>But there definitely are audio frequency "RF" transmitters, and these are
>not "ill-designed products." For example, the U.S. Navy uses what most
would
>consider as "Audio Frequencies" in their equipment designed to communicate
>with submerged submarines. Though efficiency is horrendous, the importance
>of the task makes the inefficiencies acceptable.

Not these days. There was a thread on this a while back where someone was
bemoaning that the transmitter had been turned off. It turns out, from a
very good source I have, they use blue-green lasers from spacecraft these
days, it is more secret, as in local to the signal destination, and has a
higher data rate. I suspect that they can probably do bi-directional as
well.

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