Richard Harrison wrote:
Tom, W8JI wrote:
"A traditional directional coupler works by comparing voltage across the
line at any one point to current in the line at that same point."

Almost. It compares a voltage sample to a current sample, both of which
have been converted into d-c voltages. These have been carefully crafted
to be exactly equal d-c voltages regardless of the power level in the
line.

That's absolutely incorrect Richard.

If you get out the schematic of ANY directional coupler, you will see
the current sampling device is in series with a voltage sampling
device.

The radio frequency voltage ratios of sampling system are combined
BEFORE detection.

The dc voltage level does vary with both voltage and current (power),
and that is why the meter on the front of your watt meter goes up and
down with power levels.

Only a phase detector levels voltages.

73 Tom