Ian White, G3SEK wrote:
The RF voltages are either summed or subtracted, and then the resultant
is detected by the diode.

I know you know this, Ian, but let's make sure that everyone understands
that the summation (or subtraction) is a phasor (vector) summation. It is
the phasing between the total voltage and total current that allows the
forward wave to be separated from the reflected wave, and vice versa. The
directional coupler designer assumes that the ratio of Vfor/Ifor = Z0
and that the ratio of Vref/Iref = Z0. If that assumption is incorrect,
the SWR meter will still assume that the assumption is correct.

Just one small point, though... it is not necessary that R = Z0.

That's true and is just a habit on my part. I set R=Z0 and then adjust the
voltage accordingly for calibration purposes. For awhile, I was using a
450 ohm load for ladder-line with a measured Z0 of 388 ohms. It still
worked pretty well.

AFAIK, the only situation where the "terminating" resistor truly needs
to equal Z0 is in parallel-line couplers for microwaves, when the
sampling line approaches a quarter-wavelength long and its own
characteristic impedance is Z0 too.

In most of the slotted line pickups that I have seen, the internal load
resistor is equal to the Z0 of the slotted line. I don't know if that
is necessary or not.
--
73, Cecil http://www.qsl.net/w5dxp



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