|
I am sorely tempted but will refrain from calling the whole lot of you a
bunch of old wives. Kindly desist in trying to put words into my mouth. I have NOT said that changing the source impedance changes the SWR indication. It doesn't ! I HAVE said that if the line is NOT 50 ohms then the SWR reading, even if it doesn't change, BECOMES GROSSLY INCORRECT ! So "in practice", even if there IS a line of some sort, as you don't know what the line impedance or generator impedance is, then the SWR indication and the reflection coefficient from which it is derived are MEANINGLESS, USELESS ! As Tarmo must by now be exhausted in repeating, and I have been saying for years, all the meter tells you is whether or not the complex load on the transmitter deviates in some unknown diirection from a purely resistive 50 ohms. That is what the thing is there for. Which is all anybody needs to know anyway. Even to mention SWR in the present context dis-orients novices and earners - which we all were at some time or other. So change the name of the instrument to TLI. It's not a meter anyway. I also have reservations about so-called reflected power. But that's another matter. ---- Regards, Reg, G4FGQ |
If the reflected and forward powers are known and the Bird is a short or
zero distance from the transmitter, why would anybody want to know the reflection coefficient and SWR anyway? Of what use are they except to use the graphical calculator to calculate the forward and reflected powers? It seems the Bird designer was wise enough to omit the SWR scale from the meter. --- Reg G4FGQ |
Reg`s statement:
"But it DOES affect the indicated SWR and so the indicated SWR is incorrect." does not apply to the Bird Model 43 wattmeter. =========================== I've never seen one but there's nothing special about a Bird. It behaves exactly the same as those from other manufacturers. If the line between generator and meter is NOT 50 ohms then the actual SWR on that line is NOT the value indicated by the meter. As I said, the indicated SWR is incorrect. It is important to distinguish between indicated and actual SWR's. It is fatal to worship the meter as being error free and so make the same incorrect assumptions as the meter does. ---- Reg, G4FGQ |
Reg Edwards wrote:
I am sorely tempted but will refrain from calling the whole lot of you a bunch of old wives. Kindly desist in trying to put words into my mouth. I have NOT said that changing the source impedance changes the SWR indication. It doesn't ! . . . I posted: Source impedance DOES affect the amount of energy moving in and sloshing around in a transmission line. It DOESN'T affect the ratio of forward to reflected waves, and therefore DOESN'T affect the SWR. To which you replied: =========================== But it DOES affect the indicated SWR and so the indicated SWR is incorrect. and: If the source is not what the meter expects then it gives the wrong answers. And its faithful worshippers believe it! =========================== End of quote Better check on the worm-wood content of that wine you've been drinking. Another tip is to save a copy of each message you post. Then when you encounter that blank spot in your memory, you can at least read what you posted the night before. Sad, isn't it, when the Old Wives can remember stuff you wrote better than you can. Roy Lewallen, W7EL -- Again confirmed as a Reg's Old Wife |
"Ian White, G3SEK" wrote
Everybody knows that the meter only claims to indicate rho/SWR on its "downstream" (load, antenna) side. It doesn't claim to say or know anything about rho/SWR on the upstream (generator, TX) side. =============================== Ian, there must by a dictation error in the foregoing. Would you care to correct it ? However, it must be admitted, when the only transmission line at the station is between meter and the antenna such a mistake must be fairly common amongst the ignorant. ---- Reg, G4FGQ |
Better check on the worm-wood content of that wine you've been drinking.
