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I am looking for a device that has following properties:

  • Can transmit data underwater (sub 2.5GHz,1-2m below surface)
  • Very small (max. 15mmx15mmx2mm)
  • In-built microprocessor
  • Able to read sensors

So far I found the XBee, which is too big, and the Bluegiga BLE121LR Bluetooth® Smart Long Range Module, which uses Bluetooth. I read that Bluetooth is really unreliable underwater. Is that true even for only 1-2m? http://www.silabs.com/products/wireless/bluetooth/bluetooth-low-energy-modules/ble121lr-bluetooth-smart-long-range-module

I have little experience when space becomes the limiting factor, that's why I am asking here. Thanks!

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    \$\begingroup\$ Good luck getting 2.4GHz to transmit more than a few mm underwater.... 2.4KHz maybe... en.wikipedia.org/wiki/Communication_with_submarines \$\endgroup\$ – Trevor_G Mar 29 '17 at 20:13
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    \$\begingroup\$ ...or even ultrasound. Note that the OP never exclusively asked for "RF" transmitter :) Flight Data Recorders (colloquially: "black boxes") use a battery powered ultrasound ping to allow investigators to triangulate them. Hundreds of meters of usable range and a pretty decent bandwidth if you're crafty. Dolphins seem to think it's a pretty neat idea (unlike digital watches). \$\endgroup\$ – Wossname Mar 29 '17 at 21:20
  • \$\begingroup\$ Do you only need to transmit data, or do you also want to receive it? What is the maximum distance it will have to cover? \$\endgroup\$ – Bruce Abbott Mar 29 '17 at 22:26
  • \$\begingroup\$ Ok, thank you! You have been helpful. I think that I will transmit the data when the device is above water. \$\endgroup\$ – jbuchel Mar 30 '17 at 8:19
  • \$\begingroup\$ The only time I've ever transmitted 2.4GHz under water I was using a 4" steel-braided air hose as a leaky waveguide. It was marginally successful-ish. I had more luck in the 5.8Ghz range - something to do with better E and/or H modes for the diameter pipe I was using - but I'm not an 'RF guy' so most of what I learnt about that is long forgotten. \$\endgroup\$ – brhans Mar 30 '17 at 11:28
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Depends on the water.
Saltwater is very conductive, so it will hardly work at all at any frequency above VLF.
Freshwater is better and it may work out to many meters at a couple of MHz. 2.4GHz is out of scope I'd suggest.

This amateur radio paper may help you with some ideas.

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The ESP8266 comes in a variety sizes and includes a 2.4GHZ wifi radio capable of 802.11b/g/n. The ESP-02 for example is 14.2mm square. There are even smaller versions.

More info on wikipedia

I can't vouch for how performant it is underwater. An article I found here seems to support the notion that transmission of a 2.4GHz signal beyond about 15-20cm will be next to impossible.

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    \$\begingroup\$ You can't vouch for its performance under water 'cause it ain't got any. \$\endgroup\$ – JRE Mar 29 '17 at 20:29
  • \$\begingroup\$ @JRE, possibly not, hence my statement. The OP says he only needs it to work to 1-2m. If you can back up your statement with facts, then I'd be very interested in seeing them. \$\endgroup\$ – Octopus Mar 29 '17 at 20:32
  • \$\begingroup\$ @JRE, nvm, I found an article myself. \$\endgroup\$ – Octopus Mar 29 '17 at 20:43
  • \$\begingroup\$ And if it's salt water, expect an order of magnitude less range still. \$\endgroup\$ – Brian Drummond Mar 29 '17 at 21:00

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