The human body seems to be a good antenna to pick up 50Hz, at least touching a scope's probe shows a 50Hz waveform more than a volt in amplitude. Is it possible to construct an electronic antenna that does the same thing? (Wavelength at 50Hz is 6000km)
And if there's so much energy in it (1V in 1M$\Omega$ is 1$\mu$W), then why don't high-impedance pins pick this up?

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A human body is a really crappy antenna at 50 Hz. What you are seeing is capacitive pickup, nothing like a antenna picking up a self-propagating wave.

Yes, any hunk of conductive material the size of a human body will pick up 50 Hz about the same. Since it's all capacitive coupling, its the area that matters, not the bulk. A proper size sheet of aluminum foil would do just as well, probably better. Just about anything large and metal will pick up 50 Hz, assuming this is coming from your power line and this is indoors where power lines are all around in the walls, cords running to appliances, etc. A metal desk that is not connected to ground is a good example.

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