Another tip is to save a copy of each message you post. Then when you encounter that blank spot in your memory, you can at least read what you posted the night before. Sad, isn't it, when the Old Wives can remember stuff you wrote better than you can. Roy Lewallen, W7EL -- Again confirmed as a Reg's Old Wife ============================= Dear Roy, a display of annoyance signifies weakness of argument. ---- Reg |
Reg Edwards wrote:
"Ian White, G3SEK" wrote Everybody knows that the meter only claims to indicate rho/SWR on its "downstream" (load, antenna) side. It doesn't claim to say or know anything about rho/SWR on the upstream (generator, TX) side. =============================== Ian, there must by a dictation error in the foregoing. Would you care to correct it ? No - I meant exactly what I said. The meter can only indicate the rho/SWR of whatever is connected downstream (load side) of the meter itself. Whatever is upstream is merely an RF source. Both the forward and reflected readings are determined by the power level of the source (obviously) but they are both affected in the same proportion, so the rho/SWR result stays the same - as it must, because rho/SWR is only affected by downstream conditions. However, it must be admitted, when the only transmission line at the station is between meter and the antenna such a mistake must be fairly common amongst the ignorant. We seem to be talking past each other, Reg, not to each other. Since neither of us is "ignorant", each of us must be saying something that the other one is missing. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
"Ian White, G3SEK" wrote -
No - I meant exactly what I said. The meter can only indicate the rho/SWR of whatever is connected downstream (load side) of the meter itself. ================================== (1) I'm sure we are agreed our meters will correctly indicate Rho and SWR only on 50-ohm lines. (2) Insofar as the meter is concerned the transmitter's load impedance is the input impedance of the transmission line between the meter and the antenna. There may or may not be an intervening Z-match network. (3) Insofar as the transmitter is concerned the line between meter and antenna can be ANY impedance. It is desirable only that line length with its Zo transform the antenna input impedance to somewhere near to 50 ohms (Like a G5RV on 14.15 MHz). If things become difficult then a Z-match can be inserted. Once we have selected Zo for this line we are no longer interested in its SWR. And if we WERE interested a 50-ohm SWR meter would be incapable of correctly measuring it. (4) Note that when a Z-match is located at the transmitter end of this feedline, and varied, the actual SWR on this line cannot change - yet the SWR meter responds readily to the Z-match settings. (5) The only line which, ideally, MUST be 50 ohms coax and have a small SWR, certainly if it is of appreciable length, is that between the meter and the transmitter. Otherwise the load directly presented to the transmitter would not be 50 ohms. Any other impedance would transform the 50 ohms seen immediately on the antenna side of the meter to some other value. (6) It is the SWR on this line which the meter indicates. (If this line is NOT 50-ohms then the meter incorrectly indicates the standing waves on it. Which is what I said before and a lot of people disagreed. Not that a false indication is of great consequence when it is the incorrect choice of line Zo where the problem arises.) (7) In practice at HF the length of this 50-ohm coax is often negligible. The meter is often inside the transmitter box. As the misleading idea of standingwaves on this short, even zero-length coax is nonsense the name of the instrument should be changed to TLI. (Transmitter Loading Indicator). Which is all that it is! ( 8) Then the topic of conversation on this newsgroup can then diverted back to where it belongs - the far more important non-measured SWR on the main feedline to the antenna. ---- Reg, G4FGQ |
To anybody interested.
