|
Richard,
Yes, I took a course from Chipman, but before he published his book. He used Adler, Chu, and Fano in his class. Reason I mentioned pulses is that most of what we did in class with reflections involved pulses. You may remember me giving Roy a hard time a few months ago, because I wasn't used to his way of thinking. He was right and I was wrong. I don't have the Chipman book. Tam/WB2TT "Richard Clark" wrote in message ... On Wed, 15 Oct 2003 17:18:06 -0400, "Tarmo Tammaru" wrote: Richard, I hope you are not mixing up analog steady state signals and reflections of pulses. The re reflection of a signal at a source depends not only on the impedance, but also on the voltage at the source. Tam/WB2TT Hi Tam, Found within the body of what I posted: Then of course there is more in Chapter 8 Chapter 8. Section 8.8 "Multiple Reflections." This material shows the transient analysis and sets up the steady state analysis already anticipated above in Chapter 9. Didn't you say you studied under Chipman? This is HIS material, not my derivations. Again, if I were wrong, there are enough copy holders here to correct me. That has not come to pass in lo' these several months. 73's Richard Clark, KB7QHC |
On Wed, 15 Oct 2003 19:53:46 -0400, "Tarmo Tammaru"
wrote: Richard, I went to a Bob Pease seminar a few years ago, great guy. You are missing one of the points of the simulation. I am not trying to market an SWR meter. I simulated it with ideal parts so that the instrument is not affecting the reading. How are you going to measure an SWR of 65:1 with a real meter? Hi Tam, A very good question. The Metrologist wouldn't, there are better techniques that are more accurate. I did change the source impedance, and it did not change the SWR within the limits of what I could resolve. In addition, as I told Slick a couple of months ago, I used a real meter (Kenwood SW2000) to measure the SWR with two different source impedances and two different load impedances, and the source impedance made no difference. That too is unremarkable. The difference is not resolved at one point as I have demonstrated. I don't know that the Harris transmitter is the same as what was described in Circuit Cellar. The Harris has no modulators and no linear amplifiers; just a bank of 65 CW power modules that get switched on and off and synthesize the desired envelope power at something like a 20 KHz rate. Sort of a D/A converter that runs at a power level of 50 KW. No, this misses the mark considerably. The lowest bit rate is half the Fo of the transmit frequency (typically the bit rate is equal to the Fo). It is accomplished with a ROM lookup table to achieve the modulation (much like the Circuit Cellar Ink articles by Don Lancaster but with significant differences too as his discussion was strictly CW). 73's Richard Clark, KB7QHC |
Tarmo Tammaru wrote:
There are models for both lossy and non lossy transmission line. I have not used them, so it might take some learning. I can tell you though that given a load and transmission line, if you find the Z at the meter with an HP vector impedance meter, and then put a lumped impedance of that same value at the meter, you will get the same results. Not in reality, you won't. Any TV ghosting that exists because of reflections will disappear when you go to a lumped impedance. And the noise across the lumped impedance will not be identical to the noise associated with a long transmission line. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Cecil Moore wrote: Tarmo Tammaru wrote: There are models for both lossy and non lossy transmission line. I have not used them, so it might take some learning. I can tell you though that given a load and transmission line, if you find the Z at the meter with an HP vector impedance meter, and then put a lumped impedance of that same value at the meter, you will get the same results. Not in reality, you won't. Any TV ghosting that exists because of reflections will disappear when you go to a lumped impedance. And the noise across the lumped impedance will not be identical to the noise associated with a long transmission line. Isn't the point just to test SWR dependency on source impedance? 73, ac6xg |
Jim Kelley wrote:
Cecil Moore wrote: Tarmo Tammaru wrote: There are models for both lossy and non lossy transmission line. I have not used them, so it might take some learning. I can tell you though that given a load and transmission line, if you find the Z at the meter with an HP vector impedance meter, and then put a lumped impedance of that same value at the meter, you will get the same results. Not in reality, you won't. Any TV ghosting that exists because of reflections will disappear when you go to a lumped impedance. And the noise across the lumped impedance will not be identical to the noise associated with a long transmission line. Isn't the point just to test SWR dependency on source impedance? Just pointing out that the "same results" statement above is a little too broad. Any deviation in the waves from cycle to cycle due to modulation and noise shows up as reflected energy. -- 73, Cecil, W5DXP |
