|
Roy Lewallen wrote:
And if Cecil's work leads to the conclusion that the source impedance impacts the line's SWR, then it's wrong. It doesn't. In fact, just the opposite. My examples do NOT even include a source impedance. -- 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 17:04:59 -0500, W5DXP
wrote: 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|)? Hi Cecil, Why don't you solve the first problem before presenting your own? 73's Richard Clark, KB7QHC |
|
|
"W5DXP" wrote in message ... 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 Cecil, When you do this, does the scale still display the correct SWR for conditions other than 1:1 ? Tam/WB2TT |
On Tue, 12 Aug 2003 15:52:05 -0700, Roy Lewallen
wrote: It's admittedly hard for me to follow what you've written, but it sounds like you're saying that: Hi Roy, 1. Source impedance affects line SWR, and 2. It's impossible to tell by how much. Did I get that right? No. The sources offered and the data exhibited provide a very clear answer. To state this yet again, it is the lack of knowledge in the distances traversed between reflecting interfaces that introduces the Mismatch Uncertainty. A smart lad might conspire to present any particular Power measurement if he could withhold other details from scrutiny. A lad who considered himself smart may do the same but think he invented a free power amplifier (or dissipationless load). This is simply an account of poor boundary controls that turn some folk into magicians and others into the reincarnation of Galileo. Either outcome is achieved through delusion. Incidentally, you've brought up a new topic, that of an SWR meter. Look at the Subject line. As for the aside about SWR meter reading, I have performed SWR measurements with a variety of NBS methods (many hundreds of times) - none of them described here very often, and some never at all. I doubt any here are so well versed in these methods as to challenge my data by employing them (it would only confirm the results). I would be happy to see as much effort put to it - in that it would represent a technical rebuttal rather than echoed denials. I would be happy to retract my points if someone revealed an error of commission/omission - such has not happened and discussion of that data has been wholly absent. Look Roy, skip the rhetoric (from all sources including me) and explain or refute the data I obtained. If you cannot accept it, reveal the error. If the method is too tedious to replicate - say so. This is not the first time I've broached the subject and I certainly don't expect many to care for one, or follow blithely for another. It only matters in issues of accurate Power determination. 73's Richard Clark, KB7QHC |
Tarmo Tammaru wrote:
"W5DXP" wrote: 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. When you do this, does the scale still display the correct SWR for conditions other than 1:1 ? The scale is calibrated using known loads so yes, it displays the correct SWR up to 5.83:1. Above 5.83:1 the resolution of the scale is poor because the full scale SWR is infinity. I have marks at 5.83:1 and 10:1. 5.83:1 is where the reflected power equals half the forward power. Incidentally, the Z0 of my '450' ohm ladder- line is closer to 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 80,000 Newsgroups - 16 Different Servers! =----- |
W5DXP wrote in message ...