We have a HF Transmitter + 50-ohm coax + SWR meter + Tuner + Feedline of any Zo + Antenna. Suppose it is all tuned-up and ready to go. The transmitter is loaded with exactly 50-ohms resistive. Now change the 50-ohm coax to shorter length of 75-ohm Zo. As everybody agrees (after perhaps a little meter recalibration) the SWR meter indication will not change. BUT THE TRANSMITTER WILL NOW BE INCORRECTLY LOADED. Where does the inconistency lie ? Does it lie in the change in effective source impedance? |
Reg Edwards wrote:
However, it must be admitted, when the only transmission line at the station is between meter and the antenna such a mistake must be fairly common amongst the ignorant. If I read what Richard said correctly, the Bird has a built-in 50 ohm transmission line. And what about the coax connection from the source to the Bird? The guys over on sci.physics.electromag said that two feet of 50 ohm coax guarantees a 50 ohm environment for a wattmeter. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Reg Edwards wrote:
Dear Roy, a display of annoyance signifies weakness of argument. Reg, do you ever display annoyance? :-) -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Reg Edwards wrote:
Where does the inconistency lie ? Does it lie in the change in effective source impedance? It lies in using the wrong coax Z0 for the situation. Assuming it's long enough to develop 75 ohms, it will cause 4% of the forward power to be reflected back toward the source at the meter. Guess you can't call it a TLI after all since it is faithfully reporting the 50 ohm SWR and not making the transmitter happy. :-) -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"Cecil Moore" wrote Dear Roy, a display of annoyance signifies weakness of argument. Reg, do you ever display annoyance? :-) ================================ Yes Cec, but you can't hear the sound of banging doors and breaking glass over the internet. Reg. |
Richard Clark wrote:
Observe how in another thread, a quarterwave dipole will exhibit no voltage drop because it is metallic along its length (never mind the inline radiation loss). Consider that if it were terminated with resistors that cause no reflections, it would be a traveling wave antenna and the voltage drop would indeed be very low in spite of the radiation. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Sat, 18 Oct 2003 13:57:22 -0500, Cecil Moore
wrote: Consider that if How charming.... |
On Sat, 18 Oct 2003 13:09:42 -0500, Cecil Moore
wrote: The guys over on sci.physics.electromag said that two feet of 50 ohm coax guarantees a 50 ohm environment for a wattmeter. At DC? 100Hz? 10KHz? ... .... 10GHz? 1THz? Poor bounding as usual. |
"Cecil Moore" wrote Reg Edwards wrote:
Where does the inconistency lie ? Does it lie in the change in effective source impedance? It lies in using the wrong coax Z0 for the situation. Assuming it's long enough to develop 75 ohms, it will cause 4% of the forward power to be reflected back toward the source at the meter. Guess you can't call it a TLI after all since it is faithfully reporting the 50 ohm SWR and not making the transmitter happy. :-) -- =================================== Cec, wrong! It is true the very incorrect use of 75-ohm line by the system designer is the reason for the UNDETECTED deviation from 50 ohms of the Tx load. The reason it is undetected is because the SWR meter is in error. Although the meter indication has not changed since the 50-ohm cable was in situ, it is now in error because it can measure correctly only on 50-ohm lines but the actual line being measured is 75-ohms. So the meter can still be called a TLI. The moral of the story is "If the connection between transmitter and SWR meter or TLI is of appreciable length then always use 50-ohm coax." Good to see you are still taking an interest. ;o) ---- Reg, G4FGQ |
Reg Edwards wrote:
"Ian White, G3SEK" wrote - No - I meant exactly what I said. The meter can only indicate the rho/SWR of whatever is connected downstream (load side) of the meter itself. ================================== We're agreed, down to and including point 5. (1) I'm sure we are agreed our meters will correctly indicate Rho and SWR only on 50-ohm lines. (2) Insofar as the meter is concerned the transmitter's load impedance is the input impedance of the transmission line between the meter and the antenna. There may or may not be an intervening Z-match network. (3) Insofar as the transmitter is concerned the line between meter and antenna can be ANY impedance. It is desirable only that line length with its Zo transform the antenna input impedance to somewhere near to 50 ohms (Like a G5RV on 14.15 MHz). If things become difficult then a Z-match can be inserted. Once we have selected Zo for this line we are no longer interested in its SWR. And if we WERE interested a 50-ohm SWR meter would be incapable of