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Dr. Slick wrote:
How would you explain what Cecil wrote? How are some people improving SWR by changing coax length, when in theory they shouldn't be able to do this? If I were using a 450 ohm SWR meter on 450 ohm Z0 feedline, the SWR meter would indicate no change in SWR as I change the feedline length. Assuming the 450 ohm SWR meter is accurate, the indicated SWR and the actual SWR are the same. When I use a 50 ohm SWR meter on 450 ohm Z0 feedline, the SWR meter is NOT indicating the true SWR on the 450 ohm feedline. It is indicating what the SWR would be if the line were Z0=50 ohms. All it is indicating is the impedance seen by the transmitter which indeed does change with feedline length in a system with reflections. Theoretically, the SWR on a feedline changes only slightly with changing feedline length and that is because of additional losses in the feedline as the length is increased. -- 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! =----- |
I wrote a little BASIC program.
================================ The value of a computer program resides not in the source code language but solely in the Reliability of the programmer. And Reliability is a measure of Quality versus time. And Quality is the extent to which a product conforms to pre-specified requirements. |
On Sat, 16 Aug 2003 14:50:29 -0500, W5DXP
wrote: Richard Clark wrote: Do you have a question about it following that point in time? I wrote a little BASIC program to digest your data and it decided that all your data is perfectly consistent with a constant SWR on a Z0=~40 ohm coax between the 50 ohm SWR meter and the 16.67 ohm load. What is the Z0 of your "RG-58 type hardline"? Hi Cecil, 50 Ohm. The BVT has been calibrated long before this application (8 years ago the first time) without surprises. Methods are described in Walt's Book, Reflections. I suggest you use his software rather than roll your own. I would also suggest you consider his methods as well. Point in fact, I followed his advice to measure at odd 8th wave intervals if I recall his instructions correctly. If not, it was "by the book." I also performed the calibrations at the remaining cardinal points. Tedious perhaps, but having done it through maybe three or four times without outliers, I don't expect there is any need to visit that again. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
I suggest you use his software rather than roll your own. Funny that you request responses, nay, demand responses, and then dismiss them out of hand. The data indicates an impedance incompatibility between the transmission line and the measuring instrument. Your results are perfectly compatible with an SWR spiral on a 40 ohm Smith Chart including the loss in the feedline. -- 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:
I wrote a little BASIC program. ================================ The value of a computer program resides not in the source code language but solely in the Reliability of the programmer. And Reliability is a measure of Quality versus time. And Quality is the extent to which a product conforms to pre-specified requirements. As demonstrated by reference to what external standards? I wouldn't walk too far out along that plank, Reg... -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
An additional source of VSWR error exists. It is of interest to the purist.
If I provide a perfect 50.000 ohm dummy load for typical JAN-C-17 RG type cable, example RG-9913, I have to design for a maximum VSWR of 1.06 to 1 due to tolerances in the cable!! So, I'll have a minor variation in observed VSWR as a function of length. Same applies to the design impedance of your VSWR meter. It has a tolerance on it's 50 ohms!!!!! Deacon Dave, W1MCE W5DXP wrote: Dr. Slick wrote: How would you explain what Cecil wrote? How are some people improving SWR by changing coax length, when in theory they shouldn't be able to do this? If I were using a 450 ohm SWR meter on 450 ohm Z0 feedline, the SWR meter would indicate no change in SWR as I change the feedline length. Assuming the 450 ohm SWR meter is accurate, the indicated SWR and the actual SWR are the same. When I use a 50 ohm SWR meter on 450 ohm Z0 feedline, the SWR meter is NOT indicating the true SWR on the 450 ohm feedline. It is indicating what the SWR would be if the line were Z0=50 ohms. All it is indicating is the impedance seen by the transmitter which indeed does change with feedline length in a system with reflections. Theoretically, the SWR on a feedline changes only slightly with changing feedline length and that is because of additional losses in the feedline as the length is increased. |
Cecil,
