Richard Harrison wrote:

Tom, K7ITM wrote:
"FWIW, the "aingle loop terminated in a diode" that provides both
magnetic and electric coupling at the same time" is not the only way to
make a directional coupler."

Agreed, but the Bird Electronic Corporation has been successful making
the plug-ins for their "Thruline Wattmeter" that way for about 50 years.

My old Heathkit HM-15 SWR meter has a short slotted through-line
with two parallel pick up wires located about halfway between the
center conductor and the shield. A 50 ohm resistor to ground at
one end kills the voltage in that direction. A diode at the
other end rectifies the voltage in the opposite direction.
With two resistors and two diodes on opposite ends of the two
pickup wires, they separate the forward wave from the reflected
wave.

The operation of the slotted line + pickup wires seems to be
a lot like the Thruline element in a Bird but with no slug
to rotate.
--
73, Cecil http://www.qsl.net/w5dxp