correctly measuring it. (4) Note that when a Z-match is located at the transmitter end of this feedline, and varied, the actual SWR on this line cannot change - yet the SWR meter responds readily to the Z-match settings. (5) The only line which, ideally, MUST be 50 ohms coax and have a small SWR, certainly if it is of appreciable length, is that between the meter and the transmitter. Otherwise the load directly presented to the transmitter would not be 50 ohms. Any other impedance would transform the 50 ohms seen immediately on the antenna side of the meter to some other value. .... so we're OK as far as here. The reasons you give why the impedance of that connecting line between the transmitter and the meter must be 50 ohms are all correct. But there's another reason - or at least, another way of looking at the same situation: because the SWR meter has been calibrated for a 50 ohm system reference impedance, it will not correctly indicate a 50 ohm load to the transmitter unless the connecting line is also of 50 ohms impedance (or is too short to matter). (6) It is the SWR on this line which the meter indicates. No, it isn't! The value of rho or SWR obtained from the forward and reflected readings of the meter is the rho/SWR of the *load* connected to the meter's *output* terminals. Where the confusion arises is because the SWR on that 50 ohm connecting line will be numerically equal to the SWR indicated by the meter - but that's only because they are both 50-ohm devices. It is *not* because the meter is reacting to the standing waves on its input side. (If you change the impedance of that connecting line, then as you say in (5) above, the load impedance presented to the transmitter will change, so the power output will change. However, the forward and reflected signals from the meter will both be affected in the same proportion, so the rho/SWR result will not change.) Or look at it another way: think of the meter as a bridge, fed by that same connecting line - after all, some SWR meters are literally three-resistor bridges with the unknown load impedance forming the fourth arm. The bridge balance is not affected in any way by the length or impedance of the line that is merely energizing the bridge. Only the load impedance affects the bridge balance. (If this line is NOT 50-ohms then the meter incorrectly indicates the standing waves on it. Which is what I said before and a lot of people disagreed. And they were right, because the rho/SWR reading will *not* change. What's incorrect is to believe that the rho/SWR meter is measuring "standing waves" OR ANYTHING ELSE to do with its input line. It ain't. Not that a false indication is of great consequence when it is the incorrect choice of line Zo where the problem arises.) That contradicts what you said in (5) above. If the meter falsely indicates correct transmitter loading, that certainly could be of consequence. (7) In practice at HF the length of this 50-ohm coax is often negligible. The meter is often inside the transmitter box. As the misleading idea of standingwaves on this short, even zero-length coax is nonsense Reg, you must be about the only person in the world who worries about that. The term "SWR" has almost completely freed itself from its original literal associations with standing waves. Everyone else accepts SWR as just one among the many mathematically interchangeable ways of expressing the quality of an impedance match or mismatch. None of them is either more or less valid than any of the others. the name of the instrument should be changed to TLI. (Transmitter Loading Indicator). Which is all that it is! It is indeed, so the very best of luck with your campaign to change its name... ;-) -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Reg Edwards wrote:
To anybody interested. We have a HF Transmitter + 50-ohm coax + SWR meter + Tuner + Feedline of any Zo + Antenna. Suppose it is all tuned-up and ready to go. The transmitter is loaded with exactly 50-ohms resistive. Now change the 50-ohm coax to shorter length of 75-ohm Zo. As everybody agrees (after perhaps a little meter recalibration) the SWR meter indication will not change. BUT THE TRANSMITTER WILL NOW BE INCORRECTLY LOADED. Where does the inconistency lie ? The inconsistency lies in failing to meet the requirement that: 1. the correct load impedance for the transmitter *and* 2. the system reference impedance for which the SWR meter was calibrated *and* 3. the impedance of that connecting line ....must all be the same. See the longer reply to your other message. (This is yet another way of saying the same things.) Does it lie in the change in effective source impedance? That's the one place where the inconsistency definitely does *not* come from. