SparkFun Forums

[Moggity]

Hi Guys,

I'm working on a project where I need transmission of data to and from a underwater device to a base station above the water. It is to be used in a pool so unfortunately the water is chlorinated. The device in the water will be worn on a swimmer so it will be very close to the surface vertically, but at times the line of sight transmission will be through a considerable amount of water.

I'm thinking of using some of the relatively low frequency transmitter/reciever sets (315/433 Mhz) so they at least have some sort of ability to get through the water. I have a few questions that someone here might be able to offer some advice on.

1. Do you think I can depend on refraction of the radio signal cutting the transmission length from direct to the signal travelling over the water and then being picked up like it was being tranmitted from a more vertical position.

2. Do you think there is any chance what so ever of any of the 2.4Ghz modules working, particularily the ZigBee modules so i can use mesh networks.

3. What would be a good method of testing connectivity during radio tests? Do you suppose that a simple time count sent from one Arduino to another would suffice (and then just looking where the missing counts are)?

If you would like any additional information, please ask and I'll try and answer to the best of my abilities.

Cheers

Andrew

[stevech]

put the candidate radio (Xbee or whatever) in a watertight jar and see what happens - short distance.

Google found this one - for 3GHz, close to 2.4GHz.

"The particular problem with the immersion of the RF system in water is that water is an extremely lossy material, with an attenuation of 382 dB/m at 3 GHz (Jacobi et al., 1979). This is due to the fact that water molecules are quite polar, with the poles being rotated by the alternating electromagnetic field, which in turn heats the water. This principle is used in microwave ovens and thus converts the energy that was to be used for transmission into heat."
http://www.emeraldinsight.com/Insight/V ... 40201.html

But at some low frequency, like maybe the unlicensed 49MHz band where there are R/C model airplanes, or even 27MHz CB band (voice) you should find a feasible solution. SCUBA divers use something in this area, I think, for short range radio-based intercom.

The DoD uses laser for high speed data and super low freq. RF for worldwide comms. to submarines.

[Moggity]

Thanks for your quick reply stevech.

I'm planning on doing some testing with a variety of radios, so hopefully that will give me something to work with. I might have to try and write some sort of error checking routine that will run on an Arduino that will report how badly mangled a piece of data is after being sent from within the water.

I know that the SCUBA systems and some others are acoustically based in some cases, but this won't work within a pool because of the amount of noise from the surface interface (the swimmer splashing around) and because of reflections... this is what I've been led to believe anyway, I could be completely wrong.

Cheers

[phishguy]

I sincerely doubt that 315 Mhz will even work. I would suggest a very low frequency. Where I work, I am involved in RFID at 134.2kHz. We use that underwater for tracking fish migration through fish ladders and bypasses. Even that uses near field communication. Before I started at my current job, they used to use 400kHz. It worked also, but not quite as well. Also, I'm not sure what effect chlorine will do to the attenuation, but I do know that salinity will also dramatically attenuate the signal.

[gallamine]

Moggity, have you made any progress?

[Moggity]

Hi gallamine,

I haven't made much progress recently. Work and other commitments have taken their toll on my tinkering time.

I'm going to try my 433Mhz modules and see if I can get any sort of signal from those while underwater.

I'll update the thread when I have completed my tests.

Cheers

Andrew

[leon_heller]

I just can't see that working.

I used to work on military comms. The UK Special Boat Squadron (similar to the US Navy SEALs) have antennas on their heads when on underwater ops. When they need to communicate they stick their heads up out of the water.

Leon

[Moggity]

Hi Leon,

I'm pretty sure I'm not going to have much luck trying to get such a weak signal to penetrate water. I'm hoping that with a small antenna I won't have to get a signal through the water because the antenna will always be above the surface.

That said I'll give it a go because even a weak signal will be enough. I'd really like to use a system like the Polar heartrate monitors use, but the magnetic pulses are far too weak to be recieved at any reasonable sort of distance. It might be worth reviewing them a little further tho.

Cheers

Andrew

[scubamatt]

Hey Andrew,

How much information are you sending to the base station and how quickly do you need it to get there?

