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How accurately does the signal strength measurement have to be? Why are you trying to measure signal strength? Do you realize that Bluetooth and wifi devices have automatic gain control snd click up power if the base station sends a notice that signal is week? Also, walls, humans, weather changes all cause changes to received signal strength. Other wifi devices cause signal strength to be boosted as well. I'll need more detailed description of your goal before I can add more...

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Why? what's the goal.

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Why not a low power accelerometer or pressure sensor. A pressure sensor can show a delta each step up a staircase, so much more sensitive in water. You can also have a small antenna wrapping up the line to make sure you maintain range when the bobber goes under.

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