Underwater communication trough cable

Question

I wanted to build a device which can stay and move underwater (let's say, for instance, 15m underwater).

Since RF frequencies are very attenuated by water and VLF antennas would be too high, I was thinking of using a cable which connects the device to an antenna which is on the surface. I have seen that a good idea is that of using an optical fiber, but I am totally unskilled with it (I think it will more suitable for me to choice something at RF frequencies or lower) and moreover I wanted to know if there were the possibility of finding something more flexible.

So, my questions are:

• which kind of cable do you think will be more suitable? For instance I think that a 15m coaxial cable at RF frequencies may attenuate a lot the signal.

• which voltage supply value do you advice?

• which frequencies will you choose for that application? In case of a coaxial cable, I'd say that it is good to use low frequencies, since the attenuation constant increase with frequency, but I do not know which is the best range. Moreover, I'd say that the signal, once arrived at the underwater device, should be amplified with a low noise amplifier, and since this kind of operations is difficult at high frequencies, I'd prefer not so high frequencies if it not necessary (but if it is necessary, no problem)

Obviusly it is a hobby, so my idea is that of choosing a compromise between price, flexibility and performances...

Answer 1

I'd go with the lowest frequency that still allows information transfer at the required speed. If you are planning to send live video footage then you might need to utilize pretty high frequencies. If you are just using the cable to control a few servos onboard the device you could probably get away with a much lower frequency, especially if you run parallel wires.

To the best of my knowledge the attenuation of the cable shouldn't be very affected by the water as long as it's water proofed, but someone could correct me on that if I'm wrong.

If you are not using very high frequencies regular 50 ohm RG174 cable should be fine. At 1GHz 15 meters it will do a little more than 15 dB of attenuation.

Answer 2

If you need 100 bits per second, on a 30,000 Hertz carrier with Differential QPSK to be self-synchronizing, you can do acoustic.

But why keep it simple, and just use a cable with data packets. Maybe a CAN bus (on shielded twisted pair).