Given a greater than 1:1 SWR as measured on a transmitter's output load (but low enough for full transmitter power output), is there any difference in efficiency or distortion along a Smith chart constant-SWR circle for a typical transmitter's final amp circuitry? (MOSFET or tube)
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1$\begingroup$ Mike Waters, I think he's talking all the possible combinations of pure resistances and reactances, along a VSWR constant, say 1.5:1. And would any particular combination of resistive & reactive load that gave a 1.5:1 VSWR, be any better for distortion, or efficiency, than another combination of resistance & reactance. $\endgroup$– Louis SeamanCommented Sep 19 at 20:17
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2$\begingroup$ A long time ago my company made dummy loads for testing airband transmitters, they had a switch to select 50 ohms, and then also 33, 45+j30, 45-j30, 75 Ohms, to check that the amplifier worked correctly at VSWR=1.5:1, any phase. $\endgroup$– tomnexusCommented Sep 19 at 23:18
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1$\begingroup$ @tomnexus That sound almost like a (partial) answer: that manufacturers test for the specified performance of a transmitter at all phase angles of some specified VSWR. So any differences will be above the specified performance. $\endgroup$– hotpaw2Commented Sep 20 at 1:12
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1$\begingroup$ @tomnexus presumably that was because there was potential for an amp to NOT work correctly at one of those 4 discrete phases? And if so, can you share any insight regarding the nature of the circuit (or its fix) for failing at only 1 or 2 of those discrete phases? Thanks! $\endgroup$– webmarcCommented Sep 20 at 15:37
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1$\begingroup$ @webmarc we just made the loads, not the transmitters. To throw into the mix: some amplifiers are unstable with various source/load impedance combinations, they depend on the impedance being within some mask. $\endgroup$– tomnexusCommented Sep 20 at 19:36
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1 Answer
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The same constant VSWR, with varying phases, can change the final amp transistor's drain voltage and current (both the phase and the peak values). Of course, the phase given to the transmitter's output port is not the same as that seen by the final amp transistor because of the LPF and matching network in between. That is even more complicated in the case of class E amp because the drain current/voltage phase is intentionally manipulated.
If there is insufficient safety margin, the transistor may blow under one phase while operating ok at another, both at the same VSWR. Depending on the phase, a protection circuit may kick in at different VSWR thresholds.
The amplifier's stability (oscillation) may also be affected by the phase, but if that is a concern, that final amp is too unstable for practical use.
