Transmitting video from underwater

I'm amazed that a 200yds/20' range camera/under-water range exists at all! Water is a significant lossy dielectric to fm and above. I would simply give thanks that someone markets such a thing already, and not try to re-invent that!

 

Correct. But you're unlikely to transmit video on those frequencies, and you'll need to deploy a long antenna behind your submarine, fed with a lot of power to make up for its deficiencies in transmitting inefficiency.

LF and ULF transmissions are usually one way - to the subs. The message is usually some code for 'come to the surface, we need to chat over satellite comms'!!! ULF Bit-rates are in the tens of bits per minute, rather than the kB/s you'll need as a minimum.

Best option, as you don't really need the 'stealth' of military subs (presumably!) is to deploy a floating antenna that the sub drags along the surface with it.

 

...just like 'real' subs!! I think it is a good idea to stick with what is likely to work.

 

There are various antennas that might improve your gain that are readily available commercially for wi-fi systems (weather-proof, but you'd have to ensure water-proof), but one problem might be that you don't have a stable platform so if the antenna has higher directivity (in the horizontal plane) this might actually be to your disadvantage as the sub bounces around and you end up out of the gain lobe. I'd not like to predict the effect, nor of the influence on the water surface to the antenna gain. You're in uncharted waters! Try it! Let us know how it went!

 

Very little, I expect! Water is a good absorber of the lower, red frequencies of light. (Think of all those underwater documentaries - all blue-green tinge!). Yes, I'd stick a bright flashing blue LED on the side of it. If you are worried about losing it, personally I'd also rig it to trigger into becoming postively bouyant in the event that it receives no signal for a set period of time.

If you are worried about it being visible, then I begin to wonder about the application!! I think at least in the first instance, if not for always, I would tend to think there is no big issue with attaching a trailing line to it (something that floats - e.g. polypropylene rope), maybe with a floatation device at the end of it (big orange ball), and maybe even your antenna too!

Good luck!!...

 

leepower, eye wool rarely helps us to arrive at functional engineering decisions.

Electromagentic radiation (radio frequencies) does not pass very well through water. It gets attenuated (abosorbed) quickly. As cmb pointed out above, some colors (optical frequencies) do pass with little attenuation. Blue-green is the best.

To get some idea of the propagation limitations you face, see:

http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water

If you use a powerful blue-green laser, and if you modulate it with your video signal, and if you transmit it from a depth of 20 feet directly (line-of-sight) to your receiver, and if you de-modulate the video signal, then you may be able to meet your specifications.