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Geoffrey S. Mendelson wrote:
If you have a resonant antenna, supposedly all of the power is radiated. (SWR 1:1) If you have a nonresonant antenna, some of the power is reflected back to the transmitter. (SWR 1:1) If you connect a tuner and it is "tuned", none of the power is reflected back to the transmitter. (SWR at transmitter 1:1, at antenna still 1:1) Obviously it has to go somewhere. Where? If you are designing a tuner, where would you design it to go? Thanks in advance Geoff. All of the power produced by a transmitter is either radiated or dissipated as heat, period. But you'll never understand where the "reflected power" goes until you understand where it comes from. Consider a 100 watt transmitter connected to a 50 ohm dummy load or antenna via a half wavelength of 50 ohm lossless transmission line. The transmitter sees an impedance of 50 ohms resistive. The transmitter delivers 100 watts to the transmission line. The transmission line delivers 100 watts to the load. The load dissipates 100 watts. The VSWR on the line is 1:1 The "forward power" in the transmission line is 100 watts. The "reverse power" in the transmission line is zero. Ok so far? Now replace the transmission line with one having 200 ohm impedance. The transmitter sees an impedance of 50 ohms resistive. The transmitter delivers 100 watts to the transmission line. The transmission line delivers 100 watts to the load. The load dissipates 100 watts. The VSWR on the line is 4:1. The "forward power" in the transmission line is 156.25 watts. The "reverse power" in the transmission line is 56.25 watts. By changing the line impedance, we somehow created an extra 56.25 watts of "forward power". Neither the transmitter nor the load was aware of this wonderful event, since the transmitter sees the same load impedance as before, and the load sees the same source impedance. The transmitter is delivering exactly the same amount of power as before, and the load is dissipating the same amount. (If the transmission line had loss, it would have increased very slightly, but probably not enough to measure, and certainly much, much less than 56 watts.) Likewise, neither the transmitter nor the load knows anything about the new "reverse power" which was created at the same time. So the answer to your question is that the "reverse power" goes to wherever the new excess "forward power" comes from. Once you figure out where that is, you'll have a better understanding of the topic than nearly everyone who has been arguing on this forum about it for years. As for a tuner, it has the magical property of having different amounts of "forward power" and "reverse power" at its input and output when the amount of actual power flowing is the same on both sides (neglecting tuner loss). So it can create or destroy "forward power" as well as "reverse power" by a simple twist of its knobs. Little does the tuner, transmitter, or load know that as soon as "reverse power" is created, interminable arguments will take place debating about where it goes. I now return you to your regularly scheduled programming. . . Roy Lewallen, W7EL |
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