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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 |
Reg Edwards wrote:
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. Well, duh. Anyone that doesn't know that SWR is with reference to a stated impedance or that SWR is influenced by the characteristics of a real transmission line just isn't paying attention. 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. Errr, no, the meter is telling what it sees at the point of measurement. If the measurer is so opaque that he/she doesn't take line influences into account, it is hardly the fault of the measuring instrument that what is reported is not the SWR of the antenna at the specified impedance. 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. Basically true given the stated conditions, and all that is probably of interest for the average ham. 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). Uttern nonsense; the instrument is still behaving as a SWR meter but the user is not applying it per spec and not correcting measurement error caused by line position. By this logic we have a lot of names to change. For starters: PAM has to change the name of their cooking spray to: Teenage looser get high in a can. Screwdriver manufacturers have to change the name of their product to: General prying instrument and paint can lid removal tool. You may add others. 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. Nonsense, the meter is always measuring SWR but the user is obviously not measuring the SWR that would be seen at the end of the line. You can't fault the instrument for it's misuse by the ignorant. What would you call a low impedance voltmeter used by some nimrod to measure voltage in a high impedance circuit? It is why there are perpetual arguments and misunderstandings about SWR, tuners and related matters on this newsgroup and in every other place. That is probably true since most people are opaque as to what goes on on a real transmission line, but not everyone is. 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. Clear thinking would demand that the influences of a real line on the observered SWR at an arbitrary point be explained. Air pressure indicators instead of airspeedometers are OK because air pressure actually exists. Non sequitur; SWR actually exists. Getting an accurate measurment is another issue and a matter of education. 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.) Nonsense. Makes a change from cavity magnetrons. More nonsense and not even a sentence. About the only difference between microwave and HF is that it is a lot easier to build a line, i.e. waveguide, that approximates a theoretical ideal lossless transmission line for reasonable distances at microwave than it is to build lossless coax as commonly used at HF. All the theory remains the same. Personally, I have never had any problem with understanding what it is that a SWR meter displays. ---- Reg. -- Jim Pennino Remove .spam.sux to reply. |
Ah, Reggie & Richie.
I tried SWRing once. My mother washed my mouth out with soap & water! |
Errr, no, the meter is telling what it sees at the point of
measurement. ===================================== But the meter is not seeing an SWR because an SWR does not exist. Where is the 50-ohm transmission line on which the SWR is imagined or supposed to lie? As you are unable to answer that question, the remainder of your argument (which, as I say, arises because of the SWR meter misleading misnomer) falls flat on its face. You are an intelligent person. I don't doubt you have no problems with understanding what the so-called SWR meter really indicates. But you didn't learn this from observations of the SWR meter - as you already know it tells lies! Just to reiterate, the so-called SWR meter indicates only whether or not the load on the transmitter is a resistive 50 ohms. If it is not 50 ohms it will not tell you what it actually is. Not that you need to know what it actually is because you will readjust your tuner, without thinking about it, to make it equal to 50 ohms. Which corresponds to no deflection of the meter needle. The TLI is a very useful and valuable device. It does not lead novices and old-wives (who ought to know better) into false ideas, or cause confusion and misunderstandings within the amateur fraternity. Professionals dismiss SWR for what it is worth anyway. Goodby to SWR except on lines where it matters and where it can be measured. Which, in practice, are very few. --- Reg, G4FGQ. |
Reg Edwards wrote:
Where is the 50-ohm transmission line on which the SWR is imagined or supposed to lie? On my system, there's a 50 ohm cable from the transceiver to the input of the SWR meter and another 50 ohm cable from the output of the SWR meter to the balun. Each of these cables forces the ratio of the voltage to current in each of the traveling waves to a value of 50 ohms. I have an in-line Autek WM-1 and no tuner. -- 73, Cecil http://www.qsl.net/w5dxp |