Cecil Moore wrote:
Jim Kelley wrote: Cecil Moore wrote: Tarmo Tammaru wrote: There are models for both lossy and non lossy transmission line. I have not used them, so it might take some learning. I can tell you though that given a load and transmission line, if you find the Z at the meter with an HP vector impedance meter, and then put a lumped impedance of that same value at the meter, you will get the same results. Not in reality, you won't. Any TV ghosting that exists because of reflections will disappear when you go to a lumped impedance. And the noise across the lumped impedance will not be identical to the noise associated with a long transmission line. Isn't the point just to test SWR dependency on source impedance? Just pointing out that the "same results" statement above is a little too broad. Any deviation in the waves from cycle to cycle due to modulation and noise shows up as reflected energy. Tarmo's simulation results seem to conflict with Richard's interpretation of Chipman. Lately you've been leaning toward Richard's point of view. How does the story end? ;-) 73, Jim AC6XG |
On Thu, 16 Oct 2003 10:00:38 -0700, Jim Kelley
wrote: Cecil Moore wrote: Tarmo Tammaru wrote: There are models for both lossy and non lossy transmission line. I have not used them, so it might take some learning. I can tell you though that given a load and transmission line, if you find the Z at the meter with an HP vector impedance meter, and then put a lumped impedance of that same value at the meter, you will get the same results. Not in reality, you won't. Any TV ghosting that exists because of reflections will disappear when you go to a lumped impedance. And the noise across the lumped impedance will not be identical to the noise associated with a long transmission line. Isn't the point just to test SWR dependency on source impedance? 73, ac6xg Hi Jim, It "was" but through these twists and turns, the point has become a amorphous blob. Tam has shown an appreciation for my interpretation of Chipman's work being focused on transient analysis (it has that in Chapter 8) and a worry that my data is forced to that criteria - it is not, it is strictly steady state results and also supported by Chipman's steady state formulas (found in Chapter 9). Chapter 8 reveals the obvious state of a transmission line being mismatched at both ends supporting a bedlam of wave mixings - the bedlam is simply the artifact of an unknown length in the path, otherwise it is quite predictable and formulaic. The steady state solutions presume you know this length (or in his terms, the position along the line mismatched at both ends) to find the dependency of measured voltage in terms of source AND load Z. Cecil, as usual complains without adding anything: On Wed, 15 Oct 2003 13:43:39 -0500, Cecil Moore wrote: Yes, my experiment seemed to support that assertion but you rejected it. Where this "seeming" was rejected for good reason (or poor reason depending upon the source ;-) as he had poorly bounded his example (it did not include the necessary transmission line which evidences the source Characteristic Z) and he was forever explaining minutia at the expense of the topic. 600 posting marathons of that kind of repetition is unnecessary. My data and sources can stand quite well on their own without the need for rubber crutches. Speaking of repetition, this is all covered in my new thread "The Impact of Source characteristic Z upon SWR measurement - the Galilean Defense re-revisited" As such there is actually nothing new to add, and absolutely no holders of Chipman's work have offered any rebuttal - elliptical criticism notwithstanding. :-) 73's Richard Clark, KB7QHC |
Jim Kelley wrote:
Tarmo's simulation results seem to conflict with Richard's interpretation of Chipman. Lately you've been leaning toward Richard's point of view. How does the story end? ;-) I suspect it ends up with me being mistaken when I said that instantaneous power is *as* useless as tits on a boar hog. If the instantaneous power is screwing up the wattmeter readings, instantaneous power is *more* useless than tits on a boar hog. :-) -- 73, Cecil, W5DXP |
Cecil Moore wrote: Jim Kelley wrote: Tarmo's simulation results seem to conflict with Richard's interpretation of Chipman. Lately you've been leaning toward Richard's point of view. How does the story end? ;-) I suspect it ends up with me being mistaken when I said that instantaneous power is *as* useless as tits on a boar hog. If the instantaneous power is screwing up the wattmeter readings, instantaneous power is *more* useless than tits on a boar hog. :-) The story must be like a daytime serial. :-) 73, Jim AC6XG |
Richard Clark wrote:
