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Length of Coax Affecting Incident Power to Meter?
Hey,
I've been getting different readings on my power meter that depend on the length of coax that i use to the meter (into a Cantenna). This is a push-pull VHF 300 watt transmitter with a coax stub harmonic filter on the output. My digital Daiwa will read about 310 watts incident power using a 4 foot RG-8X jumper coax (from filter to meter), and about 240 watts using a 12 foot section of RG-8X!!?? VSWR stays the same at about 1.4:1 . There is the old ham wife's tale that you can tune your transmitter for a better match if you adjust the legnth of your coax to the antenna. And from a theoretical point of view, it _should_ be only a tale, because if your antenna is not a perfect 50 Ohms, the length of the coax should still not matter, because the constant VSWR circles around the center of the Smith Chart have just that, the same SWR independant of the wavelengths away from the antenna (assuming 50 Ohm transmission lines are used). OTOH, the actual series equivalent complex impedance will be alternating from inductive to capacitive, every 1/2 wavelength. So perhaps this will tune/detune the amplifier? Any non-bullsh** advice/explainations appreciated. Dr. Slick |
With a non-resistive load, the length of the coax and placement of an
inexpensive (e.g. non-Bird) meter will cause different readings. |
"Dr. Slick", Sure, for the CB crowd, it's in increments of 3 feet. 'Doc |
Dr. Slick wrote:
This is a push-pull VHF 300 watt transmitter with a coax stub harmonic filter on the output. My digital Daiwa will read about 310 watts incident power using a 4 foot RG-8X jumper coax (from filter to meter), and about 240 watts using a 12 foot section of RG-8X!!?? VSWR stays the same at about 1.4:1 . Remember the maximum power transfer theorem? The transmitter probably works better into some impedances than into others. With an SWR of 1.4:1, the transmitter will see a resistance between 35.7 ohms and 70 ohms with a reactance between zero and about plus or minus j15. Knowing the frequency and VF of the RG-8X, you should be able to estimate the two impedances seen by the transmitter. -- 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 80,000 Newsgroups - 16 Different Servers! =----- |
"Tarmo Tammaru" wrote in message ...
You don't say what VHF frequency, but the Cantenna is probably unsuitable at that frequency. Your amp is spec'd to put out 300W into 50 Ohms, not 35 or 70. It could put out more or less power into a different impedance. Your best bet is to build a little L network right at the Cantenna, adjust it for 1:1 SWR, and then measure the power. Another approach would be to string all the RG58 you own in series with the 8X. Tam/WB2TT Less than 150 MHz. Someone else suggested that i place the meter right at the cantenna, to at least exclude the patch from meter to dummy load. Unfortunately, as i have mentioned before, the cantenna is not the greatest 50 Ohms at anything above 10-20 MHz. Slick |
W5DXP wrote in message ...
Remember the maximum power transfer theorem? The transmitter probably works better into some impedances than into others. With an SWR of 1.4:1, the transmitter will see a resistance between 35.7 ohms and 70 ohms with a reactance between zero and about plus or minus j15. Knowing the frequency and VF of the RG-8X, you should be able to estimate the two impedances seen by the transmitter. Actually, i read off of my Smith Chart about +/- j20, but you are close. Anyhow, my point is that the ham "wife's tale" of adjusting the coax length for lowest SWR may be truly just a tale. However, in my case, the SWR doesn't change much, while the INCIDENT power does!!! To me, this may be due to the fact that the PA isn't 50 Ohms at the output (I'll bet not many really are), and so a swing of +/- j20 may improve or degrade how close you are to a conjugate match. What do you dudes think? Dr. Slick |
Dr. Slick wrote:
However, in my case, the SWR doesn't change much, while the INCIDENT power does!!! Guess I wasn't clear. There are an infinite number of impedances on a constant SWR circle. The transmitter likes some of those impedances better than others. -- 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 80,000 Newsgroups - 16 Different Servers! =----- |
"Dr. Slick" wrote in message om... Someone else suggested that i place the meter right at the cantenna, to at least exclude the patch from meter to dummy load. No, you don't want to do that. Put as much loss as you can between the meter and the Cantenna. 100 feet of RG58 will have about 6.5 db of loss. It will make your load a lot closer to 50 Ohms. There are Tables that show SWR as a function of cable loss for a given mismatch. One chart that I have in "Reference Data for Radio Engineers" shows that with a load SWR of 1.4, and 6.5 db of cable loss, the SWR at the driving end is around 1.075. Note that you DO NOT want low loss coax. Tam/WB2TT |
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"Tarmo Tammaru" wrote in message ...