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Dear Ian, please forgive me. The twists and contortions in your use of the English language are too involved for me unravel. No useful purpose would be served. However, between us, you HAVE contributed to an excellent demonstration of the nonsensical mess which occurs when standing waves, reflection coefficients, conjugate matches, virtual thingammies, Cecil's guaranteed environments, etc, etc, are dragged in in futile attempts to explain what goes on on that most simple imaginable of all connections between the transmitter output and that so-called SWR meter. It is not, as you imply, a trivial derisory matter to get yourself off the hook. However you are in good company. For many years it has mis-informed and confused newcomers, CB-ers, genuine students, and has obstructed education in general about understanding the operation and adjustment of antenna systems. And the eminent 'guru's' have managed successfully to mis-inform and confuse themselves as evidenced by the incessant squabbling on this newsgroup. Fortunately, with radio, any bloody thing will work, even if you DON'T, as you should, use 50-ohm coax between Tx and TLI. It's not likely to blow up your transistorised PA. ;o) Incidentally, if you DO use 75-ohm coax because you just happen to have a reel of it lying around, then do as I did with one of my ancient home-brew transeivers and use a 75-ohm TLI. Just change the setting of one preset resistor or capacitor in a 50-ohm model. As you say, it's only a 3-arm bridge ! Swap the connections to the little toroidal current transformer and you have a crude thru-power meter PROVIDED the Tx load really is 50-ohms.. Forget about reflected power - which nobody wants to know anyway - its scale has no more use or meaning than that for the defunct SWR. Try the typist's white stuff. "Carry on London. Sweet violets." ---- Regards from a Californian-Wine-imbibing Italian Clown. ;o) ;o) ;o) ;o) ;o) ;o) ---- ======================= For Free Radio Design Software go to http://www.g4fgq.com ======================= |
Whoa, I did that experiment for Slick a month ago. I replaced the short
piece of LMR240 between the transmitter and Kenwood SW2000 meter with a precisely measured 1/4 wavelength piece of RG59 75 Ohm coax. The loading changed, as expected, but the SWR reading did NOT change for SWR values of 1:1 and 1.6:1. The transmitter now saw a load of 112.5 Ohms. The Kenwood now saw a different source impedance. I don't see any inconsistency. 100W into 112.5 Ohms requires a voltage of 106V RMS. In a transmitter with a fixed ratio output transformer that may not be doable. It is designed to put out 70.7V RMS into 50 Ohms, with some margin. Tam/WB2TT "Reg Edwards" wrote in message ... To anybody interested. We have a HF Transmitter + 50-ohm coax + SWR meter + Tuner + Feedline of any Zo + Antenna. Suppose it is all tuned-up and ready to go. The transmitter is loaded with exactly 50-ohms resistive. Now change the 50-ohm coax to shorter length of 75-ohm Zo. As everybody agrees (after perhaps a little meter recalibration) the SWR meter indication will not change. BUT THE TRANSMITTER WILL NOW BE INCORRECTLY LOADED. Where does the inconistency lie ? Does it lie in the change in effective source impedance? |
Ian said -
No, it isn't! =============== Yes, it is! --- Reg. ;o) |
The inconsistency is that the power level changed, the load on the
transmitter changed, but the SWR meter gave no indication of it. What's wrong ? |
Richard Clark wrote:
On Sat, 18 Oct 2003 13:09:42 -0500, Cecil Moore wrote: The guys over on sci.physics.electromag said that two feet of 50 ohm coax guarantees a 50 ohm environment for a wattmeter. At DC? 100Hz? 10KHz? ... ... 10GHz? 1THz? Poor bounding as usual. The frequency discussed was 10 MHz. Shall I add my family tree to the context? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Reg Edwards wrote:
The inconsistency is that the power level changed, the load on the transmitter changed, but the SWR meter gave no indication of it. What's wrong ? Nothing's wrong with the equipment. Something's wrong with the operator. If you want to make the problem even worse, remove the coax entirely. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Reg Edwards wrote at 01:18:
Dear Ian, please forgive me. The twists and contortions in your use of the English language are too involved for me unravel. No useful purpose would be served. Try reading it again, preferably at the other one o'clock... -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
On Sat, 18 Oct 2003 22:40:57 -0500, Cecil Moore
wrote: The frequency discussed was 10 MHz. Shall I add my family tree to the context? Odd that you feel it necessary, but if serves bounding then, yes, perhaps you need to do that more often. 73's Richard Clark, KB7QHC |
The only "inconsistency" is that an SWR meter is obviously NOT a "TLI",