Reg Edwards wrote:
Errr, no, the meter is telling what it sees at the point of measurement. ===================================== But the meter is not seeing an SWR because an SWR does not exist. Nonsense. Where is the 50-ohm transmission line on which the SWR is imagined or supposed to lie? In my personal case, there is 50 Ohm transmission line between the transceiver and the SWR meter, then a length of line to a coax switch, then several lines out to antennas. You seem fixated on haveing some magical length of transmission line being necessary for a SWR to exist. This is nonsense. As you are unable to answer that question, the remainder of your argument (which, as I say, arises because of the SWR meter misleading misnomer) falls flat on its face. You are an intelligent person. I don't doubt you have no problems with understanding what the so-called SWR meter really indicates. But you didn't learn this from observations of the SWR meter - as you already know it tells lies! Babble. A SWR meter indicates what is. Knowing what the reading really means is a matter of education, not veracity. Just to reiterate, the so-called SWR meter indicates only whether or not the load on the transmitter is a resistive 50 ohms. If it is not 50 ohms it will not tell you what it actually is. Not that you need to know what it actually is because you will readjust your tuner, without thinking about it, to make it equal to 50 ohms. Which corresponds to no deflection of the meter needle. With some education and multiple measurements, you can caluclate the actual impedance if one desires. What tuner? What makes you believe everyone has a tuner? The TLI is a very useful and valuable device. It does not lead novices and old-wives (who ought to know better) into false ideas, or cause confusion and misunderstandings within the amateur fraternity. Professionals dismiss SWR for what it is worth anyway. The only one I see confused is you and professionals use SWR all the time and in many systems it is extremely important. Goodby to SWR except on lines where it matters and where it can be measured. Which, in practice, are very few. If one knows what they are doing, SWR can always be measured. --- Reg, G4FGQ. -- Jim Pennino Remove .spam.sux to reply. |
Jim wrote,
If one knows what they are doing, SWR can always be measured. =================================== NOT on a line which isn't there. QED. --- Reg. |
On Mon, 26 Sep 2005 16:20:14 +0000 (UTC), "Reg Edwards"
wrote: Jim wrote, If one knows what they are doing, SWR can always be measured. NOT on a line which isn't there. QED. Hi Reg, Which only proves Jim is correct, in that you reject the facility to do it with a line. My SWR meters do in fact have a 50 Ohm line transiting from the gozinta to the comesouta. Your poor choice of vendor, or poor solution implemented in design is your own problem, not that of the world's. I see that you still enjoy its mutilated discussion to the obvious neglect of magnetrons. Much better than flogging Kelvinator, I suppose. Now there's an image, you going nose to nose with him. He'd send you back to lower 6th for missing the bloody obvious. 73's Richard Clark, KB7QHC |
Reg Edwards wrote: Jim wrote, If one knows what they are doing, SWR can always be measured. =================================== NOT on a line which isn't there. QED. --- Reg. Perhaps this is a dumb question Reg, but if the transmission line isn't there, how does RF get from the transmitter to the antenna? Thanks, ac6xg |
Reg Edwards wrote:
Errr, no, the meter is telling what it sees at the point of measurement. ===================================== But the meter is not seeing an SWR because an SWR does not exist. Where is the 50-ohm transmission line on which the SWR is imagined or supposed to lie? Why do you keep insisting that SWR can only exhist on a 50-ohm transmission line. There are a multitude of impedances in transmission lines that are other than 50-ohms. SWR is present on every and any transmission line that doesn't match the transmission element perfectly. Dave WD9BDZ As you are unable to answer that question, the remainder of your argument (which, as I say, arises because of the SWR meter misleading misnomer) falls flat on its face. You are an intelligent person. I don't doubt you have no problems with understanding what the so-called SWR meter really indicates. But you didn't learn this from observations of the SWR meter - as you already know it tells lies! Just to reiterate, the so-called SWR meter indicates only whether or not the load on the transmitter is a resistive 50 ohms. If it is not 50 ohms it will not tell you what it actually is. Not that you need to know what it actually is because you will readjust your tuner, without thinking about it, to make it equal to 50 ohms. Which corresponds to no deflection of the meter needle. The TLI is a very useful and valuable device. It does not lead novices and old-wives (who ought to know better) into false ideas, or cause confusion and misunderstandings within the amateur fraternity. Professionals dismiss SWR for what it is worth anyway. Goodby to SWR except on lines where it matters and where it can be measured. Which, in practice, are very few. --- Reg, G4FGQ. |
