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what happens to reflected energy ?
I haven't followed this thread since it's been beaten to death so many
times here before. But there's an interesting fact that might have been overlooked, and might (or might not) be relevant: If you put a directional coupler such as a Bruene circuit at the output of a transmitter, and use its "forward" output to control the transmitter power to keep the "forward" directional coupler output constant, you'll find that the power output vs load resistance characteristic is exactly the same as if the transmitter had 50 ohm output impedance. This is assuming that the directional coupler is designed for a 50 ohm system, and that the load is purely resistive. It also assumes that any load is left in place long enough for the feedback circuit to stabilize. The effective source impedance to a very rapidly varying load (that is, one changing so fast that the ALC feedback system doesn't have time to respond) would be the open-loop output impedance which could be quite different. I haven't taken the time to analyze how it behaves with a complex load. I stumbled across this some time ago when designing a rig using this ALC method and found it interesting. I believe many if not most modern solid-state transmitters use this ALC method. Roy Lewallen, W7EL |
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