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. BTW, how do you know the accuracy of your homebrew SWR meter? Slick |
Dr. Slick wrote:
Would these be the termination resistors to ground, one for each directional coupler? Yes I would think that you would have to adjust the width of the traces as well, or the thickness of the dielectric material, or the space between the couplers and the thru-line to use it for an impedance other than 50 Ohms (talking about commercial equipment, that is). I use a toroid to sense the RF current and an R=Z0 resistor to turn it into a voltage. I have a capacitive voltage divider on the RF voltage which is adjustable for calibration purposes. The only real difference between it and a 50 ohm SWR meter is the internal load resistor. It's actually a bridge circuit with a Z0 reference resistor. Unfortunately, it is not very accurate for the relatively high SWRs that I run on my ladder-line, e.g. 4:1 to 16:1 -- 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 80,000 Newsgroups - 16 Different Servers! =----- |
Dr. Slick wrote:
BTW, how do you know the accuracy of your homebrew SWR meter? I have a bunch of 50W 600 ohm non-inductive resistors that I use for calibration purposes. And I really don't know the accuracy. An SWR of 20:1 looks the same as an SWR of 25:1 on the scale. I have an upper and lower acceptable limit to the SWRs with the matching method I use. The SWR meter tells me if the SWR is outside of that acceptable range. -- 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:
As you might know, the input S11 or SWR will change when you go from an antenna analyzer or network analyzer to measuring with the actual full power PA and meter hooked up. This may be partly due to the fact that the meter is usually not a perfect 50 ohm thru, and partly due to the fact that the analyzers outputs are closer to 50 ohms than the PA. If S11 or the SWR actually does change, you've either got a nonlinear transmission line or a nonlinear load. That is, the impedance changes as the signal level changes. If you *measure* a different SWR or S11, it means that either the SWR or S11 is actually changing for the reasons I just stated, or the meter is nonlinear in the sense that its reading changes with power level (possibly due to RF ingress, but it could be a host of other things), or you're measuring with two different meters that don't agree. It's not because of the different source impedances. Sure, you can normalize a Smith chart to anything you'd like. That doesn't make the SWR change with source impedance. . . . Roy Lewallen, W7EL |
W5DXP wrote:
Dr. Slick wrote: BTW, how do you know the accuracy of your homebrew SWR meter? I have a bunch of 50W 600 ohm non-inductive resistors that I use for calibration purposes. And I really don't know the accuracy. An SWR of 20:1 looks the same as an SWR of 25:1 on the scale. I have an upper and lower acceptable limit to the SWRs with the matching method I use. The SWR meter tells me if the SWR is outside of that acceptable range. It's interesting to see an example of an SWR meter for a Z0 that isn't 50 ohms, because it helps to confirm that they all work in basically the same way. If [V] is a sample of the line voltage, and [i] is a sample of the line current, then the forward reading is the sum of two RF voltages, [V] + [i]R, where R is the resistor that converts the [i] sample into a voltage. The reverse reading is the difference, [V] - [i]R. The "calibration to Z0" procedure consists of terminating the line in the design value of Z0, and then adjusting R so that the reverse reading [V] - [i]R is zero. The RF voltages are either summed or subtracted, and then the resultant is detected by the diode. Just one small point, though... it is not necessary that R = Z0. The value required depends on the sampling factors kV and kI that relate the voltage and current in the line to the sampled values [V] and [i]. In full, the instrument is calibrated to Z0 when: kV*V - kI*I*R = 0 In a typical bridge, two out of the three constants kV, kI and R are fixed, and the third is adjustable. In a Bruene bridge, kI is fixed by the number of turns on the current-sampling toroid, R is fixed, and you calibrate the bridge by adjusting the kV factor in the voltage divider. However, it would be equally good to build-in fixed values of kV and kI, and balance the bridge by making R a small trimpot. So R really does not have to equal Z0... and in most published circuits, it doesn't. This can also be shown in a different way, by thinking of it as a Wheatstone bridge, with Z0 as one arm. The requirement for balance is only that Z0/R2 = R3/R4. It is not necessary for any of the other resistors R2, R3 or R4 to equal Z0 in order to achieve balance. AFAIK, the only situation where the "terminating" resistor truly needs to equal Z0 is in parallel-line couplers for microwaves, when the sampling line approaches a quarter-wavelength long and its own characteristic impedance is Z0 too. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Dr. Slick wrote:
As you might know, the input S11 or SWR will change when you go from an antenna analyzer or network analyzer to measuring with the actual full power PA and meter hooked up. This may be partly due to the fact that the meter is usually not a perfect 50 ohm thru, and partly due to the fact that the analyzers outputs are closer to 50 ohms than the PA. Sorry, that is exactly wrong. S11, SWR and the impedance itself, do *not* change when you connect a different instrument to the same load. All the changes you have described are due entirely to instrument errors. That's how the instrument errors are determined... by knowing for a fact that, whatever all the different instruments may say, the impedance they're trying to measure is the one thing that has *not* changed. -- 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 in message ...