Suppose I do the following experiment: a source of unknown impedance, connected to a shorted 1/4 wave line through a ~50 Ohm series resistor. When looking at the voltage on the two ends of the resistor with a 'scope, I expect them to be the same value, with no phase difference. Do you think that would be enough to satisfy the nay sayers that the source is delivering no power? I am *not* talking about you. In retrospect, my posting here is probably a variation of what Roy posted a few months ago. Didn't mean to rip him off. BTW, I was in error when I said the line charges up in 1/2 cycle; that would be true only if the source impedance was 0. Tam/WB2TT "W5DXP" wrote in message ... Tarmo Tammaru wrote: I think the only alternative in un nice; namely that there is no reflection in steady state There are an infinite number of possibilities between the rails of 100% re-reflection and zero re-reflection. I suspect a PA obeys the rules of the wave reflection model set forth in Ramo & Whinnery. -- 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! =----- |
Richard Clark wrote:
What does your software report for calibrating a software 40 Ohm line feeding a software 50 Ohm load? It doesn't report that. It is a data reduction, curve fitting program. What does your software report for calibrating a software 50 Ohm line feeding a software 50 Ohm load? It doesn't report that. It is a data reduction, curve fitting program. The only input to the program is your data. Consider that with your 16.67 ohm load, the impedance 1/2WL away from the load should repeat. But the minimum reflected power point indicates a load of about 25 ohms. So there's one error already, perhaps caused by the diode. If you plot your data as SWR points on a 50 ohm Smith Chart, it is obvious that the SWR circle (spiral) is compressed at the lowest reflected power readings. It is also obvious that the center of the SWR circle is not the center of the 50 ohm Smith Chart. That's a pretty clear indication that everything is not 50 ohms. If full scale on the Bird is 5 watts, the readings can be off by 0.25 watts. 0.45 watts plus or minus 0.25 watts is not very accurate. -- 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! =----- |
Richard Clark wrote:
What does your software report for a software 40 Ohm line feeding a software 50 Ohm load? That is, when all data points (that is each increment of 1 foot) exhibits 0 Watts returned for 5W applied? My software takes a list of reflected powers, assumes the forward power is five watts, calculates the 50 ohm SWR for each point, and plots them every 0.03WL on a Smith Chart. It was written to accept your data points. If the reflected power above was always zero watts, all those points would be the same point, i.e. a circle of zero radius at the center of the Smith Chart. Take time to plot those 50 ohm SWR points on a Smith Chart with the 0.45 watts reflected point assumed to be the lowest purely resistive point. A picture is worth a thousand words. I have seen that picture before. It was when I designed my SWR meter for a Z0 of 450 ohms and my window-line turned out to have a Z0 of 388 ohms. -- 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! =----- |
Richard Clark wrote:
Thank you. I see that for the 50 Ohm load that the line exhibits a characteristic of 50 Ohms as described by its concentricity about the reference. This is no surprise given the cable is premium short run (length, not reels) material specified out to 20 GHz (from Boeing's precision electronics facilities). I have several hundred feet brand new. Please plot your data on a Smith Chart. Since your data is obviously nonlinear, I would suggest diagnosing the nonlinearity problem before concluding anything. For instance, what is the one tenth scale RF RMS voltage before it is rectified by the diode? -- 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 Sun, 17 Aug 2003 07:25:45 -0500, W5DXP
wrote: Richard Clark wrote: Thank you. I see that for the 50 Ohm load that the line exhibits a characteristic of 50 Ohms as described by its concentricity about the reference. This is no surprise given the cable is premium short run (length, not reels) material specified out to 20 GHz (from Boeing's precision electronics facilities). I have several hundred feet brand new. Please plot your data on a Smith Chart. Since your data is obviously nonlinear, I would suggest diagnosing the nonlinearity problem before concluding anything. For instance, what is the one tenth scale RF RMS voltage before it is rectified by the diode? Hi Cecil, That is the whole point of Mismatch Uncertainty. What you call nonlinearity is the NBS documented conclusion of the mismatched conditions at both ends. Feel free to investigate and enquire further about your concern over nonlinearity. 73's Richard Clark, KB7QHC |
Roy Lewallen wrote in message ...