"Dr. Slick" wrote in message om... Someone else suggested that i place the meter right at the cantenna, to at least exclude the patch from meter to dummy load. No, you don't want to do that. Put as much loss as you can between the meter and the Cantenna. 100 feet of RG58 will have about 6.5 db of loss. It will make your load a lot closer to 50 Ohms. There are Tables that show SWR as a function of cable loss for a given mismatch. One chart that I have in "Reference Data for Radio Engineers" shows that with a load SWR of 1.4, and 6.5 db of cable loss, the SWR at the driving end is around 1.075. Note that you DO NOT want low loss coax. Tam/WB2TT Perhaps you are correct. That you would only want to use the meter right at the load if you wanted to measure the real SWR of the antenna, as the coax loss will improve the return loss (SWR). I'll try a long piece of RG-58 and see how the incident power is affected. Slick |
W5DXP wrote in message ...
Dr. Slick wrote: However, in my case, the SWR doesn't change much, while the INCIDENT power does!!! Guess I wasn't clear. There are an infinite number of impedances on a constant SWR circle. The transmitter likes some of those impedances better than others. No, i understood you alright, and your statement above is understood and agreed to. However, in this case, we only have two impedances to chose from (two different coax lengths). Slick |
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This means your shield has RF on it .
Also triming cable has nothing to do with SWR. Trimming the cable will change the impedance the transmitter sees if the transmission line has standing waves on it. "Dr. Slick" wrote in message om... Hey, I've been getting different readings on my power meter that depend on the length of coax that i use to the meter (into a Cantenna). This is a push-pull VHF 300 watt transmitter with a coax stub harmonic filter on the output. My digital Daiwa will read about 310 watts incident power using a 4 foot RG-8X jumper coax (from filter to meter), and about 240 watts using a 12 foot section of RG-8X!!?? VSWR stays the same at about 1.4:1 . There is the old ham wife's tale that you can tune your transmitter for a better match if you adjust the legnth of your coax to the antenna. And from a theoretical point of view, it _should_ be only a tale, because if your antenna is not a perfect 50 Ohms, the length of the coax should still not matter, because the constant VSWR circles around the center of the Smith Chart have just that, the same SWR independant of the wavelengths away from the antenna (assuming 50 Ohm transmission lines are used). OTOH, the actual series equivalent complex impedance will be alternating from inductive to capacitive, every 1/2 wavelength. So perhaps this will tune/detune the amplifier? Any non-bullsh** advice/explainations appreciated. Dr. Slick |
"Dr. Slick" wrote in message om... W5DXP wrote in message ... Remember the maximum power transfer theorem? The transmitter probably works better into some impedances than into others. With an SWR of 1.4:1, the transmitter will see a resistance between 35.7 ohms and 70 ohms with a reactance between zero and about plus or minus j15. Knowing the frequency and VF of the RG-8X, you should be able to estimate the two impedances seen by the transmitter. Actually, i read off of my Smith Chart about +/- j20, but you are close. Anyhow, my point is that the ham "wife's tale" of adjusting the coax length for lowest SWR may be truly just a tale. I hope this is a wifes tale that has long been discredited. Trimming cable changes the impedance seen by the transmitter with no change to SWR. It is true that there are a lot of people who dont understand the difference. However, in my case, the SWR doesn't change much, while the INCIDENT power does!!! To me, this may be due to the fact that the PA isn't 50 Ohms at the output (I'll bet not many really are), and so a swing of +/- j20 may improve or degrade how close you are to a conjugate match. What do you dudes think? Dr. Slick |
That would give you 13db of return loss which would be an SWR less than 2:1
even if the far end was a dead short. Given his current SWR I woud say this would result in a return loss of around 30db. Numbers are a lot of by guess and by golly but think I am not too far off. "Tarmo Tammaru" wrote in message ... "Dr. Slick" wrote in message om... Someone else suggested that i place the meter right at the cantenna, to at least exclude the patch from meter to dummy load. No, you don't want to do that. Put as much loss as you can between the meter and the Cantenna. 100 feet of RG58 will have about 6.5 db of loss. It will make your load a lot closer to 50 Ohms. There are Tables that show SWR as a function of cable loss for a given mismatch. One chart that I have in "Reference Data for Radio Engineers" shows that with a load SWR of 1.4, and 6.5 db of cable loss, the SWR at the driving end is around 1.075. Note that you DO NOT want low loss coax. Tam/WB2TT |