Sorry, that is exactly wrong. S11, SWR and the impedance itself, do *not* change when you connect a different instrument to the same load. All the changes you have described are due entirely to instrument errors. That's how the instrument errors are determined... by knowing for a fact that, whatever all the different instruments may say, the impedance they're trying to measure is the one thing that has *not* changed. You are right, but i never stated that the impedance we are feeding ever changes, only the measured SWR. Hey, we live in the real world, with real instrument errors. This is the case i am talking about when i started the thread. I usually DON'T measure the same SWR from antenna analyzer versus PA and meter hooked up, and this may be due to the fact that the PA has a different source impedance than the analyzer. I'm not claiming that the impedance we are feeding has changed. And if you read my original post, you will notice that the SWR didn't change when the coax length was changed, mainly the incident power. 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? Do you think the series reactance a system offers a PA may actually improve it's incident power? Slick |
Dr. Slick wrote:
And i don't think you can expect the measured SWR with the meter and PA to be exactly the same as what you get with a small-signal analyzer. It's usually a bit different. The actual conceptual SWR and the difficulty in measuring the SWR are two different things. Low power level SWRs are difficult to measure because of the diode voltage drops. Better to use a class-B amp for such where the zero-crossing points are aligned. -- 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:
"Ian White, G3SEK" wrote in message ... Sorry, that is exactly wrong. S11, SWR and the impedance itself, do *not* change when you connect a different instrument to the same load. All the changes you have described are due entirely to instrument errors. That's how the instrument errors are determined... by knowing for a fact that, whatever all the different instruments may say, the impedance they're trying to measure is the one thing that has *not* changed. You are right, but i never stated that the impedance we are feeding ever changes, only the measured SWR. Oh dear... just when I thought it was safe to go back into the waves... [On the other points, I'll reply to your second, corrected, posting.] How would you explain what Cecil wrote? Who else but Cecil would dare attempt that? :-) How are some people improving SWR by changing coax length, when in theory they shouldn't be able to do this? There are two possible reasons. One is instrument error - SWR meters are not perfect. The other possible reason is that the *outer* surface of the coax has currents on it, so it has become part of the antenna. In that case, changing the length of coax is not only changing the length of feedline (the inner surfaces of the coax), but also is changing the antenna itself. The voltage and current distributions on all the wires will shift around, resulting in a different V, I and relative phase at the top of the coax - in other words, a different feedpoint impedance. Then the SWR (as measured on the *inside* of the feedline) genuinely will change. This SWR change is usually quite difficult to predict, because you didn't mean there to be any current on the outer surface of the coax in the first place. The only practical way to see if there could be a problem is to use a clamp-on RF current meter to see how much surface current is present. If the SWR changes with feedline length *and* you have significant surface current, then you know one probable reason... but in all cases, these can also be instrument error in the SWR meter. Do you think the series reactance a system offers a PA may actually improve it's incident power? To answer your exact question: I don't think there is a valid general answer. It depends on the specific PA design, and also on what you mean by "improve". What I do know is that changes in the load impedance presented to a PA will change several of its operating conditions, all at the same time. Some of those changes will be "improvements" - but others definitely won't be. For example, reducing the load impedance will usually make the output device operate in a more linear way... but the efficiency drops and the greater heat dissipation and current are likely to shorten the lifetime of the device. Is that an improvement? -- 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:
