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Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Plasma wrote ...
Not radio waves, but 1.5volt ac can travel 1.5 meters in a bath tub, with the electrodes separated 300 mm ,50mV picked up at the opposite end of the tub.
You said UHF. Which is necessary for the bandwidth required by video. Lets say 500MHz. In seawater the wavelength will be around 7cm. You want to transmit 100m. Thats well into the far-field regime (>4 wavelengths). That is most definitely radio. The nearest estimate for RF attenuation in seawater I can find is roughly 20dB per m for those sorts of frequencies. So we're looking at something on the order of 2000dB of path attenuation. And thats not taking into account attenuation from the inverse square law. We can probably get a radio receiver to work for video at a sensitivity of -80dBm. So we'd need to transmit 1920dBm of power to account for the path loss. Thats 1x10^189 W.
Now, with all that said, RF communication under sea is possible, but only at VLF (under 30kHz). But the data rate is very low.
Your bath tub AC (50Hz/60Hz?) is a very different scenario. You're in fresh water and your wave length is ~600km so you are working in a near field regime, where you can detect the E-field directly.
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268
OK, yep I only tested with DC and 50hz AC, didn't think about increased frequency. Pulsating DC at 500Mhz have the same problems equal lly comparable to AC?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I have zero experience with underwater ROV but;
I think that you will need to spend the extra on suitable connectors for the ROV cable because the ROV would otherwise be permanently attached to the cable and you will have all kinds of transport and handling problems, the ROV and cable reel/spool/drum would be permanently attached. OK for prototypes and experimenting but a real pita for regular use.
Choice of cable and connector will of course determine ROV capabilities; e.g. the cable pointed to has four twisted pairs of 0.125 sq.mm if one pair is for uplink and one for downlink you could use one pair for 0v and one for +300V dc, (or even 230 Vac 50Hz) 2x 0.125 = 0.25 sq.mm ... say 1Adc = 300W available for a dc:dc converter.(or transformer) So this configuration would give one uplink pair, one downlink pair, and 300Wdc for lights, motors, cameras and electronics. (or 150Wdc, two twisted pairs for data and one twisted pair for video etc.)
If really desparate for power and/or data you could use two pairs for +dc and two for -dc via inductors, for 600Wdc, and all four pairs for data via capacitors or signal transformers.
You could get more power by running at greater than 4A/mm2 as the cable is water-cooled, but any cable not under water, or worse, on the reel, would overheat.
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Long distance video transfer over wire