My data and sources can stand quite well on their own without the need for rubber crutches. Certainly true about your sources standing well on their own. You should probably let them. ;-) 73 de ac6xg |
Cecil,
I thought that we were considering steady state single frequency sine waves. The whole thing becomes so much more straight forward when talking about pulses..... Tam/WB2TT "Cecil Moore" wrote in message ... Tarmo Tammaru wrote: There are models for both lossy and non lossy transmission line. I have not used them, so it might take some learning. I can tell you though that given a load and transmission line, if you find the Z at the meter with an HP vector impedance meter, and then put a lumped impedance of that same value at the meter, you will get the same results. Not in reality, you won't. Any TV ghosting that exists because of reflections will disappear when you go to a lumped impedance. And the noise across the lumped impedance will not be identical to the noise associated with a long transmission line. -- 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! =----- |
"Jim Kelley" wrote in message ... Tarmo's simulation results seem to conflict with Richard's interpretation of Chipman. Lately you've been leaning toward Richard's point of view. How does the story end? ;-) 73, Jim AC6XG Jim, It probably can't be proven, unless somebody comes up with an alternative definition for SWR. If you look at my simulator equations of a few listings back, I proved that what my model (and the Bird wattmeter) call SWR is RL/Z0. So, unless I screwed up, running any number of simulations is not going to disprove that. If you look at textbook examples where they transmit pulses, The source impedance determines what V+ is, and whether there is a second reflection from the source, but NOT what the reflection at the load end is. Lets delve on this for a second. It seems fair to say that if the source impedance determines V+, clearly it has an effect on V-. But, that does not mean it has anything to do with rho. Tam/WB2TT |
Tarmo Tammaru wrote:
Jim, It probably can't be proven, unless somebody comes up with an alternative definition for SWR. If you look at my simulator equations of a few listings back, I proved that what my model (and the Bird wattmeter) call SWR is RL/Z0. So, unless I screwed up, running any number of simulations is not going to disprove that. It's hard to imagine how Rs (Zs) could have any effect on that ratio. If you look at textbook examples where they transmit pulses, The source impedance determines what V+ is, and whether there is a second reflection from the source, but NOT what the reflection at the load end is. Unless I = 0, source impedance should certainly have an effect on source voltage. My car battery this morning comes to mind. Seemed to have developed a high internal resistance. It's doing some serious current limiting. Chipmans explanation re-reflection was eloquent I thought. Lets delve on this for a second. It seems fair to say that if the source impedance determines V+, clearly it has an effect on V-. But, that does not mean it has anything to do with rho. I don't know how else to look at it. The question that comes to mind is whether the argument is about the effect source impedance has on actual SWR, or the effect it has on measured SWR - considering the real world limitations of metering instruments. Perhaps people are talking about different things. 73, Jim AC6XG |
Tarmo Tammaru wrote:
It seems fair to say that if the source impedance determines V+, clearly it has an effect on V-. But, that does not mean it has anything to do with rho. Chipman seems to say that an SWR meter can be disturbed by a localized energy exchange between reactive values with opposite signs. The impedance of the source has an effect upon where in the transmission line those localized energy exchanges occur. -- 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! =----- |
Jim Kelley wrote:
It's hard to imagine how Rs (Zs) could have any effect on that ratio. Consider a reactive load where energy can be locally exchanged between the load reactance and the impedance looking back into the feedline. Zs can certainly affect the impedance looking back into 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! =----- |
Jim Kelley wrote:
It's hard to imagine how Rs (Zs) could have any effect on that ratio. Here's an interesting quote from _Transmission_Lines_, by Chipman, page 175: "Equation (8.27) demonstrates explicitly that the shape of a standing wave pattern representing |V(d)| as a function of d on a transmission line is in no way affected by the quantities, Vs, Zs, and rho(s) at the source." -- 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! =----- |
Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to reflected waves, and therefore DOESN'T affect the SWR. Once again, here's a way to see why. I'll restrict the discussion to a lossless line for simplicity. When you first turn the source on, a forward wave (voltage and current) travels toward the load. The source impedance does play a role in determining the size of this wave; it can be determined by analysis of a simple voltage divider circuit, with the source voltage dividing between the source impedance and the line Z0. A portion of the forward wave is reflected from the load unless the line is perfectly matched. The fraction which is reflected has nothing to do with the source impedance, and in fact it can easily be calculated from only the line and load impedances. That fraction (magnitude and angle) is known as the reflection coefficient -- you can find the formula in any transmission line text, or derive it yourself very easily. Take a look at the system just before the reflected wave returns to the source. At each point along the line we have a forward wave and a reflected wave, which vectorially add. These create standing waves and, if the line is long enough, we can calculate the SWR directly as the ratio of maximum to minimum voltage along the line. A little bit of algebra will show that the SWR is determined entirely by the ratio of forward to reflected waves -- their absolute values don't matter (except, of course, as it affects their ratio). Given a reflection coefficient, you can calculate the SWR. Ok, now suppose that some fraction of the returning wave reflects from the source and heads back toward the load. Say, X percent of it. When it reaches the load, exactly the same fraction of it is reflected as was the case for the original forward wave. That is, if the new forward wave is X percent of the original, then the new reflected wave is also X percent of the original reflected wave. If we add the new forward and reflected waves to the original ones, and take the ratio of forward to reverse, we find that the ratio of the new, combined forward wave to the new, combined reflected wave is exactly the same as it was for the first forward and reflected waves. It doesn't matter what X is -- no matter what fraction of the reflected wave bounces off the source, the same fraction of that new forward wave is reflected from the load. The SWR is the same as it was for the original pair of waves. Eventually, we build up a large number of pairs of forward and reflected waves. And the ratio of each forward wave to its corresponding reflected wave is always the same -- it's the reflection coefficient of the load. So when we add all the forward waves into a single forward wave and all the reflected waves into a single reflected wave, we get the same ratio. And that ratio doesn't depend in any way on the source impedance or what fraction of each returning wave is re-reflected from the source. One of the nice things about this way of looking at it is that it's entirely supported by the theory and equations describing transmission line operation which engineers have used to design working systems for the past hundred years or so. Roy Lewallen, W7EL Cecil Moore wrote: Jim Kelley wrote: It's hard to imagine how Rs (Zs) could have any effect on that ratio. Consider a reactive load where energy can be locally exchanged between the load reactance and the impedance looking back into the feedline. Zs can certainly affect the impedance looking back into the feedline. |
Holy mackrel, Mr. Science!
But gee, you can see that from even a casual glance at the equations you'll find in virtually any transmission line text. It's kind of like saying that wow, Terman concludes that resistance is voltage divided by current, so now we can believe it too. People who believe that SWR is affected by source impedance have either rejected established theory, or don't have the background or interest to read and understand what we consider to be very simple equations. So I'd hardly expect them to be impressed by someone pointing out what the equations and established theory say clearly and unambigously. You can't fight Ouija boards with math. Roy Lewallen, W7EL Cecil Moore wrote: Jim Kelley wrote: It's hard to imagine how Rs (Zs) could have any effect on that ratio. Here's an interesting quote from _Transmission_Lines_, by Chipman, page 175: "Equation (8.27) demonstrates explicitly that the shape of a standing wave pattern representing |V(d)| as a function of d on a transmission line is in no way affected by the quantities, Vs, Zs, and rho(s) at the source." |
Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to reflected waves, and therefore DOESN'T affect the SWR. =========================== But it DOES affect the indicated SWR and so the indicated SWR is incorrect. It is the meter which is at fault ! It is designed to indicate correctly only when the source is 50 ohms. Here's the proof - Rho = (50-Zt) / (50+Zt) - which you may have seen before. SWR, of course, is calculated from Rho and the meter scale is calibrated accordingly. If the source is not what the meter expects then it gives the wrong answers. And its faithful worshippers believe it! --- Reg, G4FGQ |