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In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote: If one knows what they are doing, SWR can always be measured. NOT on a line which isn't there. QED. In the strong sense of the definition ("A VSWR meter is a meter which measures, literally, the ratio between the voltage maxima and minima present on a transmission line"), Reg is correct. This can be done with a section of slotted line and a probe, of course, tapped into a section of a transmission line having the same characteristic impedance. That's not how a typical amateur "SWR" meter works, though - it's not either locating or measuring either the maxima or the minima of either voltage or current on the line. So, the strong, literal, pedantic sense of the term, I agree with Reg that a standard "SWR" meter is not truly measuring SWR, and that he's correct in his objection. However, I also think he's overstating the case. An "SWR" meter circuit, in the usual sense (e.g. a Monimatch or similar) can provide an accurate *indirect* measurement of SWR, *if* the conditions under which it is used are appropriate. It measures the currents and voltages flowing through it, and derives (electrically and mathematically) a number which correlates extremely well to what a true SWR measurement on a T-line of specified impedance would say. If you build and calibrate this sort of circuit accurately enough, and then put it in the middle of a section of 50-ohm line (or whatever it's calibrated for), it'll give the same numbers as a true, direct measurement of VSWR on a length of T-line at that point. Now, it's true (as Reg says) that a Monimatch or similar indirect-measurement meter can give you inaccurate or misleading numbers, if used in an environment other than what it's designed for. If you stick a Monimatch at the output of a transmitter or transmatch, with its output looking into a high-impedance balanced line, then the numbers it displays won't equal the actual VSWR on the line. You might be able to come up with a correction factor / curve for it, though. If you stick it right at the transmitter output, and it reads 1.0:1, then you can be confident that your transmitter is seeing the load that it wants to see... hence Reg's desire to have it renamed as a "TLI". Seems to me, though, that the same is true of a real "VSWR" measurement system (e.g. a slotted line) if you use it under inappropriate test conditions. If you take a slotted-line-and- probe measurement device whose internal line is 50 ohms, and stick it in the middle of a 75-ohm line, and measure the VSWR on your slotted line, you'll come up with a number which *does* equal the VSWR in the slotted line but does *not* equal the VSWR on the actual transmission line. Same problem, really. In that sense, even a "real" VSWR meter isn't a "useful" VSWR meter, if you use it inappropriately. All that being said: I conclude that there's nothing wrong with calling a Monimatch (or similar) current/voltage measurement circuit a "SWR" meter, as it *will* display correct and accurate numbers for the SWR on the line when used appropriately (within the limits of its calibration, of course). It's up to the user to understand the conditions under which this sort of measurement can be made accurately and usefully... just as it is with every other sort of test instrument I know of. Reg, I think you're tilting at windmills. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Jim Kelley wrote:
Reg Edwards wrote: Jim wrote, If one knows what they are doing, SWR can always be measured. =================================== NOT on a line which isn't there. QED. --- Reg. Perhaps this is a dumb question Reg, but if the transmission line isn't there, how does RF get from the transmitter to the antenna? Thanks, ac6xg Well there's wave guide, there's printed circuit traces, there's wire. OPPS.... those are transmission lines. Dave WD9BDZ |
