On Mon, 8 Jul 2002, Brendan Moran wrote:

>Thanks Bill & Jim
>I guess I assumed that I had something that might work, but when I tried to
>find info on using directional couplers, I pretty much came up blank.  So,
>how does a directional coupler for low RF (10-20MHz) actually work?

The same as for any frequency. A correctly terminated transmission line
does that. The easiest way to understand a d.c. is to imagine that it has
one line terminated with Z0 at both ends and the other is not matched and
fed by a generator. This will cause a standing wave in the unmatched line
and thus in the coupler.  Examine what this causes to appear on the
termination resistors of the matched side of the d.c. assuming that its
coupling coefficient is 1. This is really used to measure SWR usually but
it makes for a good example to understand what is going on. Notice that
for a matched d.c. L/lambda is irrelevant but when it is unmatched
L/lambda influences the magnitude of the signals on the matched outputs.
d.c.  for HF are usually made by winding bifilar (or more) wire on a
toroid to achieve a resonable proportion of lambda. (L is the electrical
length of the d.c. and lambda the wavelength at the frequency used).

hope this helps,

Peter

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