Tarmo Tammaru wrote:
I thought that we were considering steady state single frequency sine waves. The whole thing becomes so much more straight forward when talking about pulses..... It is still straight forward when we take reality into account. :-) Pure steady state single frequency sine waves, sans noise and/or jitter, exist only in the human mind. -- 73, Cecil, W5DXP |
Roy Lewallen wrote:
Source impedance DOES affect the amount of energy moving in and sloshing around in a transmission line. It DOESN'T affect the ratio of forward to reflected waves, and therefore DOESN'T affect the SWR. That's not the question. The question is: Does it affect the SWR meter? Does the localized energy exchange between two reactances (your own "third power" term), as alluded to by Chipman, cause erroneous SWR readings from a directional wattmeter? -- 73, Cecil, W5DXP |
Roy Lewallen wrote:
People who believe that SWR is affected by source impedance have either rejected established theory, or don't have the background or interest to read and understand what we consider to be very simple equations. So I'd hardly expect them to be impressed by someone pointing out what the equations and established theory say clearly and unambigously. You can't fight Ouija boards with math. The question remains: Does the localized exchange of energy between reactances, as presented by Chipman and by you as a third power term, cause a directional coupler error? Source---w---(-j500)---x---(+j500)---y---50 ohm load | | +---braid---------braid--------braid------+ Given the phasor addition that happens in a directional coupler, does it handle all cases of voltage and current properly? In the above example, the measured SWR is 1:1 at 'w' and 'y' but not at 'x'. I remember someone saying that an SWR meter reading is correct only when the SWR is 1:1. -- 73, Cecil, W5DXP |
Reg Edwards wrote:
Source impedance DOES affect the amount of energy moving in and sloshing around in a transmission line. It DOESN'T affect the ratio of forward to reflected waves, and therefore DOESN'T affect the SWR. =========================== But it DOES affect the indicated SWR and so the indicated SWR is incorrect. It is the meter which is at fault ! It is designed to indicate correctly only when the source is 50 ohms. Here's the proof - Rho = (50-Zt) / (50+Zt) - which you may have seen before. SWR, of course, is calculated from Rho and the meter scale is calibrated accordingly. If the source is not what the meter expects then it gives the wrong answers. And its faithful worshippers believe it! Sorry, Reg, for the last few weeks I'd believed you'd been trying to make some profound point about this. But it's rather the opposite: all you're saying is that the indication on the "SWR" scale of the meter depends on the actual power level... which is obvious. That's why the SWR result *always* has to be based on some kind of ratio between forward and reflected readings on the meter, to allow for varying power levels. 1. For a Bird-43 type of meter, you have to read the forward *and* the reflected indicated "power" levels, and plug *both* of those numbers into the little formula to calculate SWR... which involves the ratio of those two numbers. 2. With an ordinary manual SWR meter, you avoid taking a ratio by *always* tweaking the knob to adjust the forward reading to full-scale as the first step. That compensates for whatever power level you happen to be using. Then the SWR indication will read correctly on the reverse setting. If you omit that first step, then you're not using the instrument correctly. Don't blame the SWR meter for that. 3. With an MFJ-259 or similar, the RF output is electronically levelled to a constant value, so instead of the front-panel pot in (2) above there is an internal set-and-forget trimpot. 4. A computing SWR meter does the calculation for you, at whatever power level you happen to be using, so it displays an SWR reading that should not vary with power (within the design limitations of the meter). If you RTFM and use the SWR meter correctly, either you or the meter will *always* compensate for whatever power level you happen to be using. As others have said - again and again, and correctly - the source impedance of the transmitter affects only the power level; it does not in any way affect the steady-state rho or SWR. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Reg Edwards wrote:
If the source is not what the meter expects then it gives the wrong answers. And its faithful worshippers believe it! If reflections are not allowed to reach the source (typical of most ham installations) the source impedance cannot have any effect on the SWR meter readings. Most ham installations are Z0-matched to 50 ohms thus eliminating reflections between the Z0-match point and the source. You are describing systems where reflections are allowed to reach the source which is atypical of ham installations. -- 73, Cecil, W5DXP |