Perhaps this is a dumb question Reg, but if the transmission line isn't there, how does RF get from the transmitter to the antenna? Thanks, ac6xg ============================= The 50-ohm line on which the SWR is supposed to be measured is between the transmitter and the so-called SWR meter. If the transmitter is connected directly to the meter, (as it usually is, very often it is inside the transmitter on the front panel) there is no line and no SWR on it which can be measured. Neverthess, the meter still provides a reading of SWR. Obviously it is telling lies and causes confusion and misunderstanding to novices about what is really happening within their equipment. In fact, just as it has been doing since Joker, Richard Clark, entered the thread. Some people force themselves to imagine a 1/4-wavelength transmission inside the little box in a vain endeavor to explain how an SWR meter works. Actually the meter indicates whether or not the transmitter is loaded with 50-ohms - and nothing else. It is an HF resistance bridge to which its circuit reduces. But, as I say, it is a very useful, almost indispensible instrument. It is a TLI. ---- Reg. |
Reg Edwards wrote: Perhaps this is a dumb question Reg, but if the transmission line isn't there, how does RF get from the transmitter to the antenna? Thanks, ac6xg ============================= The 50-ohm line on which the SWR is supposed to be measured is between the transmitter and the so-called SWR meter. If the transmitter is connected directly to the meter, (as it usually is, very often it is inside the transmitter on the front panel) there is no line and no SWR on it which can be measured. What you're saying is that there's a line connecting the transmitter to the meter, but there isn't a line connecting the transmitter to the meter. I'd have difficulty defending such a position. Neverthess, the meter still provides a reading of SWR. Obviously it is telling lies and causes confusion and misunderstanding to novices about what is really happening within their equipment. In fact, just as it has been doing since Joker, Richard Clark, entered the thread. Some people force themselves to imagine a 1/4-wavelength transmission inside the little box in a vain endeavor to explain how an SWR meter works. Actually the meter indicates whether or not the transmitter is loaded with 50-ohms - and nothing else. I don't think the meter really knows what impedance the transmitter is loaded with. As far as I can tell, it only knows how many volts and amps gozinta one port and/or compared with how many volts and amps gozinta the other port. But I agree that some people read an awful lot into the measurements they make with one of these devices. It is an HF resistance bridge to which its circuit reduces. True enough. But, as I say, it is a very useful, almost indispensible instrument. It is a TLI. I like to think of it more as a reflectometer. 73 de ac6xg |
On Mon, 26 Sep 2005 19:49:00 +0000 (UTC), "Reg Edwards"
wrote: In fact, just as it has been doing since Joker, Richard Clark, entered the thread. Aw Reggie, You are getting emotional because you were caught spitting on Kelvinator and telling us it was the London fog. 73's Richard Clark, KB7QHC |
I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines. All I have said, somewhere, is that the usual SWR meter gives the correct answers only on 50-ohm lines. ---- Reg. |
Reg Edwards wrote:
Perhaps this is a dumb question Reg, but if the transmission line isn't there, how does RF get from the transmitter to the antenna? Thanks, ac6xg ============================= The 50-ohm line on which the SWR is supposed to be measured is between the transmitter and the so-called SWR meter. No, the SWR being measured is on the load side of the meter. If the transmitter is connected directly to the meter, (as it usually is, very often it is inside the transmitter on the front panel) there is no line and no SWR on it which can be measured. Bzzzt, wrong answer. SWR is measured on the output side of the meter, not the input. Neverthess, the meter still provides a reading of SWR. Obviously it is telling lies and causes confusion and misunderstanding to novices about what is really happening within their equipment. In fact, just as it has been doing since Joker, Richard Clark, entered the thread. Some people force themselves to imagine a 1/4-wavelength transmission inside the little box in a vain endeavor to explain how an SWR meter works. That would be inane. All one need do is realize the meter measures toward the load. Actually the meter indicates whether or not the transmitter is loaded with 50-ohms - and nothing else. It is an HF resistance bridge to which its circuit reduces. But, as I say, it is a very useful, almost indispensible instrument. It is a TLI. ---- Reg. -- Jim Pennino Remove .spam.sux to reply. |
Reg Edwards wrote:
I have not insisted, not even mentioned, that SWR can exist only on 50-ohm lines. All I have said, somewhere, is that the usual SWR meter gives the correct answers only on 50-ohm lines. ---- Reg. If I connect a 100 Ohm antenna through a 100 Ohm transmission line to a SWR meter designed for 50 Ohms and then to a transmitter which expects a 50 Ohm load, does the meter read correctly with respect to the desired transmitter loading? Of course it does. -- Jim Pennino Remove .spam.sux to reply. |