(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com... ... What do you mean by "calibrated to the line"? The SWR meter should read zero reflected power when connected to a load whose impedance is equal to the line's. Does it? If not, it's not properly calibrated. Putting it another way, what's the directionality of the bridge? You mean the directionality of the couplers inside the meter? I don't know. You can adjust the needle on the analog Daiwas to read zero watts with nothing hooked up to them. ... Not so much a surprise as a disappointment! A difference of 70 watts incident power is totally unacceptable with only 8 feet of coax length added. But the load presented to the amplifier is totally different in the two cases, most likely. Only if the amplifier's output impedance were the complex conjugate of the line's, and the line were lossless, and the amplifier behaved as a linear time-invariant system would you (should you) expect the power to remain unchanged. I'm not sure i agree with your statement as you have written it. The power would remain the same only if the load and the line were perfect 50 Ohms and also lossless, then the length of the coax should not matter at all. This is certainly not the case on our bench, which is a problem, because i want to have an approximation of how it will perform attached to the antenna before i ship it out to the customer. A difference of 70 watts is not acceptable. Tam brought up the suggestion that i try a really long piece of RG-58 from meter to dummy load, to make the Cantenna more like 50 Ohms. Gonna try it. Or just tune the load to zero reflected power after you're sure the meter is properly calibrated...but that can be a catch-22 situation that the really long RG-58 can help with. Be aware, though, that "50-ohm" line seldom is -- it can be off 5 ohms or more. It's a cause of some consternation to those of us involved in calibration of precision RF test equipment. Oh, definitely, especially the cheap stuff. But if the RG-58 went around the world a couple of times, the end could be open or shorted, and you would still measure 50 Ohms, most likely. Load-pull techniques are commonly used to characterize RF source impedances. You make known incremental changes to the load, and deduce from the change in output power what the source impedance is, assuming it's a linear time-invariant system. Note that adding length to a mismatched line is one way to make an incremental change to the load... I wouldn't necessarily say that the output impedance of a class C amplifier is meaningless, but it may well not be constant for all loads. and may depend on parameters you'd have trouble controlling from day to day. Cheers, Tom Right, they did plenty of load-pulling on FETS at my former place of employment. I think they may have been more meaningful for class A linear CDMA PAs. But finding the max power transfer impedances with small signal class A measurements doesn't guarantee that these will be the optimum impedances the transistor wishes to see in class C mode. But i don't think there is a practical way to measure the S22 of a high powered Class C amp. Most companies don't really seem to care how close or far from 50 Ohms it really is, all they care about is the %PAE and Pout and harmonics dBc going INTO a 50 Ohm system. But even ensuring that your bench will match your customer's bench is a difficult thing. Slick |
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Jimmy wrote:
I hope this is a wifes tale that has long been discredited. Trimming cable changes the impedance seen by the transmitter with no change to SWR. However, a 50 ohm SWR meter reading will change as one changes the length of the ladder-line. When the 50 ohm SWR meter reads 1:1, the transmitter is seeing 50 ohms. That's how I tune my dipole. -- 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 80,000 Newsgroups - 16 Different Servers! =----- |
"Jimmy" wrote in message . com...
This means your shield has RF on it . Also triming cable has nothing to do with SWR. In theory, yes. It would be on the constant VSWR circles, assuming 50 Ohm transmission line! Trimming the cable WILL have something to do with the series reactance that the PA will see, and may affect the impedance match. Slick |
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My 2 cents (wont even buy a lolipop any more!) From the old VHF,er
magazine, couple of thoughts: 1) Very lossey coax will show a prefect termination impedence even if open, or shorted, if sufficiant length is used (for the coax's attenuation ) . For that reason, still keep a coil of RG-8 size coax (for power dissipation), with RUBBER DIALECTRIC (was origianlly made for attenuators at the Microwave Region) Even at 80 meters, with no load , has very low SWR! At 10 meters, and up, with a UHF connector on one end, and a N connector, on the other, with NO TERMINATION, makes a great dummy load to 2-300 watts!. and, 2) Just because you think you have a great match, can be a random event. to test , place a 1/4 wavelength piece of coax (figuring with velocity factor). If the SWR stays flat, you can assume a good match, if, howerer, the SWR climbs, then you need to check for a proper match ( this will throw the worst condition into your coax , with this extra length, kinda like the opposite of trimming coax for the best match legends of old CB lore)! as info, Jim NN7K "Tom Bruhns" wrote in message m... (Dr. Slick) wrote in message . com... ... What do you mean by "calibrated to the line"? The SWR meter should read zero reflected power when connected to a load whose impedance is equal to the line's. Does it? If not, it's not properly calibrated. Putting it another way, what's the directionality of the bridge? ... Not so much a surprise as a disappointment! A difference of 70 watts incident power is totally unacceptable