Absolutely incorrect. Time for you to review your Smith Chart again. Here's an experiment for you to try. On your workbench, measure the characteristic impedance of a cable, or connect it to a source or load and measure the SWR on it. Now go over to your desk, take out your grease pencil, and change the reference impedance of your Smith chart. Go back to the bench, check the SWR and the cable Z0. Has it changed? Do you often change the impedances of your equipment by scribbling on a pad of paper? Maybe you can tell us how you do this. If *you'll* review the Smith chart again, you'll find that *if* you set the reference impedance to the Z0 of the transmission line you're analyzing, then the SWR, impedances, and so forth that your read from the Smith chart are correct. If you set the reference impedance to some other value, an SWR read from the chart certainly isn't the SWR on the transmission line. PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy load 1 2 3 Once again, if you placed an SWR meter of the same output impedance of the PA at point 1, you will measure the SWR at point 1, and it would be akin to re-normalizing the Smith. Change the PAs S22 (and the impedance of the SWR meter, and you will definitely change the SWR at point 1. Do you agree with this? I've already agreed with you that the SWR at point 3 won't change in theory, as it is surrounded by 50 ohms. What is it about this simple concept that you cannot seem to understand? Thank you, although I'm not an expert at using the Smith chart, I know my way around the circle. It's obvious you don't. Time to read up on it, Roy. I am always interested in learning. I don't use a Smith chart a great deal, Obviously. Slick |
Dr. Slick wrote:
Roy Lewallen wrote in message ... . . . If *you'll* review the Smith chart again, you'll find that *if* you set the reference impedance to the Z0 of the transmission line you're analyzing, then the SWR, impedances, and so forth that your read from the Smith chart are correct. If you set the reference impedance to some other value, an SWR read from the chart certainly isn't the SWR on the transmission line. PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy load 1 2 3 Once again, if you placed an SWR meter of the same output impedance of the PA at point 1, you will measure the SWR at point 1, and it would be akin to re-normalizing the Smith. Change the PAs S22 (and the impedance of the SWR meter, and you will definitely change the SWR at point 1. Do you agree with this? Absolutely not. The SWR meter will measure the SWR at point 1 only if it's the same impedance as the *line*. If the PA has a different impedance than the line, and the SWR meter has the same impedance as the PA, it will not be reading the SWR on the line. If you want to use a Smith chart to see what the SWR is on the line, you must normalize it to the impedance of the line. Change the PA impedance all you want. Change the Smith chart normalization all you want. Neither will have any affect on the SWR on the line. Changing the meter impedance won't have any effect on the SWR on the line, either, although it'll change the reading on the meter so that it's no longer indicating the SWR on the line. I've already agreed with you that the SWR at point 3 won't change in theory, as it is surrounded by 50 ohms. What is it about this simple concept that you cannot seem to understand? ? Thank you, although I'm not an expert at using the Smith chart, I know my way around the circle. It's obvious you don't. Time to read up on it, Roy. I am always interested in learning. I don't use a Smith chart a great deal, Obviously. I see you're reduced to insults now. Time for me to really leave this thread. I'll leave you with your conception of how things work, hoping only that the other readers aren't left with them also. Roy Lewallen, W7EL |
Dr. Slick wrote:
PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy load 1 2 3 Once again, if you placed an SWR meter of the same output impedance of the PA at point 1, you will measure the SWR at point 1, and it would be akin to re-normalizing the Smith. Change the PAs S22 (and the impedance of the SWR meter, and you will definitely change the SWR at point 1. Do you agree with this? What exactly do you think you mean by "change the SWR at point 1"? All you have done is to replace one SWR meter by a second one that has been calibrated for a different system reference impedance. You had to *choose* a value for the new reference impedance in order to pre-calibrate that second meter. When you re-normalize the Smith chart, you choose the *same* new value. So of course the SWR reading on the second meter is going to be the same as you'd calculate by re-normalizing the SWR1 circle on the Smith chart from Z1 to Z2. If that's all you ever meant, we've spent a week homing-in on something that is totally obvious. The only other problem with your example is that by mentioning the PA's S22 value, you seem to imply that it is relevant. It isn't, because variations in the source impedance will not change the SWR reading on either meter. Only the load impedance can change the SWR. Now take away all the coax and move the 50-ohm load to the outputs of the respective SWR meters. The indicated values of SWR1 and SWR2 will be exactly the same as before. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Richard Clark wrote:
