Does anybody know how wireless communication works underwater? I assume low frequency would work better than high frequency. Do you know of a system that works?
I don’t need a whole lot of data, i’d be transmitting a pressure every second or so, and only need to transmit 5 feet at most. Got ideas?
Thanks
Attenuation is very high, I doubt if it will work. The US Navy uses VLF with an antenna that is several miles long for communicating with submerged submarines, at very low data rates.
Leon
Leon is right about RF attenuation in water, but it’s ony 5 feet you’re after, so it should be fairly “easy”.
I’d suggest ultrasonic, but if you do go RF, try to use as low a frequency as possible. I believe there are dive computers etc that use RF in the order of 10’s of KHz to communicate short distances.
AM broadcast (550 to 1600kHz) works well under water to 10 feet easily.
A modified AM transistor radio and a simple transmitter should get you through the proof of concept at 550-700kHz. Goggle VLF radio from lots of design ideas for 10’s of kHz.
I did some pretty extensive testing on underwater communications. As one would expect, the path loss goes up as a function of frequency. One data point of interest: at 42" underwater, and 433 MHz, the path loss was 77 dB. Path loss is pretty linear, in dBs, with water depth.
So a standard transceiver chip, at as high as 433, but certainly at 315 MHz or lower frequency, should communicate between 5’ of water. Most transceiver chips can handle 95 or more dB of path loss (some as high as 108 dB path loss).
One problem you will have is: how do you keep the antenna working underwater? Most antennas that you would buy are designed to work in air. Loading them with the high dielectric constant of water will make the antenna look electrically too long. As such, an antenna tuned at 315 MHz in air might retune itself much lower in frequency–resulting in poor radiation efficiency. One solution would be to use a standard antenna, but put it inside of a PVC tube that is filled full of air. Another method might be to use a ceramic chip antenna, where the chip material has a very high dielectric constant, so that the surrounding water will not detune the antenna chip. A final method might be to use a non-resonant magnetic coil as the antenna (probably more likely if you choose a frequency below 20 MHz).
Get a cheap FM broadcast band transmitter and try it with various antennas.
If signals under water are that hard to do, how do the devices that can pulse the entire sea bed for its depth work? What type of technology do they use and would that not be possible to convert into a messaging format?
angelsix:
If signals under water are that hard to do, how do the devices that can pulse the entire sea bed for its depth work? What type of technology do they use and would that not be possible to convert into a messaging format?
those are audio/ultrasonic, not radio freq. (sonar).
Navies do long distance underwater comms but at very low frequency and very high power. Also moderate distances using optical/laser.
I wonder what technology the SCUBA divers with voice comms (as we see on TV) use? Or unmanned undwater vehicles (untethered). Perhaps the H field instead of E field?
Googling, it looks like these use 32,768Hz single sideband.
I’d think there’d be lots of multipath and doppler to cope with - probably using a long spread spectrum code and high correlation gain.
One article I read says that in fresh water, it’s about
100dB attenuation for a bit more than two meters of distance at 1MHz. At 10MHz this becomes less than 1m. The graph doesn’t go above 10MHz.
A 100dB path loss in air is “typical” for common radio links, but for much greater distances, e.g., free space loss is about 34+20log(distance in m )+20log(frequency in MHz).
stevech:
I wonder what technology the SCUBA divers with voice comms (as we see on TV) use? Or unmanned undwater vehicles (untethered). Perhaps the H field instead of E field?Googling, it looks like these use 32,768Hz single sideband.
good to know that these use a 32kHz signal… that’s pretty easy to implement
Don’t know how one would couple the E or H field at 32KHz to the water.
That is, what kind of antenna?
Presumably a ferrite rod with a coil of wire around it. Similar to an AM radio, but different ferrite / coil due to the lower frequency. Maybe powered iron core instead; suggest you do some reading on antenna design…
MichaelN:
Presumably a ferrite rod with a coil of wire around it. Similar to an AM radio, but different ferrite / coil due to the lower frequency. Maybe powered iron core instead; suggest you do some reading on antenna design…
32KHz is substantially lower in frequency than the MW (AM) broadcast frequencies (around 1MHz) and falls into the audio spectrum. Some form of ultrasonic transducer is needed.
Ultrasonic is a possible solution (and probably the preferred one in this case), but I wouldn’t say it is “needed”.
I’m confident that if properly designed, a coil-type antenna could be used at 32KHz underwater. Have a look at these small antennas; while I’m sure they would be de-tuned to an extent, I’m sure something like these could be used underwater:
http://search.digikey.com/scripts/DkSea … A-40-19-ND
32kHz could be an acoustical wave or an EM wave. The former is audio, technically, but the latter is RF. Acoustical waves can be generated into the MHz and is called ultra sound.
http://en.wikipedia.org/wiki/Ultrasound
And Rf (EM waves) can be generated into the very low kHz.
Check out information on the US Navies VLF transmissions to submarines and look up VLF communications.
http://www.fas.org/nuke/guide/usa/c3i/vlf.htm
http://chennaihams.blogspot.com/2009/09 … using.html
There is also a section around 137kHz that is license free by the FCC and there are quite a few people doing experimental communications at 137kHz.
http://www.dxzone.com/catalog/Operating_Modes/Longwave/
http://www.nitehawk.com/rasmit/ws1_21.html
Lots of information out there on the web.
Here in the UK we have an experimental VLF amateur band on 136 kHz. It’s mainly intended for transatlantic working using very slow Morse code and data.
Leon
I think the 32KHz SCUBA gear uses EM, not acoustics. I saw one loop antenna of just 8 inches’ diameter. Not sure how that can resonate given lambda at that freq.
stevech:
I think the 32KHz SCUBA gear uses EM, not acoustics. I saw one loop antenna of just 8 inches’ diameter. Not sure how that can resonate given lambda at that freq.
Have you ever looked at the loop-stick antenna inside an AM pocket radio?
stevech:
I think the 32KHz SCUBA gear uses EM, not acoustics. I saw one loop antenna of just 8 inches’ diameter. Not sure how that can resonate given lambda at that freq.
Like you noted, it could be an EM system and the loop is the induction coil. But I would think that the range would be very limited. The MIT work, which can communicate hundreds of meters, appear to be using acoustics, but I haven’t read the paper in depth.
We need to transmit scuba tank pressure for an air pressure alert system for a project at work. I have one of those wireless air integrated dive computers so our first idea was to copy their RF and use it for this system. It turned out that the dive computers transmitter could only go two feet or so. (I’m always getting “pressure signal lost” messages on the computer while diving too). If you’re interested in using sound instead then you can look at our acoustic modem for inspiration. It’s over here: http://www.desertstar.com/Products_prod … roductID=4