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Owen Duffy wrote:
On Mon, 26 Sep 2005 20:38:01 +0000 (UTC), wrote: Reg Edwards wrote: I have not insisted, not even mentioned, that SWR can exist only on 50-ohm lines. All I have said, somewhere, is that the usual SWR meter gives the correct answers only on 50-ohm lines. ---- Reg. If I connect a 100 Ohm antenna through a 100 Ohm transmission line to a SWR meter designed for 50 Ohms and then to a transmitter which expects a 50 Ohm load, does the meter read correctly with respect to the desired transmitter loading? Of course it does. Jim, that seems inconsistent with your earlier statemetn "No, the SWR being measured is on the load side of the meter." The load side is the side with the load, i.e. the antenna, on it. In the example you quoted with a 100 ohm load on a 100 ohm line, were the line loss low, and the line long enough to be sure to sample a fully developed voltage maximum and voltage minimum it would be found that the VSWR was 1:1. Not for a 50 Ohm system, i.e. a transmitter expecting 50 Ohms and a meter calibrated for a 50 Ohm system. Yet we would expect the "SWR meter designed for 50 Ohms" to which it is connected (on the load side) to read VSWR=2:1, so is it measuring the SWR on the load side of the meter as you earlier stated? Yep. Perhaps typical SWR meters are actually sampling current and voltage on a very short section (almost a point sample) of transmission line that is part of the instrument, and they are indicating what the observed SWR would be on an extended line of that type, provided that attenuation was insignificant, and that the extended line was sufficiently long to allow full development of the standing wave pattern. The SWR that most people care about is that of the antenna. Usually you have a 50 Ohm transmitter connected to 50 Ohm line, and then to an antenna you hope is 50 Ohms. To accurately measure the antenna SWR without any error being introduced by line losses, you have to put the SWR meter at the end of the line adjacent to the antenna. This is usually impractical and we normally put the SWR meter near the transmitter. In this case the meter measures the SWR of the entire system, i.e. the line going from the meter to the antenna and the antenna. The net practical effect of the line loss for real line and real antennas is that the observed SWR will indicate a lower value than if the meter were directly connected to the antenna. The inference of what is happening on adjacent line is ours, not the instruments, as demonstrated by your example above. Owen -- -- Jim Pennino Remove .spam.sux to reply. |
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Jim says,
No, the SWR being measured is on the load side of the meter. ========================================= Jim, I have difficulty in telling you this without hurting your feelings. Or perhaps you are just joking and pulling my leg. Try the other one - its got bells on. I will assume you are not joking. You clearly havn't the foggiest idea of how the so-called SWR meter works. Although you may be in good company - including not a few professional engineers who have picked up old-wives' tales from radio amateurs. And have been led astray by the meter being called something which it isn't. The meter indicates SWR when placed at the ANTENNA end of the line. It is the antenna which does the terminating. And the meter gives the correct answers only when the line Zo = 50 ohms. If there is a tuner or matching network in the line then the meter will indicate SWR on the line between the transmitter and the tuner - provided the meter is placed next to the tuner. When there is no appreciable length of line between transmitter and tuner (as is the very common case) there is no line on which to measure SWR. But the meter will indicate an SWR - and tells lies. But there's nothing to worry about just because the meter is indicating SWR nonsense. The meter is telling you what you really want to know, its exactly why you placed it there - whether or not the transmitter is loaded with 50 ohms. ---- Reg. |
"Owen Duffy" wrote - Your example demonstrates that a typical SWR meter does not measure, or necessarily indicate the SWR of the (actual) transmission line on the load side of itself. ---------------------------------------------------------------------- ---------- You must have been reading what I've been writing for the last 6 or 7 years. ---- Reg. |
On Tue, 27 Sep 2005 00:24:51 +0000 (UTC), "Reg Edwards"