with only 8 feet of coax length added. But the load presented to the amplifier is totally different in the two cases, most likely. Only if the amplifier's output impedance were the complex conjugate of the line's, and the line were lossless, and the amplifier behaved as a linear time-invariant system would you (should you) expect the power to remain unchanged. The transmitter does not have a mismatch sensor on it for reducing power at high SWR. It naturally does output less wattage as the transconductance is reduced with higher temperatures, but my measurements were done in close proximity to each other. Exactly how would you measure the output impedance (S22) of a PA? Drive it with a small signal (class A, linear mode) in one frequency (F1), and then inject another frequency (F2) into the output, and then measure the reflected F2 power? If i remember correctly, that's how someone was trying to measure S22 at my former place of employment. But all bets are off as soon as you go non-linear, or non-class A. S-parameters are supposed to be all small signal. I don't think you can properly measure the S22 of a class C PA. Tam brought up the suggestion that i try a really long piece of RG-58 from meter to dummy load, to make the Cantenna more like 50 Ohms. Gonna try it. Or just tune the load to zero reflected power after you're sure the meter is properly calibrated...but that can be a catch-22 situation that the really long RG-58 can help with. Be aware, though, that "50-ohm" line seldom is -- it can be off 5 ohms or more. It's a cause of some consternation to those of us involved in calibration of precision RF test equipment. Load-pull techniques are commonly used to characterize RF source impedances. You make known incremental changes to the load, and deduce from the change in output power what the source impedance is, assuming it's a linear time-invariant system. Note that adding length to a mismatched line is one way to make an incremental change to the load... I wouldn't necessarily say that the output impedance of a class C amplifier is meaningless, but it may well not be constant for all loads. and may depend on parameters you'd have trouble controlling from day to day. Cheers, Tom |
Tom
He is trying to measure the output power of an amplifier. His meter is calibrated for 50 Ohms; so, he needs a 50 Ohm load. Tam/WB2TT |
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Almost correct.
The transmitter output impedance has no effect whatsoever on the line's SWR. Roy Lewallen, W7EL Dr. Slick wrote: W5DXP wrote in message ... Jimmy wrote: I hope this is a wifes tale that has long been discredited. Trimming cable changes the impedance seen by the transmitter with no change to SWR. However, a 50 ohm SWR meter reading will change as one changes the length of the ladder-line. When the 50 ohm SWR meter reads 1:1, the transmitter is seeing 50 ohms. That's how I tune my dipole. This is very interesting, as it may indicate that the "wife's tale" may have some validity in _some_ cases. For example, if your antenna happens to have an impedance of say, 60 Ohms, and your transmission line happens to NOT be a perfect 50 Ohms (usually the case), and happens to be around 54.8 Ohms or so... then at odd integer multiples of 1/4 wavelength (velocity factor included, of course), you will be right on 50 Ohms. Additionally, as we have mentioned before, PAs rarely have 50 Ohm output impedances anyways (who actually measures this??). So an impedance transform via perfect 50 ohm coax length may indeed get you closer to a conjugate match and higher incident power, which may improve your VSWR (which is based upon the incident and reflected, of course). Slick |
(Dr. Slick) wrote in message . com...
(Tom Bruhns) wrote in message om... This could be a whole 'nuther thread. For a reference, I don't have any difficulty making a 50 ohm load with 40dB return loss out to a GHz or so for less than $10, and most of that is the connector. Does someone need to document how to do that? But I submit that if you want accurate SWR measurements on a particular line, you should calibrate your SWR meter to that line, and that doesn't take any reference except the line itself. Cheers, Tom But can you make a 50 Ohm dummy load with those specs that can handle 300 Watts? The stays at 50 Ohms out to at least 200 Megs or so? 50 +/- what? What return loss are you shooting for in this 300W dummy load? Do you really need 40dB, or is 30dB good enough? I believe it's possible to bootstrap yourself into measurements that are far more accurate than you'll need for what you are doing, and do it quite economically if you don't count your time. But you ought to first ask yourself just what accuracy you really need, and understand why. Probably one can make a quite reasonable broadband power load, at least to your 200MHz limit. There have been some good construction articles on making tapered shields for power load resistors, for example, to get good high frequency performance. But if you can make just a good low-power one, you can bootstrap your way to accurate measurements at high power. Use the low power one to insure your directional coupler is good to some tolerance, and use that to tune your load at whatever frequency you wish to check. Even your cantenna should be low enough Q when tuned with an L network that it would be acceptable over the whole of the 2-meter ham band. It's tedious, but