Brian Reay wrote:
"---dying to hear at what frequencies directional couplers suddenly
begin to exist."

It isn`t sudden.

They sure work at audio frequencies. In telephones, they are used to
prevent the user`s voice from overpowering the distant party`s voice in
the user`s ear. They are called hybrids.

Hybrids are also used to couple a 2-wire circuit which simultaneously
carries both directions of transmission with a 4-wire circuit consisting
of a transmit pair and a receive pair.

Best regards, Richard Harrison, KB5WZI