One sided mirror

(Don Klipstein) writes:

This brings me to directional couplers (a radio frequency device) which
I have heard of but I don't know exactly what they do nor how they work.

There are a number of different ways to build directional couplers, but
(as you surmised) they don't provide a perpetual motion machine, nor are
they a one-way valve. They just control where a signal goes to.

One type of directional coupler is the splitter/tap used in cable TV
systems. A signal that arrives at the "input" is distributed to each
of the "outputs" in a specified power ratio. If the splitter is on
the pole in front of your house, there's one output that receives
almost all the signal, which is the main cable feed continuing down the
street, and one or more house-feed taps that may get a signal that's 20
or 30 dB down from what's on the main cable. But it's a passive device,
so the sum of the signal power in all of the outputs is always less than
the input power.

Its use as a splitter is pretty mundane, but any passive splitter also
works as a combiner when the signals arrive from the other direction. A
signal coming in one of the "outputs" is passed through to the "input",
without also being fed to the other "outputs". In other words, a signal
headed "upstream" continues upstream, without the other "downstream"
outputs seeing it. That's why it's called a directional coupler:
direction of the signal matters.

Dave