you can re-tune for any spot frequency inside or outside the ham band. And given one accurate load, you can determine what impedance your long, lossy coax is and then use that as a dummy load (probably quite broadband). For example, if your cantenna is good enough through 30MHz, and pretty good at 54MHz, and under 2:1 SWR at 150MHz, then perhaps enough RG-58 to give you 10dB loss at 150MHz (150 feet or so), feeding that cantenna, would work fine from 1MHz to 1GHz. Just beware of power dissipation in the line itself at higher frequencies. You can even cascade large coax, small coax and the cantenna, to insure power handling. Beware of harmonics messing up your readings! Also, you'd probably do well to consider how small a change in power results from a modest load change, for various source impedances. What's the worst case? What's the best? Is there a reason it might be nice if an amplifier output was "reasonably close" to 50 ohms, or doesn't it matter at all? If you think all this through, you may realize that if your dummy load is even only 20dB return loss, it will be just fine for the measurements you need to make. But YOU should convince YOURSELF of that, or of what you really do need. And i'm still not sure what you mean by "calibrate your SWR meter to the line". All the SWR meters i have seen are all for 50 Ohms. Could you tell us the exact procedure? Of course not; I know nothing about YOUR SWR meter. If you understand how yours works, you should be able to see how to adjust it, though it may not be worthwhile. To a close approximation, practically all of them work by sampling the line current and voltage at a point. The current is somehow turned into a voltage, and if you adjust either the voltage sampling percentage or the voltage produced by a given current, you will have adjusted the calibration impedance. At the high frequency end, you may need to worry about reactive or phase-shift effects. If it's not adjustable, don't you at least want to know WHAT impedance it's calibrated for? Or at VERY least, what it reads with a 50 ohm load? Cheers, Tom |
On Tue, 12 Aug 2003 02:48:49 -0700, Roy Lewallen
wrote: Almost correct. The transmitter output impedance has no effect whatsoever on the line's SWR. Roy Lewallen, W7EL Hi Roy, Entirely incorrect. Transmitter output impedance that does not conform to transmission line Z, when presented with a mismatched load through that line, adds mismatch uncertainty in the form of an indeterminate SWR and indeterminate Power to the load. This has already been demonstrated twice. This has long been documented with NBS/NIST references going back 4 decades. There is nothing mysterious about it at all, and it conforms to the rather simple principles of wave interference so poorly presented by Cecil in months past. The authoritative site: http://www.boulder.nist.gov/div813/index.html Direct reference: "Juroshek, J. R.; A Direct Calibration Method for Measuring Equivalent Source Mismatch; Microwave J., pp. 106-118; October 1997 Obscure references: http://www.boulder.nist.gov/div813/r...00S_n2nNet.pdf "With vector measurements of the generator and meter reflection coefficients Ãg and Ãm, respectively, the power of the incident signal am can be related to the power of the source." http://www.boulder.nist.gov/div813/r...FRad_ARFTG.pdf which describes radiometer calibration (perhaps too exotic for this group) "tests are based on two assumptions. First, the network responds linearly to our signal ( no power compression), and second, the radiometer is sufficiently isolated from the source impedance." ... "One of the assumptions made in deriving eq. (2) was that the output from the radiometer is not dependent on the source impedance. In the construction of the radiometer, two isolators are inserted at the input of the radiometer to isolate the radiometer from the source." ... "The mismatch uncertainty depends strongly on the poorly known correlation between uncertainties in the measurements of different reflection coefficients, and so we use the maximum of the uncertainties obtained by assuming either complete correlation or no correlation whatsoever." "Forthcoming Paper: Influence of Impedance Mismatch Effects on Measurements of Unloaded Q Factors of Transmission Mode Dielectric Resonators" IEEE Transaction on Applied Superconductivity "Analysis of Interconnection Network and Mismatch in the Nose-to-Nose Calibration Automatic RF Techniques Group , June 15-16, 2000 , Boston, MA - June 01, 2000 "We analyze the input networks of the samplers used in the nose-to-nose calibration method. Our model demonstrates that the required input network conditions are satisfied in this method and shows the interconnection errors are limited to measurement uncertainties of input reflection coefficients and adapter S-parameters utilized during the calibration procedure. Further, the input network model fully includes the effects of mismatch reflections, and we use the model to reconcile nose-to-nose waveform correction methods with traditional signal power measurement techniques." As I mentioned, obscure references. However, given the impetus of their discussion is long known (and that I have already provided the original references they rely on), NIST presumes the investigators already have that basis of knowledge. 73's Richard Clark, KB7QHC |
He sed:
"The transmitter output impedance has no effect whatsoever on the line's SWR." --------- THIS IS ABSOLUTELY CORRECT! 73 Jack |