I work for some people that use a lot of acoustic tech and they test the systems in pools as well. There's a balance you have to reach between detection threshold levels and switching frequencies after transmissions/receptions so that you ignore the echoes but it works. I'm not sure what kinds of rates you would be able to achieve in this kind of an environment though.

Best of luck!
-Matthew

[stevech]

pulse coded Lasers are often used for underewater data comm

[Moggity]

Hi Guys,

Conducted some testing this morning at the pool and I have some interesting results.

The setup I was using was an Arduino with a 433 Mhz transmitter (http://www.jaycar.com.au/products_uploa ... 0(mod).pdf) running at 3.3V using a 1/4 wave whip (dodgy piece of wire truthfully) running the VirtualWire toolkit developed by Mike McCauley (http://www.open.com.au/mikem/arduino/VirtualWire.pdf) and another Arduino with the 433 Mhz receiver (http://www.jaycar.com.au/products_uploa ... 2(mod).pdf) and the same dodgy antenna.

The test codes were the examples provided in the VirtualWire toolkit. I can't remember the data rate off hand but it's all in the examples in the PDF I linked.

Testing showed that the radio managed to transmit to a depth of approximately 3" below the surface, regardless of the range (0 meters or 50 meters away) and could probably get a bit deeper with the current modules and some decent antennas.

This will probably be more than good enough for my purposes, but I'm going to investigate some higher power transceivers either in the 433 Mhz band or the 315 Mhz band and see if I can do any better.

Cheers

[stevech]

Hi Guys,

Conducted some testing this morning at the pool and I have some interesting results....
Testing showed that the radio managed to transmit to a depth of approximately 3" below the surface, regardless of the range (0 meters or 50 meters away) and could probably get a bit deeper with the current modules and some decent antennas.

This will probably be more than good enough for my purposes, but I'm going to investigate some higher power transceivers either in the 433 Mhz band or the 315 Mhz band and see if I can do any better.

Cheers

consider this:

The antenna 3 in into the water, you managed reception out to at least 50m.

The free space (air) path loss at 50m is about 58dB at 400MHZ. Let's assume the transmitter you have now is about 1mW (typical for such radios in this band). In dBm (decibels w.r.t. 1mw) this is 0dBm. 2mW is 3dBm; 4 mW is 6dBm, and so on.

The receiver sensitivity, for a cheap radio and primitive modulation methods, is about -90dBm. What you could do next is find where the error rate is unacceptable due to weak signal, with the ant. in 3" of water. That will tell you pretty accurately how much more range you'd get with x dBm of power (increase).

I've never fiddled with antennas in water, but I'd guess that the method of keeping the antenna separated from the water, say, in a watertight RF-transparent tube, is key. Seemingly, it would have to be a 1/4 wavelength or more.

So going from 1mW to, say, 32mW is really not a lot of dB compared to the path loss and water attenuation.

[Moggity]

Hi stevech,

Thanks for your reply! I'll have to look at the performance figures for the radios I used and see if I can figure out what sort of improvement I'll get.

Perhaps you can correct me here, but this is my understanding which, if this is true then I'll be in good shape. The attenuation of the signal is going to be some sort of a cubic function vs the power of the transmitter. So if i was to increase the power output of the transmitter by a factor of 9, I would get 3 times the penetration into the water.

Is this correct? If it is then having something like 9" of submergance would be more than enough for my needs and I'll be quite pleased.

Cheers

[cnd]

The local radio-controlled submarine club here all use 27mhz to 40mhz - all the new-fangled Xhgz stuff does not work at all.

I just tested myself - stuck a bluetooth speaker in a plastic bag - it only takes a few centimeters of (salt pool) water to totally block the signal.

[waltr]

Yep, the higher the frequency the less depth the RF will be received. Conversely the lower the frequency the greater the depth. Try doing this with an AM broadcast radio (550to 1600kHz), you will be able to receive stations from the bottom of the pool but not with an FM broadcast (88 to 108MHz) radio. The US Navy has a radio system to communicate with submersed subs that transmits at around 10kHz.
So find or build a radio that operates at 100's kHz.