Underwater transducers - RF or ultrasonic: Looking for some general info.

Thread Starter

WBahn

Joined Mar 31, 2012
27,946
I have a project that I am just starting to play around with and it would involve transmitting and receiving signals underwater over a range of up to, perhaps, 100 feet or so. I want to explore both an RF and an ultrasonic option. I know that RF signals are attenuated very heavily by water, particularly salt water, but I have no feel for whether transmitting low bandwidth signals over 100 ft using equipment that is suitable for personal use (think something that a scuba diver would use) is at all practical. On the ultrasonic side, my immediate concern is whether I need to use sensors that are specifically designed to be immersed or whether I can use cheap sensors inside a watertight enclosure and couple them to the water effectively.

Any thoughts or guidance from folks out there that have any kind of experience in this arena?
 

studiot

Joined Nov 9, 2007
4,998
Even for 30metres distance there is a problem getting a high enough radio frequency carrier for comms through.
But here is some data. Note also the antenna sizing.
subcom1.jpg

Going the ultrasonic route, you will need to seal the transducers agains moisture.
Fortunately there are many ready made ones available.

Beware of the simple ones available at les than $10 per pair since there are many designed for machinery sensors but they only reach out to 3metres, eg the Murata ones.

Look for marine echo sounder transducers, this site has lots of good information.

http://www.simrad.com/www/01/nokbg0237.nsf/AllWeb/82DFD1B75A682A52C12570D100662A70?OpenDocument

Note these require greater supply power to reach the extra distance.
 

Thread Starter

WBahn

Joined Mar 31, 2012
27,946
Thanks for the info guys. After making the post I ran across a paper talking about the 1.8 MHz ham band (I think the article was U.K.) and it indicated ~20 m range in fresh water and nothing useful in salt. It had a footnote that since the article was written that the 170 kHz range was opened up to hams and that this pushed it into the 30 m range for fresh but didn't give the numbers for salt. I'm guessing that the product I have in mind could be quite feasible even if the range ended up being limited to 15 m or perhaps even 10 m. There's a lot of market research that still has to be done. I also have to look into what frequencies I could use for unlicensed operation given the very short ranges involved (though the range when on the surface would need to be considered, too).

I'm tending to favor an ultrasonic option because the lower speed of propagation significantly works to my advantage for this application.
 

GopherT

Joined Nov 23, 2012
8,009
I stumbled upon a collection of electronics papers yesterday. One dealt with underwater, you may be interested.

http://users.tpg.com.au/ldbutler/Underwater_Communication.pdf

All the papers are well written. Here is the home page. Mostly intermediate to advanced but some are more basic. All are written for the ham or hobbiest. The transistor as voltage amplifier is very well written and saved me from re-inventing the wheel for someone else - that one is very basic.

http://users.tpg.com.au/ldbutler/
 

Papabravo

Joined Feb 24, 2006
19,617
Thanks for the info guys. After making the post I ran across a paper talking about the 1.8 MHz ham band (I think the article was U.K.) and it indicated ~20 m range in fresh water and nothing useful in salt. It had a footnote that since the article was written that the 170 kHz range was opened up to hams and that this pushed it into the 30 m range for fresh but didn't give the numbers for salt. I'm guessing that the product I have in mind could be quite feasible even if the range ended up being limited to 15 m or perhaps even 10 m. There's a lot of market research that still has to be done. I also have to look into what frequencies I could use for unlicensed operation given the very short ranges involved (though the range when on the surface would need to be considered, too).

I'm tending to favor an ultrasonic option because the lower speed of propagation significantly works to my advantage for this application.
Although there has been"talk" about an NPRM (Notice of Proposed Rule Making) from the FCC on Amateur allocations below the 160M(1.8 MHz) band, none of them has come to fruition AFIK. Given the challenges involved I would suggest that you pursue ultrasonics at the moment.
 

RichardO

Joined May 4, 2013
2,270
I'm tending to favor an ultrasonic option because the lower speed of propagation significantly works to my advantage for this application.
Me too. Of course that is some of my experience... Now to dust off a few cobwebs.

In the diagnostic ultrasound systems I worked on the speed of sound was defined as 1540 m/s for the human body. I suspect this is close enough to get started in your thinking -- think of the body as lumpy salt water.

Here is a real sketchy memory: I remember the attenuation as being roughly 1dB /cm /MHz for a round trip signal, i.e. pulse/echo.

For background, the transducers we used were PZT (lead zirconium titanate?) The impedance of the ceramic transducer was acoustically matched to the impedance of the body. This was done either with epoxy or silicone rubber or both, I am not sure. On one transducer, the silicone rubber was shaped to make a lens. The transducer also had a backing material (I think) to damp reflections from that direction.

I think you want to use a transducer made for fish finding since it is already made to work in water. Hopefully the frequency of operation works for you. Walmart has a fish finder for $70...

Here may be some useful information on ultrasonic fish finders:
http://www.furuno.com/special/en/fishfinder/
 

Geowizard

Joined Nov 30, 2014
8
Hi,

I'm new to the forum. :)

But having worked extensively in the geophysical realm, I might offer a helpful idea.

The hydrophone approach is fairly straight-forward but how about a VLF electromagnetic transmit and receiver?

Submarines communicate using ELF. Using 30 KHz or so and PSK modulation might work.
 

GopherT

Joined Nov 23, 2012
8,009
Hi,

I'm new to the forum. :)

But having worked extensively in the geophysical realm, I might offer a helpful idea.

The hydrophone approach is fairly straight-forward but how about a VLF electromagnetic transmit and receiver?

Submarines communicate using ELF. Using 30 KHz or so and PSK modulation might work.
Because there is no allowable frequency by FCC rules.
 

Geowizard

Joined Nov 30, 2014
8
Low power applications similar to hand-held metal detectors and two coil geophysical systems operate without FCC permitting or certification in terms of VLF output. The issues are related to electromagnetic interference in home and office settings.

I would suggest that remote applications done with milliwatt to one watt energies are not an issue. The electromagnetic "field of influence" or detectability is outside of that which FCC regulates.
 

Thread Starter

WBahn

Joined Mar 31, 2012
27,946
Hi,

I'm new to the forum. :)

But having worked extensively in the geophysical realm, I might offer a helpful idea.

The hydrophone approach is fairly straight-forward but how about a VLF electromagnetic transmit and receiver?

Submarines communicate using ELF. Using 30 KHz or so and PSK modulation might work.
And what kind of power and antenna size requirements would I be looking at? Remember, this is for a scuba application.
 

Thread Starter

WBahn

Joined Mar 31, 2012
27,946
Because there is no allowable frequency by FCC rules.
What frequencies can and can't be used is something that will have to be addressed. It would be an unlicensed transmitter and not an amateur transmitter, so the rules are very different.
 

Geowizard

Joined Nov 30, 2014
8
An important premise in FCC regulation is avoiding conflict in the "spectrum".

Nobody wants to create interference to other users of the spectrum. :)

There are already systems in use that provide voice communication underwater as demonstrated on Bering Sea Gold. Those systems are local two-way systems. The Base station looks a lot like a CB radio. They apparently are commercially available.

My thought was more along the line of a low power data communication network - I misunderstood.
 

studiot

Joined Nov 9, 2007
4,998
If you are thinking of rolling your own here is some information taken from

Ultrasonics by Vigoureux of the Royal Naval Scientific Services.


for magnetostrictive transducers.


ultra1.jpg
 
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