In most discussions about the consequences of high SWR, and perhaps
in this one, forward power is confused with _net_ forward power. By
_net_ forward power I mean forward power minus reverse power.
EM waves, carrying power, travel in both directions on a
transmission line. A directional coupler/bridge whose characteristic
impedance Zo matches that of the line distinguishes between and can
measure each of these two, forward and reverse, waves separately.
From the amplitudes (in volts or amps) of the two waves and Zo, a
typical "SWR" meter derives (by some analog or digital process of
calculation) the power carried by each of the forward and reverse
waves, the percentage of "reflected" (i.e., the ratio of reverse to
forward) power, and/or the VSWR.
E.g., a VSWR equal to 3 corresponds to a forwardwave amplitude (in
volts) equal to two times the reversewave amplitude (in volts);
maximum voltage on the line occurs where the amplitudes of the two
waves are inphase (2+1=3); minimum voltage on the line occurs where
the amplitudes of the two waves have opposite phases (21=1); and
reflectedpower ratio equals 25%.
Assuming for simplicity that the line is lossless and that a 100watt
transmitter is matched to the line by a lossless "antenna tuner" or
matching network, then:
(1) A meter between the transmitter and the tuner will indicate
Forward power = 100 watts;
Reverse power = 0 watts;
% refl. power = 0 %;
VSWR = 1:1; and
the _net_ forward power is 100  0 = 100 watts.
(2) A meter between the tuner and the antenna will indicate
Forward power = 133.33 watts;
Reverse power = 33.33 watts;
% refl. power = 25 %;
VSWR = 3:1; and
the _net_ forward power is 133.33  33.33 = 100 watts.
Note that the forward power exceeds the power output
from the transmitter because the 33.33 watts of reverse
power returning to the tuner is reflected from the tuner
back toward the antenna.
All of the 100 watts put out by the transmitter is delivered to the
load, i.e., the antenna, because we have assumed no losses. If the
antenna itself has no loss, and if nothing in the near field of the
antenna (e.g., the earth) dissipates power, then all of the 100 watts
put out by the transmitter is actually radiated.
In the real world, both the line and the tuner have losses, and the
story is different in several respects: First, some of transmitter's
output power is dissipated in the tuner. Second, some of the reverse
power returning to the tuner is dissipated in the tuner and not
reflected back toward the antenna. Third, some of the forward power,
and some of the reverse power, flowing in the line between the tuner
and the antenna, are dissipated in the line. The arithmetic is
somewhat complicated, but it's easy with a computer. Various
computer programs are available, some from the ARRL, and some from
the web (e.g., see <http://fermi.la.asu.edu/w9cf/index.html>).
An interesting question for you armchair lawyers out there: Does the
FCC's peaktransmittedpower limit of 1500 watts refer to the forward
power somewhere, or to the _net_ forward power?
IMO, the FCC regulation _must_ refer to _net_ forward power because
forward power can be almost anything, depending on where you measure
it, for a given transmitter power, for a given radiated power, or for
a given power delivered to the antenna.
73 de Chuck, W1HIS
