> A sample of the line voltage is balanced with a sample of the current,
> then the two signals are added, which cancels the reflected component, to
> measure "power" in the forward direction. Note that it is really the
> line voltage, not power that is being sensed -- calibrate your meter scale
> accordingly! And since this is a voltage based measurement, it will only
> be correct at the selected line impedance.
It is a voltage AND current based measurement. You said so
yourself above, and you had it right!
> The reflected "power" is measured similarly by using an opposite phase
> sample of the current in the addition.
True enough. The directional coupler measures voltage, current,
and even detects phase. You wind up with the vector sum of
current and voltage representing each power reading, so no matter
what the load if SWR is high the forward power reading goes up.
To measure power delivered to the feedline, you have to subtract
reflected power from forward.
The best way to detect a load fault is to simply watch the reflected
power. That's what radios do, and with a directional coupler it
works perfectly fine. Sample forward power to control the drive
power to the PA, and reflected power for SWR shutdown.
This always works under any load condition.
73, Tom W8JI
w8ji@contesting.com
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