The RF voltages are either summed or subtracted, and then the resultant is detected by the diode. I know you know this, Ian, but let's make sure that everyone understands that the summation (or subtraction) is a phasor (vector) summation. It is the phasing between the total voltage and total current that allows the forward wave to be separated from the reflected wave, and vice versa. The directional coupler designer assumes that the ratio of Vfor/Ifor = Z0 and that the ratio of Vref/Iref = Z0. If that assumption is incorrect, the SWR meter will still assume that the assumption is correct. Just one small point, though... it is not necessary that R = Z0. That's true and is just a habit on my part. I set R=Z0 and then adjust the voltage accordingly for calibration purposes. For awhile, I was using a 450 ohm load for ladder-line with a measured Z0 of 388 ohms. It still worked pretty well. AFAIK, the only situation where the "terminating" resistor truly needs to equal Z0 is in parallel-line couplers for microwaves, when the sampling line approaches a quarter-wavelength long and its own characteristic impedance is Z0 too. In most of the slotted line pickups that I have seen, the internal load resistor is equal to the Z0 of the slotted line. I don't know if that is necessary or not. -- 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:
Roy Lewallen wrote: Sure, you can normalize a Smith chart to anything you'd like. That doesn't make the SWR change with source impedance. I disagree on this point. You are caught up in the 50 Ohm world, which i admit is easy to do. The SWR is based on the ratio of the forward to the reflected power. If you had an analyzer that was calibrated to 20 Ohms (the same as normalizing the Smith for 20 Ohms in the center) you would certainly have reflected power and high SWR going into a 50 Ohm load. Not if the feedline has a Z0 of 50 ohms. The problem would be in believing what you believe about what the 20 ohm SWR meter is trying to tell you. It is trying to tell you that it is being misused but you are inferring that it is trying to tell you the actual SWR. It is not. And a 20 Ohm load would have a 1:1 SWR. Not if the feedline has a Z0 of 50 ohms. If a DC voltmeter gives an erroneous reading for RF voltage, do you blame the voltmeter or the user of the voltmeter? -- 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:
"Ian White, G3SEK" wrote in message ... Dr. Slick wrote: As you might know, the input S11 or SWR will change when you go from an antenna analyzer or network analyzer to measuring with the actual full power PA and meter hooked up. This may be partly due to the fact that the meter is usually not a perfect 50 ohm thru, and partly due to the fact that the analyzers outputs are closer to 50 ohms than the PA. Sorry, that is exactly wrong. S11, SWR and the impedance itself, do *not* change when you connect a different instrument to the same load. All the changes you have described are due entirely to instrument errors. That's how the instrument errors are determined... by knowing for a fact that, whatever all the different instruments may say, the impedance they're trying to measure is the one thing that has *not* changed. On second thought, i believe we are all wrong to equate S11 with SWR! Input S11 of a system will certainly never change. But the SWR is absolutely dependant on the source impedance. No! SWR, S11, return loss, rho, Y-parameters, Z-parameters, etc, etc are all different derived functions of the same two variables: an arbitrary complex impedance, and the system reference impedance Z0 (a constant which may or may not be defined as complex). Only those two variables are involved, so all of these functions are locked together. If one variable changes, all the derived functions change too. Either all change, or none change; nothing else is logically possible. As Roy says, the equations relating any one of these parameters to any other are all well known. NONE of them ever involves source impedance. If you had a network analyzer calibrated for 20 Ohms, you would certainly have reflected power and high VSWR going into 50 Ohms, and a 1:1 SWR going into 20 Ohms. This would be the same as re-normalizing the Smith Chart for 20 Ohms in the center. You certainly can do this in MIMP. I don't blame anyone for believing it's a 50-Ohm-only world! No argument about any of that... but it's a totally separate point that has no relevance whatever to your earlier statements about source impedance. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Non-linear, or time variant. I'd expect some variation going between
