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Dr. Slick wrote:
Roy Lewallen wrote in message ... The observation that changing line length changes the measured SWR is regularly reported in this newsgroup, and the explanation is as regularly provided. There are at least three ways this can happen. 1. The SWR meter is designed for an SWR that's different from the line impedance. It's easy to show that this will result in different readings for different line lengths. I've measured RG-58 at over 60 ohms characteristic impedance, so this can happen even with a perfect 50 ohm SWR meter and "50 ohm" line. In this case, changing line length isn't really changing the line's SWR, just the meter reading. But a 60 ohm transmission line transformation from a non-50 ohm load will certainly change the SWR, as it won't be on the constant VSWR circle anymore. Once again, you're confusing the SWR meter reading with the SWR on the line. When the line and SWR impedances are different, the two are *not* the same. When I speak of the SWR on the line, I mean the SWR on the line, not the meter reading. Now look at the sentence you wrote. It's not very clear to me, but it would certainly make more sense if you replaced "the SWR" with "the SWR meter reading". Do *not* confuse the two. 2. There's significant loss in the cable. In that case, the longer the distance between the meter and the load, the better the SWR. That's the obvious one. 3. There's current on the outside of the coax. This means that the outside of the transmission line is actually part of the antenna. When you change its length, it changes the effective length of the antenna, which really does change the SWR. Current on the outside of the cable can also get into a poorly shielded SWR meter and modify its reading. And this is all in agreement with established theory. So you see, theory does say you can change the SWR reading, and in some cases, the actual SWR, by changing the coax length. But only under very specific circumstances. When observations don't match theory, chances are overwhelming high that either the observation is erroneous or misinterpreted, or theory is being misapplied. Roy Lewallen, W7EL I believe the source of confusion is he PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy load 1 2 3 The "+" are connector points. You folks are saying that as you change the PA (source) impedance, that the SWR you read will remain the same, even if the incident power changes due to the change in reflected power at point 1. I'm saying that the line SWR doesn't change when you change the source impedance. I didn't say anything about incident or reflected power anywhere. And I won't. Cecil probably said something about the incident and reflected power, but explanations in those terms are strictly up to him. If you were to change the impedance of the left hand line (the one between the PA and meter), then the SWR on the left hand line would change, but the SWR on the right hand line wouldn't, and the SWR meter reading would remain the same. For that matter, you can do anything you want between the SWR meter and the PA -- add an impedance of any kind in series or parallel, change the left hand transmission line length and/or Z0, change the power, whatever you want, and it won't change either the meter's indicated SWR or the actual SWR on the right hand line. The rule is that whatever you change, it won't affect the SWR on any line that's "downstream" (toward the load) from the change you made. This i can agree with, as no matter how much incident power makes it past the impedance discontinuity at point 1, the system after this point will theoretically always reflect the same ratio of reflected power to incident. Be really, really careful when you start talking about forward and reflected power. It can very easily lead you to wrong conclusions about what's going on. Just check the postings on this group for the past few months for evidence. All the phenomena you can observe and measure can be fully explained by looking at forward and reverse voltage and current waves, and it's a whole lot less hazardous. One of the several problems with thinking in terms of forward and reflected power is that it's universally meant to refer to average power. So you've lost all time and phase information, making it impossible to clearly see how the traveling waves interact. If you must deal with "forward power" and "reverse power", do your thinking and calculations with voltage and current waves, then calculate the power when you're all done. As I said before, the ratio of forward to reflected voltage or current is independent of the source impedance. That ratio, when measured at the load, is simply the reflection coefficient at that point. But, if you place an SWR meter of the same impedance as the output of the PA at point 1, you will definitely see a change in SWR at point 1 as you change the PA impedance, as you are changing the reference impedance (center of Smith re-normalized). No, you won't see a change in the SWR at point 1 as you change the PA impedance. All the fiddling you do with your Smith chart just won't make it happen. Sorry. The SWR, voltage, current, impedance, power, reflection coefficient, waves, or anything else don't change in response to your Smith chart exercises. This is what i thought you meant when you said "change the source impedance", but you meant to say "change the source, but keep the reference impedance the same". No. When I said change the source impedance, I meant change the source impedance. Surely we don't need a discussion about what "impedance" means? When you get out your grease pencil and change the reference value of your Smith chart, it doesn't magically change the waves on the line on your workbench. Understood, assuming this is what you guys mean. It's time for me to leave this discussion. I've tried to make my statement as clearly and simply as I know how, but somehow people have decided that I really meant something else, or that there's this condition or that condition that cause exceptions to it, or that it all depends on what you scribble on your Smith chart with a grease pencil. It bears a striking resemblance to a political science (what an oxymoron!) course I took, in which we could make up any definition for anything, or any interpretation of anything anyone said or wrote (and were encouraged to do so), and all were equally valid. I've spent too much time interacting with engineers and not nearly enough with politicians and philosophers to know how to deal well with this fuzziness. Anyone who really cares can look up the equations in a couple of minutes. I'm sure they're on the web, if you have an aversion to paper media. Look up the equations, study them, understand them. If you don't believe them, make up your own equations. Then set up a couple of simple experiments to test them, and see which are right. That's how science and engineering are done. Roy Lewallen, W7EL |
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