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 shortcircuit
is a whole number of halfwavelengths away. In the case the ideal
voltage source sees an ideal shortcircuit. This is an impossible
situation unless the value of the ideal voltage source is zero,
because the voltage across an ideal shortcircuit is zero.
(2) The source is an ideal voltage source and an ideal open circuit
is an odd whole number of quarterwavelengths away. Again, the ideal
voltage source sees an ideal shortcircuit, and the situation is
impossible.
(3) The source is an ideal current source and an ideal open circuit
is a whole number of halfwavelengths 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 shortcircuit
is an odd whole number of quarterwavelengths. 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 farend termination is an
ideal short or open circuit, the impedance seen by the source is
pureimaginary, 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 sineweighted 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