or transmitter loading indicator, as you've been so kind as to graphically point out. Roy Lewallen, W7EL Reg Edwards wrote: To anybody interested. We have a HF Transmitter + 50-ohm coax + SWR meter + Tuner + Feedline of any Zo + Antenna. Suppose it is all tuned-up and ready to go. The transmitter is loaded with exactly 50-ohms resistive. Now change the 50-ohm coax to shorter length of 75-ohm Zo. As everybody agrees (after perhaps a little meter recalibration) the SWR meter indication will not change. BUT THE TRANSMITTER WILL NOW BE INCORRECTLY LOADED. Where does the inconistency lie ? Does it lie in the change in effective source impedance? |
The only "inconsistency" is that an SWR meter is obviously NOT a "TLI", or transmitter loading indicator, as you've been so kind as to graphically point out. ============================= It would be more true if you had said the SWR meter is not being used as an SWR meter because there's no SWR for it to measure. There's an inconsistency because the line between transmitter and meter is not 50 ohms. The Tx is incorrectly loaded but neither meter scale gives any indication of it to the user. It could be a serious matter but there's no warning. The operator is allowed to believe he has set up the equipment correctly. A TLI suffers from the same disadvantage as an SWR meter - it gives the correct answers only when making measurements on 50-ohm lines. This should not be surprising. They have identical circuits. But when there is no line of any impedance, just a few inches of wire, the TLI indicates correctly. Whereas the SWR meter requires at least a 1/4-wavelength of 50-ohm line before it stops being dishonest. And it doesn't stop telling white lies even on longer lengths. |
Reg Edwards wrote:
Whereas the SWR meter requires at least a 1/4-wavelength of 50-ohm line before it stops being dishonest. I asked that question over on sci.physics.electromag. A formula was provided. Using the dimensions of RG-213, two feet is long enough to establish the 50 ohm environment at 10 MHz. SWR = [sqrt(Pfwd)+sqrt(Pref)]/[sqrt(Pfwd)-sqrt(Pref)] -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
And for anyone doubting this dual definition of SWR, just take a look at
Agilent, Narda, or similar web sites, and notice that their high priced and precision terminations all come with an SWR specification. You'll also find SWR specs for test equipment inputs and many other devices not containing transmission lines. These are clear examples of Definition 2. Roy Lewallen, W7EL Ian White, G3SEK wrote: Reg Edwards wrote: The only "inconsistency" is that an SWR meter is obviously NOT a "TLI", or transmitter loading indicator, as you've been so kind as to graphically point out. ============================= It would be more true if you had said the SWR meter is not being used as an SWR meter because there's no SWR for it to measure. There's an inconsistency because the line between transmitter and meter is not 50 ohms. The Tx is incorrectly loaded but neither meter scale gives any indication of it to the user. It could be a serious matter but there's no warning. The operator is allowed to believe he has set up the equipment correctly. In that case, calling the meter a "Transmitter Loading Indicator" will only *add* to the confusion! The rho/SWR meter has only ever been claimed to measure conditions on the *load* side of itself. It knows nothing about conditions on the transmitter side; and until Reg came along, nobody ever claimed that it could. Now here comes Reg, saying "Let's all call it a Transmitter Loading Indicator!" Later, he realises that his "TLI" will only work correctly if the connecting line between the transmitter and the meter is (a) of impedance Zo, or (b) vanishingly short. And his reaction to that: blame the meter! Me, I'd go looking for that bloke who invented "TLI"... A TLI suffers from the same disadvantage as an SWR meter - it gives the correct answers only when making measurements on 50-ohm lines. This should not be surprising. They have identical circuits. But when there is no line of any impedance, just a few inches of wire, the TLI indicates correctly. Whereas the SWR meter requires at least a 1/4-wavelength of 50-ohm line before it stops being dishonest. And it doesn't stop telling white lies even on longer lengths. What Reg refuses to accept is that there are two alternative definitions and usages of the term "SWR": Definition 1. Given an impedance Z at any single point in any circuit, and a system reference impedance Zo, then SWR is a *mathematical* function of Z and Zo that tells you how close together the two impedances are. Definition 2. Given a transmission line that is terminated in an impedance Z that is different from the line's own characteristic impedance Z(line), then a standing wave of varying voltage and current will appear on the line. If the line is long enough to identify a voltage maximum and a voltage minimum (an electrical quarter-wavelength