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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 |
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