low and high power with typical dummy loads, because the resistance will change with temperature. In fact, the loss in the transmission line will change with temperature, too, and it's significant enough to be able to measure with ham-type equipment if you are careful. But I'd FIRST suspect other things going on: different calibrations between different instruments, and the fact that SWR meters that use uncompensated diode detectors will not read the same SWR at low power as at high: they will fail to read high enough at low powers. The resistance of the copper in the transmission line changes with temperature. If ambient is 20C and you put in enough power to heat up the line (center conductor) to 70C, that's a 50C change, and will result in about an 18% increase in resistance. So if you had a line which had 3dB loss at 20C, it would increase to about 3.5dB at 70C. If the load end has a 2:1 SWR, then the sending end will have about 1.40:1 SWR at 20C and about 1.35:1 at 70C. It's not a _big_ change, but it should be observable on an SWR meter that is accurate over a wide range of powers. I want to make it clear that this is in support of what Roy and Ian are saying, as an added detail, and not contrary to the notion that SWR on a line in a linear, time-invariant system with steady-state excitation does not depend on the source impedance. Cheers, Tom Roy Lewallen wrote in message ... Dr. Slick wrote: As you might know, the input S11 or SWR will change when you go from an antenna analyzer or network analyzer to measuring with the actual full power PA and meter hooked up. This may be partly due to the fact that the meter is usually not a perfect 50 ohm thru, and partly due to the fact that the analyzers outputs are closer to 50 ohms than the PA. If S11 or the SWR actually does change, you've either got a nonlinear transmission line or a nonlinear load. That is, the impedance changes as the signal level changes. If you *measure* a different SWR or S11, it means that either the SWR or S11 is actually changing for the reasons I just stated, or the meter is nonlinear in the sense that its reading changes with power level (possibly due to RF ingress, but it could be a host of other things), or you're measuring with two different meters that don't agree. It's not because of the different source impedances. Sure, you can normalize a Smith chart to anything you'd like. That doesn't make the SWR change with source impedance. . . . Roy Lewallen, W7EL |
|
|
Roy Lewallen wrote in message ...
I stand by my statement. I, and I'm sure many others, likely including the authors of certain cited papers, stand beside you. Cheers, Tom |
Tom Bruhns wrote:
So if you have 200 feet of RG-213 and 200 feet of RG-58, put those in series to the cantenna. Coil them loosely and cool them with a fan if needed, if you are running high power (or start the chain with larger coax). . . One thing to keep in mind when you use coax as an attenuator or dummy load is that the portion of the cable nearest the transmitter dissipates most of the power. If you had, say, 6 dB per 100 ft attenuation and a 200 ft cable, the first 50 ft dissipate 1/2 the power (and that's concentrated toward the transmitter end), the next 50 ft dissipate 1/4 the power, the next 50 ft 1/8, and the final 50 ft 1/16. So do as Tom says and put the heavier coax up front, and allow for more air circulation for the coax nearest the transmitter if dissipation becomes a problem. Roy Lewallen, W7EL |
Ian White, G3SEK wrote:
Dr. Slick wrote: How would you explain what Cecil wrote? Who else but Cecil would dare attempt that? :-) In my example, the SWR on the ladder-line is not changing (except for losses in the ladder-line). Changing the length of the ladder-line changes the 50 ohm SWR meter *reading*, i.e. it changes the impedance 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! =----- |
Tom Bruhns wrote:
On the other hand, knowing measured S11 and the reference impedance for it, and the line characteristic impedance, you can determine the SWR on that line. That's true for a one-port load but not usually true for a two-port impedance discontinuity in the transmission line. This also brings up another point: do YOU define S11 to be the same as reflection coefficient? S11 is the (physical) reflection coefficient when a2 equals zero. When a2 is not zero, the physical reflection coefficient, S11, will not usually equal the measured reflection coefficient, the square root of Pref/Pfwd. -- 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! =----- |
Roy Lewallen, W7EL sed...
