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Sorry, I must have misinterpreted your earlier posting.
But we seem to now have a "true SWR" as opposed to some other kind of SWR. And "true SWR connected to the tx output" doesn't have any meaning at all to me. I also have no idea of what "sample points within the transmitter" might be. So let me explain what I (and virtually all published literature) mean by SWR. If we connect a transmitter to an SWR meter, and then to a long piece of lossless cable with the same Z0 as the SWR meter, and finally to a load, the SWR meter reading will be the same as the VSWR on the cable, i.e., the ratio of maximum to minimum voltages on the line. This ratio of voltages is, by definition, the VSWR -- which equals the ISWR, and is often referred to simply as SWR. If we measure or calculate the impedance seen looking into the line, then disconnect the line from the SWR meter and replace it and the load with lumped elements of the same impedance, the SWR meter reading won't change(*). Now, I can calculate the what the SWR meter reading will be under this condition also. In both cases, the source impedance won't affect the SWR meter reading, the positions or relative magnitudes of the maximum and minimum voltages on the line, or the voltage or current within the SWR meter line section. (This last condition assumes that the net power delivered by the source stays the same; otherwise, the ratio of voltage to current, and their phase angles, stay constant, regardless of the power delivered.) I have no idea how all this relates to your "true SWR". But do you agree with what I've said above? If not, I'll describe a couple of simple experiments which will test it against any alternative view you might propose. (*) We can also replace them with a load at the end of a line of different Z0. As long as we choose the load Z and the line length to make the impedance seen at the line input the same as before, the SWR meter will read the same as before -- even though it no longer equals the actual VSWR on the transmission line. The SWR meter is really indicating the impedance seen looking into the line, not in this case the actual line VSWR. (That's the essence of Reg's objection to the SWR meter designation. Of course, if I connect my ammeter across a resistor, it's not measuring the current through the resistor, either.) Roy Lewallen, W7EL Richard Fry wrote: "Roy Lewallen" wrote Let me suggest an additional exercise for Richard and anyone else that believes that source impedance affects the SWR. (etc) ____________________ Just one sec, please. I didn't say that the true SWR connected to the tx output connector was affected. I said that the RF power measured at the sample point(s) in the transmitter can be affected by the source and load impedances of the tx, for the reasons stated. The true load SWR does not change under these conditions, but it cannot then be determined by such a meter. Attempting to do so will yield some value, but it will be wrong. RF |
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