Medium Distance - Single Wire Transmission

Hi all,

Have an idea for a project that involves transmitting data underwater. We are talking 20-50m line of sight in salt or freshwater.

Whilst higher data rate is best, I understand the RF options even in the 900mhz range will not cut the mustard. Additionally it appears that acoustic methods and laser have expensive patents wrapped over them.

Whilst running a seperate wire underwater is something I want to stand clear of for this project, there is a anchor chain that I think may be of some use as it naturally travels from the boat (where I want to receive the data) to the transmitter at the bottom.

Does anyone have any advice on transmitting data through 1 wire essentially? This is galvanised chain so I am a little unsure of it’s electrical transmision characteristics (with essentially thousand of linked connections) but I was thinking of a 1 wire protocol and a mossfet to ramp up the current.

Keen for any thoughts or alternative solutions!

brodiej:
Additionally it appears that acoustic methods and laser have expensive patents wrapped over them.

I seriously doubt that you could infringe on any sonar patents with any simple acoustic communications setup. Not unless you study and copy the methods for optimum performance. Nobody owns the patents to basic techniques - patent law explicitly states that the methods must be “non-obvious”.

aarg: I seriously doubt that you could infringe on any sonar patents with any simple acoustic communications setup. Not unless you study and copy the methods for optimum performance. Nobody owns the patents to basic techniques - patent law explicitly states that the methods must be "non-obvious".

Absolutely I agree your right. I more was referencing pre-assembled 'modules' for sonar or laser transmission.

From what I could see there was no way to utiilise for example MaxSonar to transmit / receive rather than sense distance. Doesn't seem to be a 'listening' mode.

brodiej: Whilst higher data rate is best, I understand the RF options even in the 900mhz range will not cut the mustard.

Indeed. Those frequencies will get you a range of millimetres, or centimetres at best. You'll have to go for much, much lower frequencies - in the kHz range - to stand a chance. The big problem here is the size of the antennas, and the very low data rates.

Additionally it appears that acoustic methods and laser have expensive patents wrapped over them.

That must be for very specific applications.

Laser is anyway not likely to work for 20-50 meter distances underwater. In seawater it may work, it can get that clear, but fresh water is usually much more turbid due to algae and silt.

Whilst running a seperate wire underwater is something I want to stand clear of for this project, there is a anchor chain that I think may be of some use as it naturally travels from the boat (where I want to receive the data) to the transmitter at the bottom.

You ALWAYS need at least two wires: one signal and one ground is the bare minimum to create a circuit. The anchor chain should work well as ground connection, especially in salt water which is an excellent conductor by itself, but you'd need to run a separate wire for the data. You may be able to attach this to the anchor chain.

Added complication: if running DC you may get electrolyses and corrosion along the chain, so you'd have to run AC, and that of at least 3 kHz frequency. You would be able to create an AC signal by connecting a capacitor to a PWM output of the Arduino. It doesn't have to be a nice sine or so, as long as you have current go in both directions. Your chain is likely to not be the only current carrier (contact between shackles will be poor at places due to stuff growing on it), with the seawater providing a second route for current to flow.

So as you'll have to run a wire along the chain anyway, you better take a two-core wire for your one-wire communication.

brodiej:
Whilst higher data rate is best…

What data rate is that?

From what I could see there was no way to utiilise for example MaxSonar to transmit / receive rather than sense distance. Doesn't seem to be a 'listening' mode.

That is because that module is designed for distance sensing. It is quite easy to send data via an ultrasonic TX and RX but you need to either design a circuit to do this or find a module that is designed to do this and not confuse this with a distance sensor.

Underwater drone tether is available with 2 wires. Uses HomePlug adapters for high speed data. Just a thought in case you need Mbits/s speeds.

https://store.openrov.com/collections/2-8-replacement-parts