On Fri, 17 Oct 2003 09:11:42 +0000 (UTC), "Reg Edwards"
wrote: | Source impedance DOES affect the amount of energy moving in and sloshing | around in a transmission line. It DOESN'T affect the ratio of forward to | reflected waves, and therefore DOESN'T affect the SWR. | |=========================== | |But it DOES affect the indicated SWR and so the indicated SWR is incorrect. | |It is the meter which is at fault ! It is designed to indicate correctly |only when the source is 50 ohms. | |Here's the proof - Rho = (50-Zt) / (50+Zt) - which you may have seen |before. | |SWR, of course, is calculated from Rho and the meter scale is calibrated |accordingly. | |If the source is not what the meter expects then it gives the wrong answers. |And its faithful worshippers believe it! I hope you meant to say that the meter "expects" to see the correct line Zo. Rho, SWR, RL, etc. are figures of merit for how well the load matches the transmission line impedance (Zo), thus to derive this figure of merit the meter's internal reference should be the same as Zo not Zs. Your "50" in the equation above is simply a special case. Let Zs, Zo and Zl(Zt) all equal 75. The line is perfectly matched, but plug 75 into your formula and see what happens. This brings up an interesting paradox: all real lines have some loss, thus Zo = Ro-jX. Unless Zl = Zo = Ro-jX the line is mismatched. Likewise, the meter reference should also equal Ro-jX if the actual line condition is to be measured. As far as the source Z having any influence on SWR, Roy is (as usual) exactly correct. Wes N7WS |
"Cecil Moore" wrote in message ... Consider a reactive load where energy can be locally exchanged between the load reactance and the impedance looking back into the feedline. Zs can certainly affect the impedance looking back into the feedline. -- 73, Cecil http://www.qsl.net/w5dxp Cecil, I did that, and the SWR did not change. But, then you could say this was by definition because the meter measured RL/Z0. Tam/WB2TT |
Tarmo Tammaru wrote:
"Cecil Moore" wrote: Consider a reactive load where energy can be locally exchanged between the load reactance and the impedance looking back into the feedline. Zs can certainly affect the impedance looking back into the feedline. I did that, and the SWR did not change. But, then you could say this was by definition because the meter measured RL/Z0. Chipman alludes to the problem not occurring with a resistive load or with lossless lines. If the load is 50+j500 and the impedance looking back into the lossy line is 50-j500, this seems to be the correct conditions to cause the localized resonant energy transfer problem. -- 73, Cecil, W5DXP |
Cecil,
You saw my simulations of your example. I got an SWR of 66.3 for ZS=0 and 69.1 for ZS= j400. That is as close as I can resolve. I had thought that if it was going to change, I would get an SWR of 1:1 for the conjugate matched case. What I had neglected, and maybe you also, is the fact that when you connect an SWR meter into the middle of a resonant series tuned circuit, the current is 90 degrees out of phase with the voltage. Tam/WB2TT "Cecil Moore" wrote in message ... Tarmo Tammaru wrote: It seems fair to say that if the source impedance determines V+, clearly it has an effect on V-. But, that does not mean it has anything to do with rho. Chipman seems to say that an SWR meter can be disturbed by a localized energy exchange between reactive values with opposite signs. The impedance of the source has an effect upon where in the transmission line those localized energy exchanges occur. -- 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! =----- |
Huh?
Humor me, in what way does that equation constitute proof? Where is the source impedance in that equation? An SWR meter will correctly indicate the SWR on a transmission line if the transmission line connected to its output equals the design impedance of the meter, regardless of the source impedance. If the transmission line connected to its output doesn't equal the design impedance of the meter, the meter won't correctly indicate the SWR on the the transmission line, again regardless of the source impedance. Why does this seem so complicated? Roy Lewallen, W7EL Reg Edwards wrote: Source impedance DOES affect the amount of energy moving in and sloshing around in a transmission line. It DOESN'T affect the ratio of forward to reflected waves, and therefore DOESN'T affect the SWR. =========================== But it DOES affect the indicated SWR and so the indicated SWR is incorrect. It is the meter which is at fault ! It is designed to indicate correctly only when the source is 50 ohms. Here's the proof - Rho = (50-Zt) / (50+Zt) - which you may have seen before. SWR, of course, is calculated from Rho and the meter scale is calibrated accordingly. If the source is not what the meter expects then it gives the wrong answers. And its faithful worshippers believe it! --- Reg, G4FGQ |
Huh?
