly after, were just chicken feed. Air Marshal "Bomber" Harris was never knighted for services rendered. The main beneficiaries of R & B's invention of the cavity magnetron, done amid the high-explosives and incendiaries falling on Birmingam, have been the Japanese and other Far Eastern peoples who have manufactured many millions of cheap, reliable, microwave ovens. And of course the many millions of people like you and I who benefit from daily hot meals. I detest barbiques. There was held in the Kensington, London, Science Museum, the original prototype of the cavity magnetron without its magnet. It was in a securely locked mahogany and glass case and looked, as I recollect, like a small dirty can of baked beans with things sticking out of it. It may still be there. Makes a change from so-called SWR meters. ---- Reg, G4FGQ. |
On Sat, 24 Sep 2005 06:21:07 +0000 (UTC), "Reg Edwards"
wrote: Makes a change from so-called SWR meters. Ah Reggie! Hardly, SWR was the second most considered technical hurdle in the development of RADAR. 73's Richard Clark, KB7QHC |
Makes a change from so-called SWR meters. Ah Reggie! Hardly, SWR was the second most considered technical hurdle in the development of RADAR. 73's Richard Clark, KB7QHC ================================= Ah Rich!, Yet again you deliberately distort my meaning in your amusing game of 0ne-Upmanship. For the benefit of lurkers, there's a great difference between meters which purport to measure SWR at HF, but do no such thing and tell lies, and probes inserted in waveguides at 3 GHz which tell the truth. ---- Reg. |
Reg Edwards wrote:
Copied from the UK Amateur Radio Newsgroup. =================================== "Joe McElvenney" wrote Randall and Boot's cavity magnetron didn't really come onto the scene until about 1940. =================================== R & B's cavity magnetron was developed at Birmingham University in the midst of the air raids on that industrial city. We can imagine R & B having to hide under the workbench whenever a descending bomb was heard, culminating in a loud bang and broken glass. The ingenious device generated a peak pulse power of 50 kilowatts at 3000 MHz. Pulse repetition frequency 400 Hz. Pulse width 1 microsecond. So far as I can remember there were 6 or 8 cavities milled into the copper block. Alternate anodes surrounding the cathode, and close to it, were strapped together at their ends via copper bars. The block diameter was about 2" and about 1" thick. The magnet was a U-shape with pole-pieces which closely fitted the flat ends round the block such that the magnetic field was parallel to the cathode. Because the luftwaffer in 1940/1 had more bombers than the RAF, and in view of its potential as a war weapon, Churchill personally banned installation in RAF aircraft in case the top-secret device should be shot down over Germany and fall into the hands of German scientists and engineers. So Churchill handed the cavity magnetron to Roosevelt as a free gift in return for 50 rusty, old, WW1 destroyers. The manufacturing capacity of the US radio industry far exceeded that of the UK. Not to be outdone, the Americans soon produced a 10,000 MHz version. I first held one in my hands in 1945 by which time centimetric radar had been installed in RAF Catalina and Sunderland flying boats on convoy-escort duties in the Battle of the Atlantic. By 1945 German submarine crews were on suicide missions like kamikazi pilots, only 1 U-boat in 10 returned to base. There are more than 100,000 merchant ship and U-boat crew-members sharing Davy Jones locker at the silent bottom of the North Atlantic Ocean. Thus was the ferocity of the war. Once having detected a centimetric radar beam, and being accurately located themselves, submarine commanders preferred to remain on the surface, uncover the guns, and fight it out, day or night. During most of the war there had been little effect on German industrial production by RAF raids. Many bombs fell on open fields and sometimes killed cattle. But by 1944 RAF navigatigators had maps of rivers and cities laid before them. More than a 1000 heavy bombers, Lancasters, could be put into the air, night after night. With radar they couldn't miss whole cities and individual districts. Nevertheless on one occasion more than 100 bombers, complete with crews, failed to return to base. Such occurrences greatly exceeded the capacity of factories to produce them and to train aircrews. During the last 12 months of the war, radar equipped RAF bombers killed more German civilians than died in the concentration camps. Hiroshima and Nagasaki, which came shortly after, were just chicken feed. Air Marshal "Bomber" Harris was never knighted for services rendered. The main beneficiaries of R & B's invention of the cavity magnetron, done amid the high-explosives and incendiaries falling on Birmingam, have been the Japanese and other Far Eastern peoples who have manufactured many millions of cheap, reliable, microwave ovens. And of course the many millions of people like you and I who benefit from daily hot meals. I detest barbiques. There was held in the Kensington, London, Science Museum, the original prototype of the cavity magnetron without its magnet. It was in a securely locked mahogany and glass case and looked, as I recollect, like a small dirty can of baked beans with things sticking out of it. It may still be there. Makes a change from so-called SWR meters. ---- Reg, G4FGQ. Reg; Thank you for your report. And thank your country men for the many inventions that they have contributed to mankind. Dave, WD9BDZ |
On Sat, 24 Sep 2005 08:04:34 +0000 (UTC), "Reg Edwards"