"The transmitter output impedance has no effect whatsoever on the line's SWR." --------------------------------------------- Of course. Good grief, is there still someone out there who does not know this??? Jack K9CUN |
-- ======================= Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.g4fgq.com ======================= "JDer8745" wrote in message ... Roy Lewallen, W7EL sed... "The transmitter output impedance has no effect whatsoever on the line's SWR." --------------------------------------------- Good grief, is there still someone out there who does not know this??? ----------------------------------------------- Here comes the $64,000 question . . . . Of what possible use is the SWR when they think they've got it ??? |
Dr. Slick wrote:
I disagree on this point. You are caught up in the 50 Ohm world, which i admit is easy to do. The SWR is based on the ratio of the forward to the reflected power. That's not correct. The SWR (more correctly VSWR) is, by definition, the ratio of the highest to lowest voltages which appear on a line long enough to have both a maximum and minimum. It can be calculated from the forward and reverse voltage waves. ISWR, the current standing wave ratio, is numerically equal to the VSWR. If you had an analyzer that was calibrated to 20 Ohms (the same as normalizing the Smith for 20 Ohms in the center) you would certainly have reflected power and high SWR going into a 50 Ohm load. And a 20 Ohm load would have a 1:1 SWR. Loads do not have an SWR, only transmission lines do. The fact that you get a reading on an SWR meter when it's connected to a resistor doesn't alter that. You have to realize that an SWR meter isn't really measuring SWR, as Reg has repeatedly pointed out. It's actually measuring an impedance, and reporting that on a scale marked SWR. So you have to be careful to avoid making the mistake of confusing an SWR meter reading with the SWR on a cable it's connected to. The two correspond only if the cable's Z0 equals the SWR meter's. Likewise, you have to realize that you don't change the SWR or "reflected power" when you change the normalization of your network analyzer or Smith chart. Those things are a function only of the load and transmission line impedance, not on your measurements or calculations. Roy Lewallen, W7EL |
Dr. Slick wrote:
On second thought, i believe we are all wrong to equate S11 with SWR! Input S11 of a system will certainly never change. But the SWR is absolutely dependant on the source impedance. I give up. People will believe what they want to believe, no matter what -- it's like arguing religion. But I hope some of the lurkers have learned that SWR is independent of source impedance, even if some of the active posters just can't seem to. . . . Roy Lewallen, W7EL |
"Ian White, G3SEK" wrote in message ...
Input S11 of a system will certainly never change. But the SWR is absolutely dependant on the source impedance. No! SWR, S11, return loss, rho, Y-parameters, Z-parameters, etc, etc are all different derived functions of the same two variables: an arbitrary complex impedance, and the system reference impedance Z0 (a constant which may or may not be defined as complex). Correct, but it doesn't have to be 50 Ohms. Only those two variables are involved, so all of these functions are locked together. If one variable changes, all the derived functions change too. Either all change, or none change; nothing else is logically possible. How about a 50 Ohm resistor, which is always 50 Ohms (impedance doesn't change), fed with 20 ohms? Or 75? Cecil is correct in saying that the SWR meter would then have to be designed for 20 or 75 ohms, but that is beside the point. SWR doesn't have to be strictly 50 ohms, and will involve TWO impedances. If your source doesn't match your reference impedance (normalized center of Smith Chart), then you won't be measuring the reflected power coming right off the source. And because most PA are not 50 ohms output, and most SWR meters are 50 Ohms, there is problem. As Roy says, the equations relating any one of these parameters to any other are all well known. NONE of them ever involves source impedance. Assuming the source impedance is 50 ohms, which it usually isn't with most PAs. If you had a network analyzer calibrated for 20 Ohms, you would certainly have reflected power and high VSWR going into 50 Ohms, and a 1:1 SWR going into 20 Ohms. This would be the same as re-normalizing the Smith Chart for 20 Ohms in the center. You certainly can do this in MIMP. I don't blame anyone for believing it's a 50-Ohm-only world! No argument about any of that... but it's a totally separate point that has no relevance whatever to your earlier statements about source impedance. it's very relevant if you consider the port on a network analyzer to be 50 ohms or not... It should be, but your PA may be quite far off. Slick |