wrote: You must have been reading what I've been writing for the last 6 or 7 years. Reg, Yes, I have probably read and learned a good deal from stuff you have written, but I have skipped over a lot of what you have written... probably most noticeably when you and others trade kicks to the groin to see who is the last man standing. Back on topic: A point that you hinted at, but might have been overlooked by some is that it can be relatively unimportant that the SWR bridge's sampling line has the same characteristic impedance as the impedance at which its detector has been nulled. For example, a typical SWR meter designed originally for 75 ohms, with a 0.1m long ideal 75 ohm sampling section, but with the detector adjusted to read nil reflected power with a 50+j0 load on the "antenna" terminals of the meter, will in most cases operate just as well as a 50 ohm SWR meter on 7MHz, as the detector will truly show when it has a 50 ohm load, the indicated VSWR for other loads will substantially correct (ie within typical accuracy for the type of instrument), and the insertion VSWR (~1.02:1) because of the 0.1m of 75 ohm line will be insignificant in practice. In many amateur reflectometer designs (and in some commercial implementations), very little attention has been given to the characteristic impedance of the sampling section, and in some cases to the insertion VSWR (that results). I recall testing a relatively expensive SWR meter rated from 1.8 to 150MHz, and noting that whilst it indicated a VSWR1.1 at 144MHz on a good dummy load a Bird 43 ahead of it indicated an insertion VSWR 1.5:1. So whilst it was good at indicating a 50+j0 ohm load on its "antenna" terminals, it was not very capable of delivering that load to its "transmitter" terminals. Owen -- |
"Cecil Moore" wrote On my system, there's a 50 ohm cable from the transceiver to the input of the SWR meter and another 50 ohm cable from the output of the SWR meter to the balun. Each of these cables forces the ratio of the voltage to current in each of the traveling waves to a value of 50 ohms. I have an in-line Autek WM-1 and no tuner. -- ==================================== Cec, You can't measure SWR on a line which is less than 1/4-wave long. Preferably it should be as long as 1/2-wavelength to ensure the max and min voltage points both occur on it. But what do you do with the SWR when you have measured it? Of what use it? All anybody needs to know is whether the transmitter is terminated or is not terminated with 50-ohms. This is very important. And when describing to a novice how your station works there's no need to mention SWR or reflections. They are quite unecessary complications and can be dispensed with except perhaps for trolling on newsgroups. ---- Reg. |
Reg Edwards wrote:
Jim says, No, the SWR being measured is on the load side of the meter. ========================================= Jim, I have difficulty in telling you this without hurting your feelings. Or perhaps you are just joking and pulling my leg. Try the other one - its got bells on. Meaningless babble. I will assume you are not joking. You clearly havn't the foggiest idea of how the so-called SWR meter works. Although you may be in good company - including not a few professional engineers who have picked up old-wives' tales from radio amateurs. And have been led astray by the meter being called something which it isn't. I assure you I have a very good understanding of how SWR meters in their various incantations work and how SWR works. The meter indicates SWR when placed at the ANTENNA end of the line. It is the antenna which does the terminating. And the meter gives the correct answers only when the line Zo = 50 ohms. A SWR meter indicates the SWR of whatever is connected to the load side of the meter relative to the reference impedance the meter is designed for. It matters not if the thing connected to the load side of the meter is an antenna, an antenna with a transmission line, a transmission line with a dummy load on the end, or two popsicle sticks and a bubble gum wrapper. If you are only interested in the SWR of the antenna and don't, or can't, correct for the effects of real transmission line in between the meter and the antenna, then you have to connect the antenna to the load end of the meter directly. If there is a tuner or matching network in the line then the meter will indicate SWR on the line between the transmitter and the tuner - provided the meter is placed next to the tuner. Tuners have nothing to do with the issue. You can put a 1956 Buick on the load side of the meter; you still read SWR of the system at the point of measurement. It is you that is trying to confuse the issue with arm waving. When there is no appreciable length of line between transmitter and tuner (as is the very common case) there is no line on which to measure SWR. But the meter will indicate an SWR - and tells lies. Babbling nonsense. But there's nothing to worry about just because the meter is indicating SWR nonsense. The meter is telling you what you really want to know, its exactly why you placed it there - whether or not the transmitter is loaded with 50 ohms. That is not what I would want to know. I know the output impedance of the transmitter and the line (assuming it is good) impedance. What I want to know is the antenna SWR and the measurement of that is trivial though it seems to have you totally confused. ---- Reg. -- Jim Pennino Remove .spam.sux to reply. |
