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## [TowerTalk] VSWR & Power

 To: [TowerTalk] VSWR & Power ccc@space.mit.edu (Chuck Counselman) Sun Mar 2 22:45:21 2003
 ```At 1:44 PM -0500 3/1/03, Martin Ewing wrote: >And if you have an open or short circuit, VSWR is infinite, and so >are the forward and reverse power! In real life, Tx power goes >somewhere - into tuner and transmission line resistance - and VSWR >is finite. It is true that if you have an open or short circuitat the far end of the line, and if the transmission line is lossless, then VSWR is infinite. However, it is not true that the forward power, or the reverse power, is infinite -- except in four impossible special cases. First, let's dispose of the special cases. I assume sinusoidal steady state (which assumption is implicit in any discussion of VSWR). (1) The source is an ideal voltage source and an ideal short-circuit is a whole number of half-wavelengths away. In the case the ideal voltage source sees an ideal short-circuit. This is an impossible situation unless the value of the ideal voltage source is zero, because the voltage across an ideal short-circuit is zero. (2) The source is an ideal voltage source and an ideal open circuit is an odd whole number of quarter-wavelengths away. Again, the ideal voltage source sees an ideal short-circuit, and the situation is impossible. (3) The source is an ideal current source and an ideal open circuit is a whole number of half-wavelengths away. In the case the ideal current source sees an ideal open circuit. This is an impossible situation unless the value of the ideal current source is zero, because the current through an ideal open circuit is zero. (4) The source is an ideal current source and an ideal short-circuit is an odd whole number of quarter-wavelengths. Again, the ideal current source sees an ideal open circuit, and the situation is impossible. Now, about the forward and the reverse power: Ignoring the impossible special cases and continuing to assume that the transmission line is lossless and that the far-end termination is an ideal short or open circuit, the impedance seen by the source is pure-imaginary, nonzero, and finite. The current flowing into the line is plus or minus 90 degrees out of phase with the voltage across the input to the line, and the source delivers zero real power. The reactive power is finite because the voltage and current at the source are both finite if the source is finite. The voltage and current are finite everywhere in the line, because they are just cosine- and sine-weighted averages of the voltage and current at the source end. The forward power in the line equals the reverse power at every point in the line, and both are finite. -Chuck, W1HIS ```
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