Cecil, W5DXP wrote:
"Knowing the current at one point tells you absolutely bothing about
SWR."

The reflection coefficient depends on a mismatched load at the end of
the line. Reflection coefficient is more or less constant in a low loss
transmission line.

Bird`s Model 43 does not contain a toroid, but it does sample the
current at a single spot.

Just as its forward and reverse watt indications can be scaled to SWR
readings, it could be calibrated to indicate SWR and scaled to give
watts.

Forward and reverse power flows on a line are nearly constant, except
for slight line loss, no matter where the line is sampled.

The design trick in the Model 43 is making samples derived from amps and
volts exactly equat to each other. So, when their polarity is the same,
their total is double the value of either by itself. When their
polarities are reversed from each other, they cancel completely.

It so happens that a reflected wave always undergoes a reversal of
polarity of either its volts or amps, but never both. This is the
distinguishing difference between the forward wave and the reflected
wave.

The phase reversal of the reflected wave, makes its sample of volts and
amps in the 43`s directional coupler (sampling unit) cancel, while the
sample of the forward wave has double the magnitude of either sample,
volts or amps, by itself. This permits a reading proportional to forward
power only.

To get an indication of the reflected power alone, it is necessary ro
flip a switch (in the Model 43, this is reversal of the slug`s
direction). This switch reverses polarity so that now the forward power
samples cancel and the reverse (reflected) power samples produce a
double of either of their values alone.

Bird gets very satisfactory results without a toroid. Using only a small
pickup loop inside the sampling element to sample the line current is
enough. Line voltage is sampled by this tiny coil`s capacitive coupling
to the precission short 50-ohm line segment inside the meter case.

In normal low-loss transmission lines the forward power is the same at
either end of the line. Same for reflected power. So it makes no
difference where they are measured in the transmission line.

Best regards, Richard Harrison, KB5WZI