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Richard Clark wrote:
To this point I have seen no counter demonstrations, nor counter proposals, nor counter argument ... Not unusual since your above deadline for counter demonstrations, counter proposals, and counter arguments was 66 minutes from the time you made the original posting. :-) You are in a room with a 150 ohm (lossless) transmission line coming through a hole in the wall and with a 50+j0 ohm load in the center of the room. Are you telling us that the SWR is not (1+|rho|)/(1-|rho|)? -- 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 80,000 Newsgroups - 16 Different Servers! =----- |
On Tue, 12 Aug 2003 14:13:44 -0500, W5DXP
wrote: Richard Clark wrote: To this point I have seen no counter demonstrations, nor counter proposals, nor counter argument ... Not unusual since your above deadline for counter demonstrations, counter proposals, and counter arguments was 66 minutes from the time you made the original posting. :-) Hi Cecil, I see you still live in a dimensional aberration where you experience 22 days as 66 minutes. You must be oxygen starved by now. 73's Richard Clark, KB7QHC |
(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com... But can you make a 50 Ohm dummy load with those specs that can handle 300 Watts? The stays at 50 Ohms out to at least 200 Megs or so? 50 +/- what? What return loss are you shooting for in this 300W dummy load? Do you really need 40dB, or is 30dB good enough? I believe it's possible to bootstrap yourself into measurements that are far more accurate than you'll need for what you are doing, and do it quite economically if you don't count your time. But you ought to first ask yourself just what accuracy you really need, and understand why. +/- 5 ohms or so. Something that doesn't swing up to 65-70 ohms or 40 ohms. And i'm still not sure what you mean by "calibrate your SWR meter to the line". All the SWR meters i have seen are all for 50 Ohms. Could you tell us the exact procedure? Of course not; I know nothing about YOUR SWR meter. If you understand how yours works, you should be able to see how to adjust it, though it may not be worthwhile. I didn't think you could tell us. I've never seen an SWR meter that you could "calibrate" to 50 or 75 ohms, or less. Slick |
Egad.
Given only a line's characteristic impedance and the load impedance, I can tell you exactly what the SWR is for a lossless line. For a lossy line, I only need to know, in addition, the line's length and the amount of loss per unit length. In no case do I need to know the source impedance. If, as you insist, the source impedance affects the SWR on the line, please provide an equation that gives the SWR on the line, with source impedance being one of the variables. It's such a simple thing, surely such an equation appears in one of the several references you cite. I did notice that SWR doesn't appear in any of the titles or the quoted passages, though, so you may have to dig a little. And if Cecil's work leads to the conclusion that the source impedance impacts the line's SWR, then it's wrong. It is, for those who are interested, very easy to see intuitively why the source impedance doesn't affect the SWR. Consider the situation that occurs when the source is first turned on. A forward voltage wave travels down the line toward the load. A reflected wave, whose magnitude and phase are determined by the reflection coefficient at the load end of the line, returns. If we stop time just as the reflected wave is returning, we can calculate the SWR, so far, on the line, solely from the ratio of the forward and reflected waves -- it's the interference between these waves that create the standing waves. Turning time back on again, the returning wave reflects off the source (assuming a source mismatch), producing another forward wave. Let's watch this wave as it travels toward the load, reflects, and returns. Exactly the same proportion of this wave is reflected as for the original forward wave. So, when this new forward wave reflects and its reflected wave returns, we've got a total of two forward waves and two reflected waves. The forward/reflected ratio of the second pair is exactly the same as the forward/reflected ratio of the first pair -- it's the reflection coefficient at the load end. So the ratio of the total forward wave to the total reverse wave is the same for the first pair, the second pair, and the sum of the two pairs. In other words, the second pair of waves hasn't changed the SWR from what we calculated from the original pair of waves. You can continue this observation for each forward-reverse pair, and see that the SWR never changes (at least when observed when each reflected wave just returns) from the original value. And the original value was determined only by the load mismatch, not the source. The source mismatch determines how big the total forward and reflected waves end up being when all the reflections have died out to a negligible value. But it has nothing to do with the forward/reverse ratio, which determines the SWR. Roy Lewallen, W7EL Richard Clark wrote: On Tue, 12 Aug 2003 02:48:49 -0700, Roy Lewallen wrote: Almost correct. The transmitter output impedance has no effect whatsoever on the line's SWR. Roy Lewallen, W7EL Hi Roy, Entirely incorrect. Transmitter output impedance that does not