wrote: Makes a change from so-called SWR meters. Ah Reggie! Hardly, SWR was the second most considered technical hurdle in the development of RADAR. 73's Richard Clark, KB7QHC ================================= Ah Rich!, Yet again you deliberately distort my meaning in your amusing game of 0ne-Upmanship. For the benefit of lurkers, there's a great difference between meters which purport to measure SWR at HF, but do no such thing and tell lies, and probes inserted in waveguides at 3 GHz which tell the truth. Ah Reggie, Yet again, you deliberately distort my meaning in your amusing game of One-Downmanship. For the benefit of lurkers, there's absolutely no difference between meters which purport to measure SWR at any frequency. You are simply fumbling around with one of your conceits, a troll in the lingua franca of the Internet. What you now describe was a flicker in time between bombs and crashing glass that was quickly discarded as an awkward technique when RADAR went into production. Such troglodyte methods were long gone before you even wrapped your mitts around a magnetron. If we pursue this with your absurd reductionist habit of arguing blind absolutes in place of practical reality (something Lord Kelvinator would sneer at as a foppish mannerism); then what you describe as "probes" are measuring nothing about SWR but are doing what any probe could accomplish: measuring a common unit of voltage, or current (and only by inference of the actual through rectification and filtering). The SWR only arrives by a second (or significantly more than two) reading, and then FURTHER only after various calculations. Even then, barring calculations (something no one does except squinty-eyed scientists and trolls), those same METERs employed were marked in SWR. Imagine, within very few months of RADAR emerging from the lab, SWR METERs ruled the production line, and the field kit. And to be sure, did they measure SWR? As much as any instrument and to your fulminating frustration, to no obvious difference that would be observed by Maxwell's demon (or Schrodinger's cat) craftily turned to this mischievously scientific validation. SWR arrived in its full glory of attention with RADAR. They were born simultaneously and absolutely no one gave a fig before on this topic. Further, it taught a generation of engineers the importance of matching production designs (which had been long inbred into the AC power production community - simply a rediscovery of a "truth" that had never been lost). This was probably because the consequence of SWR is so dramatic in the 100s of KW, when it occurs in the locality of the workbench in a system as small as the span of your arms. Even the Old Wives notice it if they, in error, try to microwave a product wrapped in a crumpled foil such as butter is wrapped. Their startled reaction evokes an immediate response, just as my post caused your knee to jerk reflexively beneath your apron. 73's Richard Clark, KB7QHC |
Rich, you sure have an extensive vocabulary.
But try as I can, I can't make any sense out of your long message about what can only be a trivial matter of your chosen ideas of gamesmanship. Kaput! I give up. ---- Yours, Punchinello, G4FGQ. |
Reg Edwards wrote:
Makes a change from so-called SWR meters. Ah Reggie! Hardly, SWR was the second most considered technical hurdle in the development of RADAR. 73's Richard Clark, KB7QHC ================================= Ah Rich!, Yet again you deliberately distort my meaning in your amusing game of 0ne-Upmanship. For the benefit of lurkers, there's a great difference between meters which purport to measure SWR at HF, but do no such thing and tell lies, and probes inserted in waveguides at 3 GHz which tell the truth. ---- Reg. Sorry, I don't see any difference between making voltage measurements with a directional coupler and calculating SWR through meter calibration and making voltage measurements on a slotted line and calculating SWR with a calculator or pencil and paper. Best I can tell is you are saying there is no such thing as a SWR meter. That's like saying there is no such thing as an airspeed meter in an airplane; the meter really measures air impact pressure. If that is your point, so what? -- Jim Pennino Remove .spam.sux to reply. |
On Sat, 24 Sep 2005 18:07:19 +0000 (UTC), "Reg Edwards"
wrote: But try as I can, I can't make any sense out of your long message Ah Reggie, As Dr. Johnson would paraphrase himself "claims of illiteracy is the last refuge of the troll." [Not to deny that you are in plenty of company - but you would shrug off that association.] This is notable in that you assert: about what can only be a trivial matter which, of course, means you understood enough not to be able to deny Lord Kelvinator harrumphing at your feigned attitude. It is an ill fitted cloak. of your chosen ideas of gamesmanship. This is the truly amusing part, you deliberately raised two topics (nothing had to be said about SWR meters, certainly - that injection is your trademark invitation), and you had two respondents answering to each of them. Even the sewer rats of Rio could see that you considered the more interesting topic as the one that you have now three times pursued. Such are the games being played, bucko! ;-) C'mon, if I hadn't responded you would have been sorely disappointed and would have had to sneer at David as an american suck-up trying to soothe an olde codger. You need a lightning rod to keep your current flowing and your response is the thanks I get. You're welcome, Old Son! 73's Richard Clark, KB7QHC |
Jim,
To measure SWR on the line it is necessary to place the SWR meter at the antenna end of the line. Even then it gives the correct answer only when the line impedance is 50 ohms. But the SWR meter is always placed immediately adjacent to the transmitter. Whatever the meter indicates it is not SWR because there is no line on which to measure it. The meter is telling lies. The meter indicates only whether or not the transmitter is loaded with a resistance of 50 ohms. Which is ALL you want to know. It tells you nothing more and nothing less. This is, of course, a very valuable function of the instrument. But it is NOT behaving as an SWR meter. Its name should be changed to Transmitter Loading Indicator (TLI). To use the name "SWR meter" and to imagine it is actually measuring an SWR is seriously misleading and is a source of confusion about what is really going on. It is why there are perpetual arguments and misunderstandings about SWR, tuners and related matters on this newsgroup and in every other place. Change the name to TLI, which is what it really does. Novices will not be lead astray, clear thinking will prevail, false ideas will not take root to remain embedded for the remainder of one's radio career. Air pressure indicators instead of airspeedometers are OK because air pressure actually exists. SWR meters are NOT OK because there is no line for SWR to exist on. (At least not where the meter is imagined or supposed to measure it.) Makes a change from cavity magnetrons. ---- Reg. |
On Sun, 25 Sep 2005 03:44:00 +0000 (UTC), "Reg Edwards"
wrote: Ah, Reggie, and Richie---- You two ought to go on the road together--your humor beats Bob Hope's hands down. You'd have em laughing their guts out in the aisles! Walt, W2DU |
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