50 Hz antenna for monitoring mains frequency fluctuations
The only worthwhile component of the EM wave created by AC power is the magnetic component and this is really easy to pick-up. Use a coil of wire - maybe ten turns or more with a diameter of say 10cm and then an op-amp amplifier configured as a sallen key low pass filter set at 50Hz. For added high-frequency noise immunity, put a 10uF non-polarized capacitor across the coil. If you added more turns you might even be able to get it to tune at 50Hz (or 60Hz) but you'll need an inductance of about 10 henry to do so but, using ferrite isn't out of the question.

You should be able to pick up ac power at a few centimetres.