conform to transmission line Z, when presented with a mismatched load through that line, adds mismatch uncertainty in the form of an indeterminate SWR and indeterminate Power to the load. This has already been demonstrated twice. This has long been documented with NBS/NIST references going back 4 decades. There is nothing mysterious about it at all, and it conforms to the rather simple principles of wave interference so poorly presented by Cecil in months past. The authoritative site: http://www.boulder.nist.gov/div813/index.html Direct reference: "Juroshek, J. R.; A Direct Calibration Method for Measuring Equivalent Source Mismatch; Microwave J., pp. 106-118; October 1997 Obscure references: http://www.boulder.nist.gov/div813/r...00S_n2nNet.pdf "With vector measurements of the generator and meter reflection coefficients Ãg and Ãm, respectively, the power of the incident signal am can be related to the power of the source." http://www.boulder.nist.gov/div813/r...FRad_ARFTG.pdf which describes radiometer calibration (perhaps too exotic for this group) "tests are based on two assumptions. First, the network responds linearly to our signal ( no power compression), and second, the radiometer is sufficiently isolated from the source impedance." ... "One of the assumptions made in deriving eq. (2) was that the output from the radiometer is not dependent on the source impedance. In the construction of the radiometer, two isolators are inserted at the input of the radiometer to isolate the radiometer from the source." ... "The mismatch uncertainty depends strongly on the poorly known correlation between uncertainties in the measurements of different reflection coefficients, and so we use the maximum of the uncertainties obtained by assuming either complete correlation or no correlation whatsoever." "Forthcoming Paper: Influence of Impedance Mismatch Effects on Measurements of Unloaded Q Factors of Transmission Mode Dielectric Resonators" IEEE Transaction on Applied Superconductivity "Analysis of Interconnection Network and Mismatch in the Nose-to-Nose Calibration Automatic RF Techniques Group , June 15-16, 2000 , Boston, MA - June 01, 2000 "We analyze the input networks of the samplers used in the nose-to-nose calibration method. Our model demonstrates that the required input network conditions are satisfied in this method and shows the interconnection errors are limited to measurement uncertainties of input reflection coefficients and adapter S-parameters utilized during the calibration procedure. Further, the input network model fully includes the effects of mismatch reflections, and we use the model to reconcile nose-to-nose waveform correction methods with traditional signal power measurement techniques." As I mentioned, obscure references. However, given the impetus of their discussion is long known (and that I have already provided the original references they rely on), NIST presumes the investigators already have that basis of knowledge. 73's Richard Clark, KB7QHC |
Sorry, it's not clear from what you've said just what you're trying to
match to what. Truly, if you connect a 50 ohm load (be it a resistor, or some impedance transformed by a transmission line, tuner, or other means -- makes no difference) to a transmitter, and the transmitter's output impedance isn't 50 ohms, there will be a mismatch at the transmitter output. But who cares? Most transmitters are designed to work properly when terminated with 50 ohms, but that doesn't at all mean that their output impedances are 50 ohms. And it certainly doesn't affect the SWR on any transmission line connected to the transmitter. If you were to discover that your transmitter's output impedance were, say, 10 ohms and you connected it to a 10 ohm load, you'd have a very unhappy transmitter. All you have to worry about is presenting the transmitter with the load it was designed to work into, not what the internal impedance of the transmitter is. Roy Lewallen, W7EL Dr. Slick wrote: Roy Lewallen wrote in message ... Almost correct. The transmitter output impedance has no effect whatsoever on the line's SWR. Roy Lewallen, W7EL My point is that if the output impedance really was at a perfect 50 Ohms, then using a perfect 50 ohms transmission line, you can never match to anything other than 50 ohms. If the S22 is something other than 50 Ohms (usual case), then a transformation via 50 ohm coax about a constant VSWR is possible, for the purposes of matching to a not-quite-perfectly-50 ohms antenna. Slick |
Richard Clark wrote:
I see you still live in a dimensional aberration where you experience 22 days as 66 minutes. I see that you still ignore the technical questions so I will repeat mine: Are you saying that SWR doesn't equal (1+|rho|)/(1-|rho|)? -- 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 80,000 Newsgroups - 16 Different Servers! =----- |
Dr. Slick wrote:
I didn't think you could tell us. I've never seen an SWR meter that you could "calibrate" to 50 or 75 ohms, or less. The calibration of the SWR meter is controlled by the internal sampling load resistor, the 'R' in Peter's V + IR equation. I have a home-brewed SWR meter that measures SWR on both balanced 450 ohm feedlines and on 300 ohm feedlines simply by changing the internal load resistors. -- 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 80,000 Newsgroups - 16 Different Servers! =----- |
Had a look at the refs. I'm curious, did you actually read the