Reflections reach the source at my station any time the SWR isn't 1:1. But source reflections have no effect on SWR. I explained why in a recent posting. Roy Lewallen, W7EL Cecil Moore wrote: Reg Edwards wrote: If the source is not what the meter expects then it gives the wrong answers. And its faithful worshippers believe it! If reflections are not allowed to reach the source (typical of most ham installations) the source impedance cannot have any effect on the SWR meter readings. Most ham installations are Z0-matched to 50 ohms thus eliminating reflections between the Z0-match point and the source. You are describing systems where reflections are allowed to reach the source which is atypical of ham installations. -- 73, Cecil, W5DXP |
Ian, G3SEK wrote:
"1. For a Bird-43 type of meter, you have to read the forward "and" the reflected indicated "power" levels, and plug "both" of these numbers into the little formula to calculate SWR...which involves the ratio of those two numbers." Yes. Bird gives: "VSWR = 1+sqrt Pref/Pfwd / 1-sq rt Pref/Pfwd To eliminate calculations, the Model 43 instruction book includes charts which give VSWR when Pref and Pfwd intersect on a chart. A VSWR slide-rule has been produced by Bird which does the same as the charts, and more. Bird has published a useful series of technical papers, "Watts New From Bird" Revisited. One paper, "The Directional Wattmeter", says: "What is the effect of load impedance on the accuracy of the THRULINE? The design formulas show that the only imopedance influeincing the output voltage is Zo, the characteristic impedannce of the line at the point of measurement. Since each THRULINE wattmeter is supplied with a section of 50-ohm line, this Zo is accurately known. The load impedance only affects the forward and reflected power levels which the THRULINE measures. Where should the weattmeter be inserted? Again referring to the formulas, we see that the elements extract a voltage proportional to either Ef or Er. While the total E varies along an improperly terminated 50-ohm line, the component voltages do not. This is simply another way of saying that the energy contained in the forward wave remains the same from the source to the load where some or all of it is reflected (unless the load is 50 ohms) and the reflected energy remains the same from the load back to the source. Our directional power meter can, therefore, be placed anywhere between the source and the load. Reg`s statement: "But it DOES affect the indicated SWR and so the indicated SWR is incorrect." does not apply to the Bird Model 43 wattmeter. Best regards, Richard Harrison, KB5WZI |
Tarmo Tammaru wrote:
Cecil, You saw my simulations of your example. I got an SWR of 66.3 for ZS=0 and 69.1 for ZS= j400. That is as close as I can resolve. I had thought that if it was going to change, I would get an SWR of 1:1 for the conjugate matched case. What I had neglected, and maybe you also, is the fact that when you connect an SWR meter into the middle of a resonant series tuned circuit, the current is 90 degrees out of phase with the voltage. Well, almost 90 degrees. V*I*cos(theta) still has to equal the forward power minus reflected power even at that point. A quick and dirty phasor diagram seems to indicate that the SWR meter bridge circuitry would get pretty screwed up at point 'x' in the following: Source---w---(-j500)---x---(+j500)---y---50 ohm load | | +----------------braid---------------------+ The SWR meter would indicate close to 1:1 at points 'w' and 'y' but would detect a forward and reflected power of approximately five times the delivered power at point 'x', offscale on both needles, which is what happened when I installed my SWR meter at that point. The SWR bridge circuit phasor adds the two phasors. Even when they are 90 degrees out of phase, they add up to a large magnitude that gets rectified and routed to the meter. You may be correctly predicting the actual SWR but I doubt that you are correctly predicting the response of the bridge circuitry in the SWR meter. -- 73, Cecil, W5DXP |
Roy Lewallen wrote:
Why does this seem so complicated? The SWR bridge circuitry may not correctly display the actual SWR. Please see my latest response to Tarmo. The SWR bridge samples the current and voltage and performs a phasor addition and subtraction to get voltages proportional to the forward and reflected powers. If there is a high voltage caused by reactive components, it will be close to 90 degrees away from the current. But phasor adding these two values gives something slightly greater in magnitude than the high reactive voltage. That high voltage gets rectified and displayed as the forward power when it is not actually the forward power but reactive power flowing from one reactance to another. -- 73, Cecil, W5DXP |
Roy Lewallen wrote:
