Hello Guys, do you know any low-frequency radio transceiver modules that could be used under water? A wireless link of around 1m would be enough. 2.4GHz modules are not an option due to the high frequency, 868MHz for LoRa and GSM is still pretty high. Maybe there are modules available for 433MHz or preferrably even lower.
Thank you!
Would it work to use a float carrying an antenna and a link from Your sub up to the float?
ELF or VLF (3 to 50 kHz), at very high power, with a very long or large antenna is used to transmit to submerged military subs.
Wikipedia says the lowest hobby bands - typically around 27MHz/40MHz - can penetrate several feet of water at short distances. Radio-controlled submarine - Wikipedia Look for 27 MHz RC controllers. Check whether legal where you live.
You could also try comm via sonar. Or blue-green laser. 
DaveEvans:
ELF or VLF (3 to 50 kHz), at very high power, with a very long or large antenna is used to transmit to submerged military subs.
Where bandwidths are so low that the best they can do is send a few bits per second, often used to just tell the sub "I have a message for you, please stick your antenna above the waves so I can send it to you!"
Maybe try air bubbles to a surface relay station? Joking!
Hi,
There are some very cheap and simple 433MHz tx/rx units around, it may be worth the couple of dollars to get a pair and experiment.
Tom....
Railroader:
Maybe try air bubbles to a surface relay station? Joking!
The underwater version of smoke signals
Hi Guys,
Thank you for your helpful comments! 433Mhz modules are widely available, 27MHz transceivers require large antennas to transfer over a reasonable distance. I think radio wave propagation and water doesn't like each other. Maybe there are other options like using light or ultrasonic.
However, I haven't found any ultrasonic modules.
Thank you!
What sort of data rate are you looking to achieve?
FWIW, scuba tank pressure transmitters achieve wireless connections between first-stage (usually somewhere behind the diver's head) and a wrist-worn computer, albeit at very low rates. (And, in my experience, not always reliably 
)
Any year now we'll have entangled photon communication... and cold fusion to power it.
Radio waves don't travel well through water indeed. The lower the frequency the better, that's why those extremely low frequencies are used with submarines. In case of 433 MHz you may get a few mm in water.
Ultrasound is possible - look for sonar type systems. Takes quite a bit of power, but they can come pretty far. Data rate remains low of course, and both ends must be submerged. The sound doesn't cross the water/air barrier.
Light is also possible - as long as the water is clear enough, of course. That has the potential for the highest data rate, but in a typical river you're not getting anywhere with light as the water is too murky. In the open ocean you have better chance. Again both sides will have to be under water for reliable communication. The surface of the water will scatter quite a bit of the light, in both directions.
Sound moves well and quickly in water. Suppose a submersible loudspeeker and a surface floating microphone and frequency shift?
It's hard to imagine that these 27 MHz antennas are "too long," but then the OP hasn't quantified the project requirements and limitations...
Couple of problems here. Water is a conductive medium. Air is not. Also the speed of light(propagation of radio waves) is much slower in water than air. Antennas don't work well in a conductive medium. At a minimum, the antenna has to be insulated from the water. Then an antenna cut for use in air must be a lot longer for the same frequency if used under water.
So, if a CB antenna in air is too long, it will definitely be too long under water.
Paul
What about induction?
Cavers use Molefones, which uses induction, IIRC.
Maybe adapt equipment used for induction loop audio systems.
More from the link in reply #2:
the lowest hobby bands - around 27MHz/40MHz - can penetrate several feet of water at short distances - typically less than 45 meters. Penetration at these frequencies is better in fresh water - a lake or swimming pool, and difficult to impossible in sea water...
For underwater radio to work, even at these frequencies, the receiving aerial needs to be completely insulated from the surrounding water....depending on water conditions, positive control can be maintained at perhaps 3 meters depth
There are several 27MHz subs for sale on Amazon...without huge antennas...
Have a look at underwater drones. There are a couple available with wireless controls. The specifications will give you an idea what frequencies can be used and what distance you can get. There are quite a couple that use cables, so wireless is probably not easy to do reliably.
If swimming pool distance and simple control is good enough, the cheapest way (from $30) might be to get a RC Submarine Toy from Amazon and extract the control electronics.
TheMemberFormerlyKnownAsAWOL:
What about induction?
Cavers use Molefones, which uses induction, IIRC.
Never heard of those, interesting.
A quick Google search got me to a.o. this article which talks about the molephone and a related design, the ogophone. The first is secret, the other supposedly available. I didn't look into it.
They mention it uses VLF for communication (like submarines), and an antenna made out of a 64-turn coil (like an inductor) at 1m diameter (can't call that "small", though it is for a VLF antenna). There is also mention of the receiver and submitter having to be right above one another, suggesting it's highly directional.
This sounds a bit like a cross-over between inductive coupling and regular radiowaves which I remember is a kind of capacitive coupling between transmitter and receiver. All in all an interesting technique, but obviously very hard to do tesitng unless you have easy access to a deep cave system.
There is mention of mineral veins severely disrupting the signal - interesting as minerals are generally not conductive, so don't know what's going on there.
Whether it's a solution for the OP's robot is hard to say. Gonna take a lot of research to build the thing, and even so it may only work in fresh water, not the far more conductive sea water.
mbobinger:
Hello Guys, do you know any low-frequency radio transceiver modules that could be used under water?
If you figure this out, the U.S. Navy will pay you millions for the technology.