away) then SWR is defined as the ratio Vmax/Vmin. That was the original definition of SWR, but it has severe limitations. For a lossless line with a characteristic impedance the same as Zo, the two definitions of SWR are related by a simple, fixed mathematical formula. But that's a special case; in all other cases, definition 2 has to be applied with care (or you may even judge that it isn't valid at all). Definition 1 is by far the most common definition and usage of "SWR" - it's just one of several alternative measures of impedance match (others including rho, return loss, S11 etc). All these different alternatives are inter-related by defined conversion formulae. RF/microwave engineers move freely between all of them, using whichever term is most convenient at the time, or more normal in that particular area of electronics. It's no big deal. Life is full of examples where the same word is used to mean many subtly different things. The whole of the electronics/RF/microwave engineering profession is very comfortable with "SWR" meaning more than one thing. Most amateurs never need to think about that difference; but if they do, they don't find it difficult. AFAIK, Reg is the only person on the planet who understands all of this perfectly, but refuses point-blank to accept it. |
"Roy Lewallen" wrote And for anyone doubting this dual definition of SWR,
just take a look at Agilent, Narda, or similar web sites, and notice that their high priced and precision terminations all come with an SWR specification. You'll also find SWR specs for test equipment inputs and many other devices not containing transmission lines. These are clear examples of Definition 2. ==================================== Roy, When trying to sell high-priced instruments, including those which are not associated with transmission lines, manufacturers would be stupid not to communicate with prospective customers in terms which customers imagine they understand. Mention of SWR no doubt sounds highly technical, but is confusing, perhaps beyond comprehension, to people who have no connection with lines. Manufacturers are not paid to educate their customers. They may claim it's a part of their job in the sales blurb but we are familiar with sales-blurb writers standards of education. For example, only suckers accept antenna performance specifications as being definitions of anything. Roy, you really are scraping the bottom of the barrel to find some support for the nonsense and confusing-to-learners waffle which fogs and enshrouds what goes on on that short length of wire between HF transmitter and so-called SWR meter. Old habits die hard. --- Reg, G4FGQ |
Reg Edwards wrote:
"Roy Lewallen" wrote And for anyone doubting this dual definition of SWR, just take a look at Agilent, Narda, or similar web sites, and notice that their high priced and precision terminations all come with an SWR specification. You'll also find SWR specs for test equipment inputs and many other devices not containing transmission lines. These are clear examples of Definition 2. ==================================== Roy, When trying to sell high-priced instruments, including those which are not associated with transmission lines, manufacturers would be stupid not to communicate with prospective customers in terms which customers imagine they understand. Mention of SWR no doubt sounds highly technical, but is confusing, perhaps beyond comprehension, to people who have no connection with lines. Manufacturers are not paid to educate their customers. They may claim it's a part of their job in the sales blurb but we are familiar with sales-blurb writers standards of education. For example, only suckers accept antenna performance specifications as being definitions of anything. Roy, you really are scraping the bottom of the barrel to find some support for the nonsense and confusing-to-learners waffle which fogs and enshrouds what goes on on that short length of wire between HF transmitter and so-called SWR meter. Old habits die hard. If we could wipe the slate clean and start again, RF engineering could probably manage quite well without the term "SWR". But we can't. In the real world of radio communication, "SWR" is used everywhere. Even absolute beginners have already heard about it. Nothing will change that. Roy and I are starting from where people actually a they've already heard about SWR, so now they need to know what it means. And in the end, it isn't all that hard to understand. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Ian, misguiding learners, undermining the foundations of radio education, is