Huh? Huh, indeed. The scope of my statement is less than yours and my statement is a sub-set of yours. If my statement is wrong, then so is yours. However, it is within the bounds of logical possibility that my statement might be correct and yours might be wrong. I'm not asserting that is the case, just that it is within the bounds of logical possibilities. Reflections reach the source at my station any time the SWR isn't 1:1. But source reflections have no effect on SWR. I explained why in a recent posting. Yes, but you didn't prove that source reflections have no effect on an SWR meter. If reflections are not allowed to reach the source (typical of most ham installations) the source impedance cannot have any effect on the SWR meter readings. You say source reflections have no effect on SWR. I say if reflections are not allowed to reach the source, the source impedance cannot have any effect on the SWR meter readings. My statement is a sub-set of yours and of lessor scope than yours. If my statement is wrong, yours must also, by the rules of classical logic, be wrong. :-) -- 73, Cecil, W5DXP |
Cecil Moore wrote: The SWR bridge samples the current and voltage and performs a phasor addition and subtraction to get voltages proportional to the forward and reflected powers. If there is a high voltage caused by reactive components, it will be close to 90 degrees away from the current. But phasor adding these two values gives something slightly greater in magnitude than the high reactive voltage. That high voltage gets rectified and displayed as the forward power when it is not actually the forward power but reactive power flowing from one reactance to another. -- 73, Cecil, W5DXP The power flow fairy sure has a lot of warts. :-) 73, jk ac6xg |
Roy, W7EL wrote:
"But source reflections have no effect on SWR." That`s right. The load is going to take a fixed percentage of energy imposed on it and reflect the rest regardless of the actual magnitudes. All the source impedance can do is to affect the magnitude imposed on the load. SWR is based only upon percentage of energy reflected regardless of its actual magnitude, not how big or small the energy making the ratio. 1/2, 2/4, and 4/8 are all the same ratio. Cecil has a valid point. If Roy had a properly tuned tuner between his transmitter and his feedline, the SWR seen by the transmitter would be 1:1, and that means no reflected energy reaching the transmitter. Reflections reaching the tuner are either wasted in the tuner or re-reflected by it if the tuner is properly adjusted and no reflections make it to the transmitter. Best regards, Richard Harrison, KB5WZI |
Cecil Moore wrote:
Roy Lewallen wrote: Why does this seem so complicated? The SWR bridge circuitry may not correctly display the actual SWR. Please see my latest response to Tarmo. The SWR bridge samples the current and voltage and performs a phasor addition and subtraction to get voltages proportional to the forward and reflected powers. If there is a high voltage caused by reactive components, it will be close to 90 degrees away from the current. But phasor adding these two values gives something slightly greater in magnitude than the high reactive voltage. That high voltage gets rectified and displayed as the forward power when it is not actually the forward power but reactive power flowing from one reactance to another. Even in this unusual situation, the behavior of the meter is completely predictable, including the incorrect power indication. Remember that the meter doesn't actually measure power - it is only *calibrated* to *indicate* power. When placed in a situation where its calibration is not valid, then of course it won't indicate power correctly. But even its wrong indication can be predicted if you know the detailed values to plug into a circuit model. There's really no mystery about it. -- 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:
Even in this unusual situation, the behavior of the meter is completely predictable, including the incorrect power indication. There's really no mystery about it. My point exactly! So, in the same vein, can the source impedance adversely affect the SWR meter reading? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Cecil Moore wrote:
Ian White, G3SEK wrote: Even in this unusual situation, the behavior of the meter is completely predictable, including the incorrect power indication. There's really no mystery about it. My point exactly! So, in the same vein, can the source impedance adversely affect the SWR meter reading? It will affect both the forward and reverse readings, but in equal proportion, so it won't affect the indicated or calculated SWR (unless there are nonlinearities in the meter, or the meter is not being used correctly). -- 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:
Cecil Moore wrote: My point exactly! So, in the same vein, can the source impedance adversely affect the SWR meter reading? It will affect both the forward and reverse readings, but in equal proportion, so it won't affect the indicated or calculated SWR (unless there are nonlinearities in the meter, or the meter is not being used correctly). How about what Chipman says? -- 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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