On Wed, 13 Aug 2003 16:13:13 -0700, Roy Lewallen
wrote: Believe me, Ian, I know how frustrating it can get. But remember all the lurkers out there who benefit from your insightful postings. Please keep it up -- it is worth while. For their sake. Roy Lewallen, W7EL Hi Roy, It is nice of you to commend Ian to continue the good effort. Why didn't you do it directly instead of posting him through me? Are the lurkers to take some lesson by this breach of netiquette? :-) 73's Richard Clark, KB7QHC |
(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com... impedance, you can determine the SWR on that line. But your "SWR" meter isn't really an S11 meter; at best it's a |S11| meter. Of course. We have no phase information. Only the magnitude of the voltage reflection coefficient. This also brings up another point: do YOU define S11 to be the same as reflection coefficient? Cheers, Tom I define the S11 as the complex impedance, which never changes, but S11 can also refer to the magnitude of the reflection coefficient at a particular phase, AT A PARTICULAR NORMALIZED REFERENCE IMPEDANCE. If you define the reflection coefficient as having phase information, then yes, they are the same, but only at a particular reference impedance. Change the reference impedance, and you will have a new reflection coefficient, but the complex impedance will still be measured to be the same. You can do this is Motorola's Impedance Matching Program (MIMP). What this all comes down to is that your 50 Ohm SWR meters only measure reflected powers after it, not before, so any mismatch from PA to reference impedance (50 ohm coax from PA to meter) is not measured. Slick |
|
"Ian White, G3SEK" wrote in message ...
How would you explain what Cecil wrote? Who else but Cecil would dare attempt that? :-) I can understand your fear... Do you think the series reactance a system offers a PA may actually improve it's incident power? For example, reducing the load impedance will usually make the output device operate in a more linear way... but the efficiency drops and the greater heat dissipation and current are likely to shorten the lifetime of the device. Is that an improvement? By improve, i mean increase the incident power. I think this is possible, and i've actually measured it, if you read my original post. If you can improve the incident power (tuning) of a PA with varying the coax length, you might be able to adjust the SWR too, in certain cases... though in my case, the SWR stayed about the same. Slick |
I apologize for the discourtesy. It did indeed set a bad example.
Roy Lewallen, W7EL Richard Clark wrote: On Wed, 13 Aug 2003 16:13:13 -0700, Roy Lewallen wrote: Believe me, Ian, I know how frustrating it can get. But remember all the lurkers out there who benefit from your insightful postings. Please keep it up -- it is worth while. For their sake. Roy Lewallen, W7EL Hi Roy, It is nice of you to commend Ian to continue the good effort. Why didn't you do it directly instead of posting him through me? Are the lurkers to take some lesson by this breach of netiquette? :-) 73's Richard Clark, KB7QHC |
Roy Lewallen wrote:
Dr. Slick wrote: I disagree on this point. You are caught up in the 50 Ohm world, which i admit is easy to do. The SWR is based on the ratio of the forward to the reflected power. That's not correct. The SWR (more correctly VSWR) is, by definition, the ratio of the highest to lowest voltages which appear on a line long enough to have both a maximum and minimum. It can be calculated from the forward and reverse voltage waves. ISWR, the current standing wave ratio, is numerically equal to the VSWR. For Dr. Slick: Knowing the forward and reflected powers, one can use the following equation to obtain SWR. 'Sqrt' means "square root of". 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 80,000 Newsgroups - 16 Different Servers! =----- |
Dr. Slick wrote:
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. If the forward power out of a PA is 100 watts and the reflected power into the PA is 50 watts, the PA is generating 50 watts, *by definition*. Given that definition, the implication is clear. All PA's, by definition, must re-reflect 100% of the incident reflected energy. Thus, everything you are worried about has already been defined out of existence. :-) -- 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! =----- |
Ralph Mowery wrote:
I have not seen a whole lot of equations on SWR but the few I have seen never mention the source at all. When someone defined the generated power as the forward power minus the reflected power, the entire problem was defined out of existence. :-) -- 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! =----- |
| All times are GMT +1. The time now is 09:35 AM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com