That is the whole point of Mismatch Uncertainty. What you call nonlinearity is the NBS documented conclusion of the mismatched conditions at both ends. I suspect many of the irregularities would be greatly diminished if you ran the experiment at 50 watts forward instead of 5. -- 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 Sun, 17 Aug 2003 15:01:44 -0500, W5DXP
wrote: Richard Clark wrote: Feel free to attempt it. You are one of the more qualified to do this by having a variable transmission line like mine. Unfortunately, my variable transmission line is Z0 = 388 ohms and I wouldn't trust my homemade 388 ohm SWR meter for wattmeter accuracy. The lengths of my T-line sections are also not super accurate. Let me get straight what you are saying. As I understand it, you are saying that if you chop off one foot of coax at a time when driving a fixed amount of forward power into a 3:1 mismatched load, the SWR will change from, e.g. 3.8:1 to 1.9:1. Is that correct? Hi Cecil, Your trepidation about the accuracy is well taken, but perhaps overstated for a first pass approximation. I will leave you to judge that, but I would point out, that if you can conspire to load both ends to mismatch as I have described (or worse), you should see the same symptoms. That your lines do not fall on cardinal points; I have faith in that your craftsmanship is of high enough caliber that it won't make any difference if you can replicate the same wavelength interval (which means you will probably need to work this out above 20 MHz). Error is not inescapable, but your regression analysis should reveal the same trend. If you in fact do not know the characteristic source Z of your transmitter, then forcing a known value imparts a basic maximum. In other words, what ever unknown value in parallel with a known value could never exceed that known value. A simple 50 Ohm load at the transmitter output guarantees that the source could never exhibit any higher value. This feeding your 388 Ohm line is by far and away more severe than any of the reports or tests I have offered. With the far end terminated in another 50 Ohm load, then you should see extremely wide variation. Your regression analysis should reveal the same pattern as you found with my data (what you call nonlinearity). You may then compare your regressions' chi-squares. Or simply measure the line loss in your standard configuration and compare it to the severely mismatched arrangement I describe above (both ends terminated in 50 Ohm loads). Accuracy demands a lot of front work, but a first pass test such as I describe should present a quick resolution to whether or not you want to commit beyond to engage in that labor. The worst of this is how much you can (or your transmitter can) tolerate working into these deliberate mismatches. 73's Richard Clark, KB7QHC |
"W5DXP" wrote in message ... How about looking at the voltage across the resistor? i.e. the difference between the voltages at each end of the resistor? -- 73, Cecil http://www.qsl.net/w5dxp That is what I mean, By looking at both ends at the same time with a dual trace scope. I guess I could also go A - B. I have the stuff to do it with. About 60 feet of 9913 that I can short the far end on, a dual trace 100 MHz scope, and MFJ analizer as a signal source. The latter is also usefol for finding exact 1/4 wavelength frequency, as I can't measure the coax length. ( Goes through several walls). Tam/WB2TT |
Richard Clark wrote:
Accuracy demands a lot of front work, but a first pass test such as I describe should present a quick resolution to whether or not you want to commit beyond to engage in that labor. The worst of this is how much you can (or your transmitter can) tolerate working into these deliberate mismatches. My SGC-500 is speced to work into a 50 ohm SWR of 6:1. -- 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! =----- |
Richard,
The frequency is going to end up around 4 MHz, which is not too bad for ALT. We will see what happens. I have a pair of 10X probes, which I can match. I guess we both know I am going to end up measuring the loss of the coax Tam/WB2TT |
Roy Lewallen wrote in message ...
let me rephrase: PA----+SWR meter1+----50 ohm line----+SWR meter2+----50 ohm line----+50 ohm (50 Ohms) dummy load 1 2 3 If you measure the SWR at point 1, you let Z1 be 50 ohms, and Zo is the output impedance of the PA. SWR meter1 should be characterized to Zo. I've agreed that SWR meter2s SWR reading will not change as you change PAs impedance, but the SWR at point 1 definitely will. So, Does the SWR change as you change the source impedance? IT DEPENDS ON WHERE YOU MEASURE IT! From Pozar's Microwave Engineering (Pg. 606): Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo) It should be easy to see from this well known equation that a perfect 50 Ohm Z1 will definitely change SWR as you change Zo away from 50 ohms. Slick BTW Roy, comparing someone to the intelligence of your adolescent kid who waits for trains because you told him you are an engineer is indeed an insult. |
Dr. Slick wrote:
From Pozar's Microwave Engineering (Pg. 606): Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo) Unfortunately, the reflection coefficient is also defined as: Sqrt(Pref/Pfwd) which is usually only equal to your equation in a one-port network. For a two-port network, those two values of reflection coefficient are not usually equal. Better define things better. For a two-port network, Z1 is NOT the physical Z1. It is the apparent (virtual) Z1 calculated from the total voltage to total current ratio which is often different from the physical Z1 above. -- 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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