# Short range - low bitrate data transmission under water

I am looking to get a hold of small transmitters for use under water (sea water). They will be rigged to my sensor equipment and attached to fish for scientific monitoring, so size is very important.

The transmitters will need to:

• Transmit digital data packages of around 256 bits once per second.
• Have a minimum range of approx. 150 meters for good measure.
• Be able to work in an system with 10 or more active transmitters at the same time.
• Be as small as possible.
• Be as energy efficient as possible

My research indicates that my best bet is to build fixed frequency am-transmitters. (Availability, cost, size etc.) with small crystal oscillators. Is this a viable plan or are there better solutions out there?

Also, does anyone know if a setup like this could work under water? Keep in mind that I only need a range of 150 meters.

Since I only need a bitrate of about 512 bits / second per transmitter (I want overhead for more data), would a lower frequency transmitter give me lower power consumption?

There are no size or power restrictions on the receivers.

EDIT: I am getting a lot of good information. The attenuation in sea water is much higher than I had expected, thanks for the graph.

I see that even my initial plan, using frequencies in the LF-band is out of reach at this range.

1KHz at 20 meters range with more receivers placed seems more realistic, but even this does not look very promising. It seems that EM transmission just isn't that feasible under sea water. Maybe I'll be better off with an acoustic transmitter or some form of passive telemetry.

Thank you all.

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Normally I try to be positive and supportive. However, in this case, I have to advise you that what you are trying to do is impossible (in practical terms). RF does not propagate well in seawater, electronics are hard to make waterproof enough for your purposes, fish are too small to tag, etc, etc. – mkeith Jan 11 at 22:44
Waterproofing the kit won't be a problem. The fish are big salmon, and can take on a bit of equipment. Small sub-sea data transmission systems do exist, but just not for my exact needs. So this is absolutely feasible, I am just looking for an optimal solution. – Pjottur Jan 11 at 23:00
@Pjottur I don't have enough experience to post a real answer, but did you consider the idea of scraping the radio transmitter completely and moving to ultra-sound? I know that there is quite a bit of research being done on using "high" data rates with ultra-sound under water and that it does in fact work. – AndrejaKo Jan 12 at 7:07
Why have you accepted such a downvoted answer and not accepted any of the significantly upvoted answers ? Is there a special badge for this ? – Autistic Jan 12 at 9:40
@Autistic he accepted the first answer given - when I saw it I had to leave my answer to compensate!! – Andy aka Jan 12 at 10:59

Yes it sounds interesting and there is some good data on transmitting radio in sea water. Let's start with a graph: -

The base of the graph is carrier frequency and the Y axis is attenuation in dB per metre. There are two plots: one is sea water (4 Siemens per metre conductivity) and one is Adelaide (Australia) fresh water (0.0546 Siemens per metre). The graph is derived from this document - it contains the formula for the electric field attenuation as: -

Attenuation$^1$ dB/m = 0.0173$\sqrt{f\sigma}$ where sigma is conductivity and f is in Hz.

For sea-water, at a carrier frequency of 1 MHz the attenuation is about 33 dB per metre. At 100 kHz this is down to about 10 dB per metre.

A 100 kHz carrier can easily support a bit rate of 256 bits per second so it's a contender (or is it?). However, given a range of 150 metres, that's an attenuation of 1500 dB so it's out of the question as far as I see. So maybe a 10 kHz carrier can work - it will attenuate about 3.5 dB per metre giving a maximum attenuation over 150 metres of 525 dB (yuk).

It's not looking good. How sensitive can a radio receiver be is the question that now springs to mind and there is a fairly widespread and useful formula that relates data rate to sensitivity: -

Power (dBm) needed by a receiver is -154 dBm + 10$log_{10}$(data rate)

At only 256 bps the sensitivity (if designed correctly) is -130 dBm.

To get this level of signal over a link that loses 525 dB means a power input to the fishes transmitting antenna of (525 -130) dBm or an UNFEASIBLE AMOUNT OF TRANSMIT POWER (we are talking 10^36 watts).

For instance, Voyager II from deep space in September 2013 produced an attenuation that was roughly 245 dBm to receive antennas on earth. OK, it transmitted 22 watts with a high gain dish antenna and we used football pitch sized dishes to receive the data but it was do-able.

So, my advise is to have a major rethink and possibly consider using underwater cameras to do what you want OR, if you are still into radio, then a number of localized receivers scattered around the "fish pond".

If you follow this 2nd approach you may be able to use magnetic transmitter coils and magnetic receiver coils. The attenuation of a mag field is a cube law but in the vicinity of 5 metres it should work.

Forget about anything that has MHz or GHz appended to the carrier frequency.

$^1$ the formula given above might need some explanation. It comes from realization of what "skin depth" is. As frequency gets higher a current will penetrate less and less into a conductive medium, preferring to stay at the surface. The skin depth at which the current has attenuated to 1/e (8.6859 dB)is: -

Length = $\sqrt{\dfrac{1}{\pi f\mu_0\sigma}}$

So for the case of 1 MHz, and assuming magnetic permeability of water is 4$\pi$ x 10$^{-7}$ with a conductivity of 4 S/m, length = 0.2516 m.

So that's an attenuation of 8.6859 dB per 0.225 m or 34.5 dB per metre as per the graph above for seawater at 1 MHz.

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One idea that I had in mind was to let the sensors stay inside the water and keep the radio on water surface attached with wire to the sensors. The communication will occur in air and OP might be able to achieve the desired range with a feasible amount of power. – Whiskeyjack Jan 12 at 6:27
But again, how to submarines stay in contact with their control station? – Whiskeyjack Jan 12 at 6:28
@Whiskeyjack They don't really, at least not at depth. They can receive ELF signals but the data rate is extremely limited. You might consider sending a submarine's ID to call it to periscope depth and receive data through a fast link to be roughly the limit -- say a handful of bytes, and it wouldn't matter if this took several seconds to receive. VLF is limited to a few metres depth but can also be used. – Chris H Jan 12 at 9:15

What has been proposed in earlier comments and answers is unrealistic. Just Google "attenuation of radio waves in sea water" and you will find that the attenuation, even at 10 MHz, is already approaching 100 dB/meter. Using radio waves for your application will limit your range to just a few meters at best. That is why sound transmission has been used in the past for applications like yours. You will still have problems because of the combination of the range you need and the data rate (there is a strong trade-off of these 2 parameters because of the high attenuations, even of sound, in water). I would try to contact other researchers in your area, as well as a good Internet search effort, to learn how what you want has been done in the past.

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Hey don't count me in your "What has been proposed in earlier comments and answers is unrealistic"!!! – Andy aka Jan 12 at 0:04
@Andy Sorry about that. Your answer had not yet been posted as I was composing mine. – Barry Jan 12 at 0:06
You are quite forgiven! – Andy aka Jan 12 at 0:08

RF is right out for underwater use on a fish farm, there is just no way to get those distances (But 150M also feels long to me, with the RX hung below the feeder 20M would probably do).

A possibly silly thought, but the cages are often equipped with extensive video systems (to keep an eye on predation by seals amongst other things), how about a coin cell and a couple of visible LEDs as a tag? Pot the thing in Polyurethane resin (Well thats how the sonar guys keep water out) and use openCV or something to track the moving lights, optical is also possible as a comms method if you were to add a little micro and fet to control the LED.

Sonar is of course a possibility, and does work well, but size might be an issue.

Regards, Dan.

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