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what happens to reflected energy ?
On Mon, 07 Jun 2010 13:36:41 -0700, Roy Lewallen
wrote: 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 Hi Roy, This application that you describe was written up in exactly the same terms within the recent HP Journals I have posted extracts here. HP used Directional Couplers (the Bruene circuit, also called a bridge, qualifies too but uses a non-wave design) to separate out the forward from the reverse power reflected from the mismatch to create a reference power. Later, HP and others strapped the signals back into the source in much the manner you describe. The rudimentary version can be found in HP Journal v.6 n.1-2. HP Journal v.12 n.4 strengthens the concept with hard copy sweeps of the reflection coefficient of a load. By HP Journal v.16 n.6, we have the description of automatic level control. For the 45 years beyond that last article, more refinements. 73's Richard Clark, KB7QHC |
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what happens to reflected energy ?
On Mon, 07 Jun 2010 15:49:04 -0700, Richard Clark
wrote: The rudimentary version can be found in HP Journal v.6 n.1-2. HP Journal v.12 n.4 strengthens the concept with hard copy sweeps of the reflection coefficient of a load. By HP Journal v.16 n.6, we have the description of automatic level control. For the 45 years beyond that last article, more refinements. HP Journal Nov. 1970 is dedicated to all system elements going into one box, 8620A, 8632A (with options for an external Directional Coupler and Power Meters). 73's Richard Clark, KB7QHC |
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