Owen Duffy wrote:
On Mon, 26 Sep 2005 21:39:27 +0000 (UTC), wrote: Jim, that seems inconsistent with your earlier statemetn "No, the SWR being measured is on the load side of the meter." The load side is the side with the load, i.e. the antenna, on it. In the example you quoted with a 100 ohm load on a 100 ohm line, were the line loss low, and the line long enough to be sure to sample a fully developed voltage maximum and voltage minimum it would be found that the VSWR was 1:1. Not for a 50 Ohm system, i.e. a transmitter expecting 50 Ohms and a meter calibrated for a 50 Ohm system. I am sorry Jim, the VSWR is a property of the transmission line and its termination, and the VSWR on that 100 ohm line with a 100 ohm termination is 1:1. The VSWR could be *MEASURED* on that line by sampling the magnitude of the voltage at different points on the line and it would be found that the magnitude of the voltage was constant, which means VSWR=1:1. No, the measured SWR is relative to the design impedance of the SWR meter which is normally 50 Ohms. If you use a 100 Ohm SWR meter you get 1:1. The SWR is a function of the TOTAL SYSTEM impedance connected to the load side. Replace the 100 antenna with a 100 Ohm resistor and the reading doesn't change. Eliminate the line and connect the 100 Ohm resistor directly to the meter and the reading doesn't change. Yet we would expect the "SWR meter designed for 50 Ohms" to which it is connected (on the load side) to read VSWR=2:1, so is it measuring the SWR on the load side of the meter as you earlier stated? Yep. No, it isn't. The SWR meter in your example reads 2:1 when the SWR on the 100 ohm line is 1:1. Your example demonstrates that a typical SWR meter does not measure, or necessarily indicate the SWR of the (actual) transmission line on the load side of itself. The SWR of the SYSTEM, line and antenna, is NOT 1:1 for a 50 Ohm reference. Owen -- -- Jim Pennino Remove .spam.sux to reply. |
Owen Duffy wrote:
On Tue, 27 Sep 2005 00:24:51 +0000 (UTC), "Reg Edwards" wrote: You must have been reading what I've been writing for the last 6 or 7 years. Reg, Yes, I have probably read and learned a good deal from stuff you have written, but I have skipped over a lot of what you have written... probably most noticeably when you and others trade kicks to the groin to see who is the last man standing. Back on topic: A point that you hinted at, but might have been overlooked by some is that it can be relatively unimportant that the SWR bridge's sampling line has the same characteristic impedance as the impedance at which its detector has been nulled. For example, a typical SWR meter designed originally for 75 ohms, with a 0.1m long ideal 75 ohm sampling section, but with the detector adjusted to read nil reflected power with a 50+j0 load on the "antenna" terminals of the meter, will in most cases operate just as well as a 50 ohm SWR meter on 7MHz, as the detector will truly show when it has a 50 ohm load, the indicated VSWR for other loads will substantially correct (ie within typical accuracy for the type of instrument), and the insertion VSWR (~1.02:1) because of the 0.1m of 75 ohm line will be insignificant in practice. In many amateur reflectometer designs (and in some commercial implementations), very little attention has been given to the characteristic impedance of the sampling section, and in some cases to the insertion VSWR (that results). I recall testing a relatively expensive SWR meter rated from 1.8 to 150MHz, and noting that whilst it indicated a VSWR1.1 at 144MHz on a good dummy load a Bird 43 ahead of it indicated an insertion VSWR 1.5:1. So whilst it was good at indicating a 50+j0 ohm load on its "antenna" terminals, it was not very capable of delivering that load to its "transmitter" terminals. Owen -- Inaccurate crap equipment has nothing to do with the arguement. -- Jim Pennino Remove .spam.sux to reply. |
Reg Edwards wrote:
You can't measure SWR on a line which is less than 1/4-wave long. Because your measurements are taken 1/4WL apart, your SWR value will be in error for all feedlines except lossless ones. SWR can be had at any point on a transmission line. Measure the forward power and reflected power at any point. SWR = [SQRT(Pfor)+SQRT(Pref)]/[SQRT(Pfor)-SQRT(Pref)] The transmission line length must only be long enough such that the V/I ratio is forced to the Z0 value. According to some pretty smart guys I asked, that's about 2% of a wavelength. -- 73, Cecil http://www.qsl.net/w5dxp |
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