my main objection to the ill-educated do-gooder discussions which take place on this newsgroup. Although at first I really did suspect your typist had made a translation error between your dictation and her keyboard, I now see you insist you were right first time. You said - No - I meant exactly what I said. The meter can only indicate the rho/SWR of whatever is connected downstream (load side) of the meter itself. Ian, this statement will mislead and undermine students of radio for the remainder of their careers. (You were one yourself.) For their (and your) benefit it must be stated the so-called SWR/Rho meter will indicate NOTHING about what lies on the antenna side of it EXCEPT, indirectly, the magnitude RELATIVE to 50 ohms of the INPUT IMPEDANCE the meter immediately sees looking towards the antenna. Obviously, the impedance seen by the meter is that which, at HF, terminates the other length of line, if one exists, between the transmitter and meter. If, AND ONLY IF, the transmitter-to-meter line is 50-ohms then the 50-ohm meter will indicate |Rho| relative to 50-ohms at the meter end of that line. If the line is not 50-ohms then, although the meter may indicate SWR =1 or Rho = TLI = 0, the transmitter will be loaded with something different from 50 ohms. Highly unsatisfactory! The impedance of this line must NOT be neglected, as somebody said, on the grounds that it forms part of the source impedance as seen by the meter, and can be treated like the internal resistance of the transmitter as irrelevant. Alternatively, if the 50-ohm Tx-to-Meter line is long enough then that's the line the meter indicates the SWR on. It is common knowledge SWR/Rho determinations MUST begin by determining the value of the reflection coefficient at the point where the reflection originates. ie., where the meter is located along the uniform length of line. The G5RV demonstrates the meter is ignorant of what may be happening on the antenna side of the meter. Whatever the meter indicates it will NOT correspond to |Rho| or SWR on the antenna's high-Zo balanced feedline. But that's OK. Nobody wants to know anyway. (Sooner or later someone will re-discover the special case that when the transmission system is 50 ohms all the way from Tx to antenna, the meter appears to measure |Rho| in both directions. This is because |Rho| is sensibly constant in magnitude along the whole line. Actually |Rho| is indicated looking back towards the transmitter and, by reversing the meter in the line, thru-power is indicated looking towards the antenna. This special case is the norm at UHF where line and connector dimensions are critical and engineering economics force simplifications, including simplification of underlying formulae. Chains of matrix algebra fit very nicely into number-crunching computer software. Now expect sombody who is still missing the point to say "not if the line is 1/2-wavelength long." But who wants a half-wavelength of line on 1.9 MHz between the transmitter and swaarrr meter. ---- Reg, G4FGQ |
We should embrace your definitions and perceptions rather than "scraping
the bottom of the barrel" by seeing how terms are used by manufacturers of professional test equipment? Sorry about the blow to your ego, Reg, but I do put more weight on how Agilent uses technical terms than how Reg does. I guess Agilent has their audience and you have yours. Each to his own. And, um, Agilent (formerly HP) and Narda do sell products which are "associated with transmission lines". Roy Lewallen, W7EL Reg Edwards wrote: "Roy Lewallen" wrote And for anyone doubting this dual definition of SWR, just take a look at Agilent, Narda, or similar web sites, and notice that their high priced and precision terminations all come with an SWR specification. You'll also find SWR specs for test equipment inputs and many other devices not containing transmission lines. These are clear examples of Definition 2. ==================================== Roy, When trying to sell high-priced instruments, including those which are not associated with transmission lines, manufacturers would be stupid not to communicate with prospective customers in terms which customers imagine they understand. Mention of SWR no doubt sounds highly technical, but is confusing, perhaps beyond comprehension, to people who have no connection with lines. Manufacturers are not paid to educate their customers. They may claim it's a part of their job in the sales blurb but we are familiar with sales-blurb writers standards of education. For example, only suckers accept antenna performance specifications as being definitions of anything. Roy, you really are scraping the bottom of the barrel to find some support for the nonsense and confusing-to-learners waffle which fogs and enshrouds what goes on on that short length of wire between HF transmitter and so-called SWR meter. Old habits die hard. --- Reg, G4FGQ |
Roy Lewallen wrote:
I guess Agilent has their audience and you have yours. Agilent has some very good papers on s-parameters. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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