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what happens to reflected energy ?
On 7 jun, 22:36, 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 Hello Roy, If you have sufficient headroom and under the conditions you mentioned, you mimic a 50 Ohms source. I think it also works for any (complex) loads (I couldn’t find why not). The difference between a real 50 Ohms circuit may be that the phase of the belonging EMF may change, in many amplifiers phase shift is somewhat excitation dependent, but who bothers? I expect such scheme in combination with VSWR measurement also, as several PAs have a reverse power indicator present (the other half of a the Bruene circuit). You need the reverse power indication to avoid destroying active devices and/or intermodulation distortion due to voltage saturation. Imagine that you have full reflection |RC| = 1 and it appears at your active device as RC=-1. You want to maintain the original forward power. Your active device has to deliver in that case double the current at zero collector/plate voltage to maintain same forward power as under matched condition. The actual power delivered to the load is zero (as the active device supplies current, but no voltage, RC=-1 means a short circuit). This will result in massive dissipation in the active device. In case of RC=+1, it has to provide double the voltage with no current. In other circumstances you will have a significant phase shift between current and voltage resulting also in increased device dissipation and inconvenient combinations of instantaneous voltage and current. So above some value for VSWR, you may have to reduce the forward power I had a discussion recently about the power control scheme for TETRA terminals, but we couldn't find the answer to what is happening under high VSWR (so we have measure it). It only states VSWR2. Best regards, Wim PA3DJS www.tetech.nl without abc, PM will reach me. |
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