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Richard Clark wrote:
How curious of you to have missed this pæan to your style: Not curious at all, Richard. My Mother is in the hospital with terminal leukemia. -- 73, Cecil, W5DXP |
Richard,
Thank you very much for posting that reference to John Juroshek's article. It happens to cover a topic very near and dear to my heart, and I called John and we had a nice chat about it. In addition, I told him where I saw the reference, and asked him if he would say that the SWR on a line, in steady state excitation, with a source at one end and a load at the other, depended on the source's source impedance. He said it of course does not, and cannot understand how you would have interpreted it that way. Should I check with any of the other authors? 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 |
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On Thu, 14 Aug 2003 14:56:27 -0700, W5DXP
wrote: Richard Clark wrote: You among the many have done more in less time. I can accept negative, informed response. Can he explain my posting? That is the more telling as it is MY statement, not your summary of it. A couple of questions about your data. What kind of coax and connectors are used for the BVT? hardline of RG-58 dimension; only two connectors, the gozinta, and the comesoutta (never changed throughout any variation nor calibration). What kind of wattmeter was used for the readings? Bird. The load seems to be 50/3 = 16.67 ohms. I would like to see the same data for a load of 50 ohms. That is already discussed in the report. Seems you are getting the same resulting effect that I do (on purpose) when I vary the length of my ladder-line while watching the 50 ohm SWR. Your results may repeat every 1/2WL. What if you had gone out to 20 feet instead of stopping at 15 feet? I have no reason to think it would offer any change in a periodic variation already in evidence. One last thought. Did you consider common-mode current possibilities? Yes. 73's Richard Clark, KB7QHC |
W5DXP wrote in message ...
By definition. I wonder who invented that definition? It seems pretty obvious that not all PA's are Z0-matched so they will always re-reflect 100% of the incident reflected power. But that is exactly what that definition implies. Your vocabulary is very confusing here, Cecil. "Incident" usually refers to the forward power, so when you say "incident reflected" it's extremely confusing. Slick |
Roy Lewallen wrote in message ...
Dr. Slick wrote: Agreed. Then any mismatch loss from PA to the 50 ohm coax is never measured at the meter. So we never really measure the reflected power coming right out of the PA, even if we attach the meter directly to it's output. No "reflected power" comes out of a PA. Roy Lewallen, W7EL Maybe that why you can't measure it. Slick |
Roy Lewallen wrote in message ...
The observation that changing line length changes the measured SWR is regularly reported in this newsgroup, and the explanation is as regularly provided. There are at least three ways this can happen. 1. The SWR meter is designed for an SWR that's different from the line impedance. It's easy to show that this will result in different readings for different line lengths. I've measured RG-58 at over 60 ohms characteristic impedance, so this can happen even with a perfect 50 ohm SWR meter and "50 ohm" line. In this case, changing line length isn't really changing the line's SWR, just the meter reading. But a 60 ohm transmission line transformation from a non-50 ohm load will certainly change the SWR, as it won't be on the constant VSWR circle anymore. 2. There's significant loss in the cable. In that case, the longer the distance between the meter and the load, the better the SWR. That's the obvious one. 3. There's current on the outside of the coax. This means that the outside of the transmission line is actually part of the antenna. When you change its length, it changes the effective length of the antenna, which really does change the SWR. Current on the outside of the cable can also get into a poorly shielded SWR meter and modify its reading. And this is all in agreement with established theory. So you see, theory does say you can change the SWR reading, and in some cases, the actual SWR, by changing the coax length. But only under very specific circumstances. When observations don't match theory, chances are overwhelming high that either the observation is erroneous or misinterpreted, or theory is being misapplied. Roy Lewallen, W7EL I believe the source of confusion is he PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy load 1 2 3 The "+" are connector points. You folks are saying that as you change the PA (source) impedance, that the SWR you read will remain the same, even if the incident power changes due to the change in reflected power at point 1. This i can agree with, as no matter how much incident power makes it past the impedance discontinuity at point 1, the system after this point will theoretically always reflect the same ratio of reflected power to incident. But, if you place an SWR meter of the same impedance as the output of the PA at point 1, you will definitely see a change in SWR at point 1 as you change the PA impedance, as you are changing the reference impedance (center of Smith re-normalized). This is what i thought you meant when you said "change the source impedance", but you meant to say "change the source, but keep the reference impedance the same". Understood, assuming this is what you guys mean. Slick |
Roy Lewallen wrote in message ...
If you ever do see an equation which shows transmission line SWR as a function of source impedance, please let me know where you see it. I'll add it to my list of untrustworthy sources. A simple experiment could be set up to disprove it. Roy Lewallen, W7EL I believe we are both right, and there is simply a misunderstanding here. I believe the source of confusion is he PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy load 1 2 3 The "+" are connector points. You folks are saying that as you change the PA (source) impedance, that the SWR you read will remain the same, even if the incident power changes due to the change in reflected power at point 1. This i can agree with, as no matter how much incident power makes it past the impedance discontinuity at point 1, the system after this point will theoretically always reflect the same ratio of reflected power to incident. But, if you place an SWR meter of the SAME IMPEDANCE AS THE OUTPUT of the PA at point 1, you will definitely see a change in SWR at point 1 as you change the PA impedance, as you are changing the reference impedance (center of Smith re-normalized). This is what i thought you meant when you said "change the source impedance", but you meant to say "change the source, but keep the reference impedance the same". Understood, assuming this is what you guys mean. Slick |
W5DXP wrote in message ...
I was thinking about how you changed the impedance of your SWR meter, and I doubt that simply changing the terminating resistors to ground on both directional couplers is all you have to do. I believe this because the SWR/power meter needs to be a Zo ohm Thru, with a microstrip line of a certain dielectric thickness, and particular width and spacing from the couplers. And you cannot change these very easily. Do you have a schematic for us to see? Slick |
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