Would it be possible to make radio controled subs > few feet depth? - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: September 22, 2018, 08:40:53 pm
content: How does radio work under water? I know for real subs the only way to communicate is ULF maybe VLF which results in short text no real time communications. But what if you wanted to make a hobby sub that goes 10 or 20 feet down? Like a radio controlled sub you could use off your boat or shore to look at things in a lake? I wish I knew my father better I knew he made a system for the navy subs that was digital and went over some insanely long wire towed behind the subs it had error correction and all kinds of stuff much more advanced then the previous teletex system that somehow could process much more data then thought possible.What frequency would work at different depths and how far away on land could you be? Im guessing that 99% of the signal power would be to travel the last few feet underwater so the transmitter would be close by on the shore. Make for an intersting range map. Would LW do the trick? The other problem would be getting any type of signal back. Maybe if you want two way you would have to tow a floating bouy with a HF/VHF whip on it. With the water around it would that work to your advantage as a ground plane how does salt water change that? Are some bands much better then others? Currently I like to design all kinds of things on paper, and research/do the math as best I can then when the time/resources come I like to select one and build it.I'm actually starting to make them into a library of those hardcover blank books you can buy at the craft store, one day I hope to have a book shelf full of inventions that people can borrow and try to build.

author: langwadt
author_rank: Super Contributor
timestamp: September 22, 2018, 09:33:38 pm
content: use ultrasound?

author: ejeffrey
author_rank: Super Contributor
timestamp: September 22, 2018, 09:53:54 pm
content: This page has a table:https://em.geosci.xyz/content/maxwell1_fundamentals/harmonic_planewaves_homogeneous/skindepth.htmlLooks like 1 kHz probably works (skin depth 9 m), 1 MHz probably doesn't (skin depth .25 m).

author: Marco
author_rank: Super Contributor
timestamp: September 23, 2018, 12:00:02 am
content: Looking at thelossesit should be possible to get a signal to/back from it in the bps range ... but there doesn't seem a point if you want to receive images from it. Laser comms seem the most viable for high bandwidth.

author: Beamin
author_rank: Super Contributor
timestamp: September 23, 2018, 03:56:43 pm
content: In sea water at 1khz depth is 8.76 m at 1mhz depth is 0.277 m. What's the smallest band width for a video signal or highly compressed digital signal? analog TV uses way too much bandwidth maybe 0.5 to 1mhz? It says 8.76m at 1kz but signals may only be useable to a fraction of that. Its interesting the rock isn't a huge blocker of radio even into the ghz at 111m for igneous rock. That's how ground penetrating radar must work.

author: dmills
author_rank: Super Contributor
timestamp: September 23, 2018, 05:02:47 pm
content: Wide bandwidth link to a ROV? Use fibre, cheap and you could basically use a network switch on the vehicle and another on the surface...Need no wires? Acoustic links are the way to go, or blue light free space optical, neither is going to easily give you realtime video bandwidths, but SSTV may be practical.Regards, Dan.

author: iMo
author_rank: Super Contributor
timestamp: September 23, 2018, 05:36:57 pm
content: Skin depth in sea water does not mean your signal is fully lost. It says the signal is 0.37 of the original.When your RxTx system can handle say -xxxdB signals you may go _much_ deeper, imho..

author: ogden
author_rank: Super Contributor
timestamp: September 23, 2018, 05:42:55 pm
content: Quote from: Beamin on September 23, 2018, 03:56:43 pmAs you don't specify resolution, frame rate and color depth, shall we assume it is 8x8 pixels, 1fps and 4 bit monochrome "video"?

author: Lord of nothing
author_rank: Super Contributor
timestamp: September 23, 2018, 06:47:07 pm
content: Depend who big your feet is.

author: SeanB
author_rank: Super Contributor
timestamp: September 24, 2018, 12:09:18 pm
content: Hobby sub you can trail a float and have an antenna on it. Flexible plastic coated steel cable to a fishing float, a constant tension spring on the submarine model and then a short whip antenna will get you the depth you want and still give somewhat usable communications, using standard radio control electronics. Just accept you have a horrid path loss, but will work, and if you use old VHF TV channel 2 as transmission band you can do standard NTSC video as well, and have short antenna length with off the shelf receiver and reasonable size antenna.10m of steel plastic coated cable acts like a coax cable ( lossy though it is, it will still work though you might have 10dB attenuation on it) in water, and will at least work providing you use a long range transmitter ( higher power) on the sending side, and a long range receiver (lower noise, better data recovery and much larger AGC range) on board, and the bands are far enough apart that a reasonable compromise on separation of the 2 can be made with low cost components in a shielded RF combiner on the boat, and shore side you just need a tripod for the TV antenna and use a broad beam mesh VHF antenna to get the coverage.

author: coppercone2
author_rank: Super Contributor
timestamp: September 24, 2018, 03:30:29 pm
content: lol, are there rules about underwater transmitters, legally speaking?Like if you sink a high power transmitter into a lake, how do you quantify the effective radiated limit based on depth and frequency?Say you want to operate a sub down to 100 feet, what if you put a powerful transmitter like 50 feet under water, so it effectively does not radiate much energy in any direction?

author: Lord of nothing
author_rank: Super Contributor
timestamp: September 24, 2018, 03:38:13 pm
content: how big is your feet?

author: LaserSteve
author_rank: Super Contributor
timestamp: September 25, 2018, 07:31:54 pm
content: DIVE! DIVE! DIVE!see:andhttps://youtu.be/KSXDZ2KzNOg?t=234Looks like local RF field goes down at least a meter or so to me..Steve

author: Beamin
author_rank: Super Contributor
timestamp: September 25, 2018, 10:39:55 pm
content: Quote from: LaserSteve on September 25, 2018, 07:31:54 pmI wonder how that black sub surfaces. Maybe it has a CO2 cartridge that it inflates into a tank. I can't see it using little air compressors to fill the tanks. Problem with that hobby is you can't see it.

author: coppercone2
author_rank: Super Contributor
timestamp: September 25, 2018, 11:04:06 pm
content: why cant you just have a ballast tank with waterbut, why don't you send a friendly email to the CIA?https://www.cia.gov/library/video-center/video-transcripts/charlie-cias-robotic-fish.html