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'[OT] RF modules - singlechip version'
1998\12\07@151230 by William M. Smithers

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On Mon, 7 Dec 1998, Craig Lee wrote:

> I believe there are several application notes on the RF Monolithics website.
> http://www.rfm.com
>
> Then just direct drive it with the TX output of a PIC!

This is pretty much what I've been looking for, only
my application is to conduct RF digital comms under
water.  I only need to reliably go about 5 feet with the data,
and can get by with 1200 baud or less....anybody
have any idea what frequency would be a good choice
for this, with low power consumption in mind?

Thanks,

-Will

1998\12\08@103506 by Nigel Orr

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At 15:06 07/12/98 -0500, you wrote:
>This is pretty much what I've been looking for, only
>my application is to conduct RF digital comms under
>water.

Yich!  Good luck ;-)

>  I only need to reliably go about 5 feet with the data,
>and can get by with 1200 baud or less....anybody
>have any idea what frequency would be a good choice
>for this, with low power consumption in mind?

I would expect (though acoustics in water, rather than RF, is my thing),
that lower frequencies will be better, but even 5 feet could be a problem-
if it isn't, I'll be interested to hear more details.

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?

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             \______/ \(

1998\12\08@110049 by William M. Smithers

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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

{Quote hidden}

1998\12\08@115117 by mwalsh

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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:

{Quote hidden}

1998\12\08@120156 by dave vanhorn
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>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.

You considered the case where several buddies are diving together, right?


>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.

Might be magnetic coupling rather than E-field. Whang a coil..



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1998\12\08@152551 by Peter L. Peres

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On Mon, 7 Dec 1998, William M. Smithers wrote:

> water.  I only need to reliably go about 5 feet with the data,
> and can get by with 1200 baud or less....anybody

Provided there is no noise 455 kHz should work. I did use a short link
like this, with two ferrite antennas, a 150 mW oscillator and a direct
detecting receiver, but for switching, not data. It should be about 9 kHz
wide so data will pass. Both antennas were ferrite bars, tuned to 455 kHz,
and had to be aligned to within +/- 20 degrees or so. Range == 3-4 meters
in air (very wet air - washing tunnel). I don't know about sea water.

Peter

1998\12\08@160044 by Harrison Cooper

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Intersting.....I redesigned something similar, 455KHz, both FSK and FM, for
use in mines.  Used the hoist cable as the transmission medium.  Isn't it
strange how things seem run in circles...

{Quote hidden}

1998\12\09@024747 by ftpMaster

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The US Navy uses extremely long waves <30kHz for communications with
(underwater) submarines all over the world. If You use a frequency about
455kHz (IF for AM radios) it will probably work, and also use common
components like ferrite antennas, resonators, ceramic and LC filters.

{Original Message removed}

1998\12\09@075951 by Martin McCormick

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       One good aspect of this problem is that magnetic fields are
not effected by water very much.  It is my understanding that radio
signals have two component fields.  There is a near field which is
magnetic in nature and a far field which is electrical.  Signals which
bounce off the ionosphere and travel great distances do so via the
electrical field while antennas that use the magnetic field such as
the rod antennas being discussed work best on magnetic pickup.  I
would think that the water would not effect the magnetic field much so
the close range communications would still work.

       Anybody who knows the theory which I am talking about is free
to step in here and say that I am full of hot air, but this is the way
I understand it.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group
ftpMaster writes:
>The US Navy uses extremely long waves <30kHz for communications with
>(underwater) submarines all over the world. If You use a frequency about
>455kHz (IF for AM radios) it will probably work, and also use common
>components like ferrite antennas, resonators, ceramic and LC filters.

1998\12\09@094742 by Nigel Orr

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At 10:54 08/12/98 -0500, you wrote:
>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.

What I mentioned was that you'd need DSP at 30km- at 5 feet, I would have
thought you could get OK data with a very simple on-off keyed or FM
ultrasonic signal and some error detect/correction.

>bursts.  Really low power - one works for several years at
>400 hours of use per year on a couple of button cells.

That's impressive- if you find more info on them, could you email me
privately with it, unless others would be interested?

Nigel
--
Nigel Orr                  Research Associate   O   ______
       Underwater Acoustics Group,              o / o    \_/(
Dept of Electrical and Electronic Engineering     (_   <   _ (
    University of Newcastle Upon Tyne             \______/ \(

1998\12\09@103900 by Peter L. Peres

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On Tue, 8 Dec 1998, Harrison Cooper wrote:

> Intersting.....I redesigned something similar, 455KHz, both FSK and FM, for
> use in mines.  Used the hoist cable as the transmission medium.  Isn't it
> strange how things seem run in circles...

They run in tiny circles around basic physical principles and
mass-produced ceramic filters in the proper frequency range ;)

Peter

1998\12\09@105551 by Sean Breheny

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In my class on classical wave theory, we were taught that it was the skin
effect which limits
the propagation of RF under water. What you get is an exponentially
decaying wave as you go further under the water. How fast it decays with
depth is dependent upon water conductivity and frequency. Here is the
relation:

P=Po*exp(-2d/s)

Where:

P=Power at depth d
Po=Power at surface
d=depth
s=skin depth
(d,s in meters,P and Po in whatever units you like, just keep them the same)

s=sqrt(1/(pi*f*g))

Where:

s=skin depth (meters)
pi=3.141
f=frequency (in Hertz)
g=conductivity (in mhos/meter)


Sean

At 09:42 AM 12/9/98 +0200, you wrote:
>The US Navy uses extremely long waves <30kHz for communications with
>(underwater) submarines all over the world. If You use a frequency about
>455kHz (IF for AM radios) it will probably work, and also use common
>components like ferrite antennas, resonators, ceramic and LC filters.

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