The US Navy used to have a transmitter south of Sacramento that
broadcast to submarines with a carrier of around 200 kHz.  I also
recall hearing about a transmitter in the 60 kHz range that used a
long wire antenna strung between two mountains.

This sort of makes sense.  If you are in the microwave region,
the energy ends up heating the water instead of being propogated
through it.  I would suspect that the lower the frequency you
can reasonably use the better off you are.

Mark Walsh



William M. Smithers wrote:

> On Tue, 8 Dec 1998, Nigel Orr wrote:
>
> > At 15:06 07/12/98 -0500, you wrote:
> >
> > Considered ultrasonics?  At that rate, a standard 40kHz ultrasonic link
> > might be easier to do?  And you could probably run it from a PIC pin (or
> > between 2 pins as push-pull) directly?  At the other end, just detect the
> > on-off keyed signal with a pre-amp and rectifier.  This assumes there isn't
> > alot of other noise in the water in the same frequency range... what's the
> > application?
>
> I'm worried that ultrasonics wont't work, - the application
> is a "hoseless" scuba diving decompression computer.  "Hoseless"
> because the air pressure hose from the tank to the processor is replaced
> by a radio link that broadcasts the tank pressure to your wrist.
> But it's not necessarily line-of-sight, and as you mentioned,
> there's usually a DSP component when you're talking about acoustics,
> which I'd really like to avoid.
>
> There are a couple units out there
> that do this already via RF, but they aren't telling
> their freq data, of course.  They use low-power intermittent
> bursts.  Really low power - one works for several years at
> 400 hours of use per year on a couple of button cells.
>
> -Will
>
> >
> > Our systems could probably manage about 30km underwater at 1200 baud, but
> > the DSP cards might be too big for your needs ;-)
> >
> > Nigel
> > --
> > Nigel Orr                  Research Associate   O   ______
> >         Underwater Acoustics Group,              o / o    \_/(
> > Dept of Electrical and Electronic Engineering     (_   <   _ (
> >      University of Newcastle Upon Tyne             \______/ \(
> >