I have not looked at the manual but I expect that there is a loop,
a resistor, and a diode detector inside the slug.
The loop does produce a voltage proportional to current in the feed-line
center conductor. But, it also has capacitance to it as well.
This capacitance and inductance are such that when all the power is going
out to a 50 ohm load the voltage produced via the capacitive coupling and
the inductive coupling are equal.
If a 50 ohm load is connected these voltages add if the slug is
pointing into the forward direction. However, when pointing into the
reverse direction the capacitive coupling remains unchanged but the loop's
induced voltage is 180 degrees out of phase. The voltages cancel out.
So when the slug is pointed in the forward direction the meter
receives current from the diode detector. But when in the reverse direction
there is no current from the diode to the meter. These are the ideal
conditions for the directional watt meter to correctly measure power. All
in the forward direction and none in the reverse direction.
Beyond 1:1 SWR the forward and reverse power measurements are
only an approximations. Closer at lower SWR's and worse at higher SWR's...
73
Bill wa4lav
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