sentence that Roy wrote? Cheers, Tom Richard Clark wrote in message . .. On Tue, 12 Aug 2003 02:48:49 -0700, Roy Lewallen wrote: Almost correct. The transmitter output impedance has no effect whatsoever on the line's SWR. Roy Lewallen, W7EL Hi Roy, Entirely incorrect. Transmitter output impedance that does not conform to transmission line Z, when presented with a mismatched load through that line, adds mismatch uncertainty in the form of an indeterminate SWR and indeterminate Power to the load. This has already been demonstrated twice. This has long been documented with NBS/NIST references going back 4 decades. There is nothing mysterious about it at all, and it conforms to the rather simple principles of wave interference so poorly presented by Cecil in months past. The authoritative site: http://www.boulder.nist.gov/div813/index.html Direct reference: "Juroshek, J. R.; A Direct Calibration Method for Measuring Equivalent Source Mismatch; Microwave J., pp. 106-118; October 1997 Obscure references: http://www.boulder.nist.gov/div813/r...00S_n2nNet.pdf "With vector measurements of the generator and meter reflection coefficients Ãg and Ãm, respectively, the power of the incident signal am can be related to the power of the source." http://www.boulder.nist.gov/div813/r...FRad_ARFTG.pdf which describes radiometer calibration (perhaps too exotic for this group) "tests are based on two assumptions. First, the network responds linearly to our signal ( no power compression), and second, the radiometer is sufficiently isolated from the source impedance." ... "One of the assumptions made in deriving eq. (2) was that the output from the radiometer is not dependent on the source impedance. In the construction of the radiometer, two isolators are inserted at the input of the radiometer to isolate the radiometer from the source." ... "The mismatch uncertainty depends strongly on the poorly known correlation between uncertainties in the measurements of different reflection coefficients, and so we use the maximum of the uncertainties obtained by assuming either complete correlation or no correlation whatsoever." "Forthcoming Paper: Influence of Impedance Mismatch Effects on Measurements of Unloaded Q Factors of Transmission Mode Dielectric Resonators" IEEE Transaction on Applied Superconductivity "Analysis of Interconnection Network and Mismatch in the Nose-to-Nose Calibration Automatic RF Techniques Group , June 15-16, 2000 , Boston, MA - June 01, 2000 "We analyze the input networks of the samplers used in the nose-to-nose calibration method. Our model demonstrates that the required input network conditions are satisfied in this method and shows the interconnection errors are limited to measurement uncertainties of input reflection coefficients and adapter S-parameters utilized during the calibration procedure. Further, the input network model fully includes the effects of mismatch reflections, and we use the model to reconcile nose-to-nose waveform correction methods with traditional signal power measurement techniques." As I mentioned, obscure references. However, given the impetus of their discussion is long known (and that I have already provided the original references they rely on), NIST presumes the investigators already have that basis of knowledge. 73's Richard Clark, KB7QHC |
On Tue, 12 Aug 2003 14:25:47 -0700, Roy Lewallen
wrote: If, as you insist, the source impedance affects the SWR on the line, please provide an equation that gives the SWR on the line, with source impedance being one of the variables. Hi Roy, Your crafted requirement reveals the shortfall inherent in the problem. It is distance based too, and without that discussion there is no way to forecast what SWR or Power reading you would obtain from simply knowing both the load and source's Z's. This is why it is called Mismatch Uncertainty. I have provided NBS and NIST documents both recently and in the past. Recent offerings are obscure, as I have already admitted. The earlier citations I provided were direct and to the point and serve as the basis of the recent work. I have provided data that exhibits the effect. I have provided the test protocol in how to achieve that data. I have also described that this data is also, theoretically, achievable through standard interference math also presented by me in the past. It takes little imagination to observe that there is a zone of confusion that lies between two reflecting interfaces when the path is not fully described. My data showed that path in one foot increments of transmission line over an interval of a quarter wavelength or more. We have been offered evidence of this Mismatch Uncertainty by Dr. Slick if I am to trust his postings - be that as it may, because it requires no further proof. Simply put, bald assertions that SWR is unaffected when read between two discontinuities is wrong without a concomitant description of all paths leading to the SWR meter. This is a commonplace of interference plain and simple. I have observed no one describing this detail (except Dr. Slick). As all this is part of the historical record entitled: "The Cecilian Gambit, a variation on the Galilean Defense revisited" I do not see how its repetition here brings anything new to the mix. None the less, this recent example has been fun. :-) 73's Richard Clark, KB7QHC |
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