Underwater RF communication

I am working on a transmission of data underwater by using XBee pro s1 (2.4 GHz). By this, I got the result 0.17-meter distance between two nodes. But I need to enhance the distance more than 2 meters. So what can I do? Either change the RF module to low-frequency module or use low pass/bandpass filters or anything else?

Does it have to be RF?
What data rate do you require?

A little bit of googling suggests that the higher the frequency, the less range you'll get. Hence the VLF systems used to communicate with submarines. Try a lower frequency.

Acoustic modems are a possible alternative.

People have reported reception of up to a couple of meters in FRESH water using 27 MHz RC radios.

This is what I do for a living (kind of - underwater geophysics, positioning, survey etc).

Underwater comms of more than "arms reach" using RF is, essentially, not possible with sensible sized equipment. Drop your frequency down, way down. Bigger wavelength is the key, but big wavelength cones with big antennas. But the lower the frequency the better luck you will have, more power works to a point, but more important is a really, really sharp receiver and signal to noise ratio.

But really, ultrasonic is the way to go. You can get data transmission for hundreds of meters with the correct ultrasonic transceivers. Light isn't a silly idea either depending on distance and data rates.


I think this shows that visible wavelengths, around the green/blue spectrum, are by far the least affected by absorption in liquid water. At first it seems a coincidence that this dip coincides with visible wavelengths. But then if you think that the eyes of animals originally evolved in the oceans, and our eyeballs are filled with a liquid which is mostly water, then it suddenly seems much less of a coincidence!

How do whales communicate? at what frequency?

robtillaart:
How do whales communicate? at what frequency?

Species dependent. The ranges are huge, and it depends what the whales are "doing" (hunting, navigating, communicating, mating etc). Also, it's the wrong way of thinking, lower frequency = lower data rate. it's fine for language between animals, but with computers it gets harder because you either have to modulate the frequency or the amplitude with audio to sent data and extract the envelope. If you are using audio that low down, the data rates would be very very slow. Ultrasonic has more sensible rates, but lower range. Since we aren't talking about sending data over an ocean here (I don't think?), we don't need to think that big.

PaulRB:
\I think this shows that visible wavelengths, around the green/blue spectrum, are by far the least affected by absorption in liquid water. At first it seems a coincidence that this dip coincides with visible wavelengths. But then if you think that the eyes of animals originally evolved in the oceans, and our eyeballs are filled with a liquid which is mostly water, then it suddenly seems much less of a coincidence!

I think this is mostly coincidence, as visible light is also the radiation that best penetrates the atmosphere. UV is filtered by the ozone layer; most above that by the earth's magnetic field; and lower frequencies (IR) don't have much energy so are much harder to detect by chemical reactions (which is what eyes - and photosynthesis - do). UV and up also get so energetic that it quickly becomes destructive, tearing chemical bonds apart.

Earth's magnetic field has no effect on light.
We get plenty of UV on Earth's surface.

Molecules absorb light frequencies by their size and their outer electron's ability to absorb the photon by moving to a higher state. Since the electron states are fixed only certain photons can be absorbed.

Water is very good at absorbing microwaves for instance but not so good at absorbing blue light.

jremington:
People have reported reception of up to a couple of meters in FRESH water using 27 MHz RC radios.

Thanks.Get it I want to know is 27 MHz RC Radio compatible with Arduino?

I reckon if i was playing with this, i’d use something like a sonalert or high powered piezo transducer and use some simple low-rate ‘chirp’ modulation.

The first question for OP is what data rate do you need to get across.
Range is already known... >2m

This will definitely require some checksummng, and packet repetition / handshaking to minimise transmission errors.

Nice challenge.

Radio waves don't transmit well through water. Water can also get extremely murky so light waves are out. I agree with @lastchangename that sound waves are the best option.

Even with sound waves there may be issues with thermoclines and haloclines. Sounds waves may bounce off, rather than pass through, water layers that have different temperatures or salinity. These boundaries can be very sharp and often happen near where rivers meet the sea.

I would try speaker, microphone, audio amplifier and dtmf decoder module. Ok, it's just 20 baud enough to transmit control for rudder, elevator and motor power levels and receive back depth and magnetic heading every couple seconds. Underwater movement is sluggish anyway.

Water is about 1000x as dense as air.
Speed of sound underwater is about 4x as high as in sea level air, 1498 m/s as opposed to 343 m/s..
Sound underwater is that many times as solid as sound in air.

Sluggish? It's the thin stuff that sound moves slow in.
It's like air is 1.1V as compared to water is 24V. Voltage in wire is a kind of force density.
Which will transmit farther on wires?