# Antenna for contactless 50/60Hz AC detector

Many simple circuits can be found online for building an AC detector that look something like the circuit below. These are simple devices to e.g detect AC lines in your walls. They feature a short piece of wire (10-20cm) wrapped in a coil of about 5-10mm. I wondered, is there a better design for an antenna for such a circuit, hand how would you go about designing one?

So far I had the impression that the antennas for a specific frequency are usually of the size in the order of their wave length (or like 1/2 or 1/4 the wavelength), but for 50Hz or 60Hz this is definitely not feasible if you want to end up with such a compact device.

simulate this circuit – Schematic created using CircuitLab

• Note that the power source is a floating 9V battery. It likely helps to hold the device via the metal battery case. It's "antenna" is a voltage-probe...sensitive to electric field via capacitive-coupling. Not at all sensitive to magnetic field. Calling the probe an antenna is a bit misleading. Oct 31, 2021 at 15:28

It's not an antenna, as you say, it's a capacitor that couples to the house wiring.

You could increase the capacitance and hence the sensitivity or range of the circuit by using a flat metal disk, about 30 mm diameter.

Increased sensitivity might not be what you want though - indoors you're already surrounded by mains frequency signals, a sensitive circuit will pick them up from 10 or 100 m away. What you want is discrimination, that the electric field strength exceeds some value that means you're within 20 mm of a wire. For that, a narrow pointy capacitor might be better than a big plate.

Remember the "ground" of the circuit is the capacitance from the battery to your hand, and your body being away from the electric field, is at a lower potential.

Coupling you finger to the base on a pad or foil into a plastic plug to insert makes an the e-field antenna by capacitance coupling into a FET buffer.

Any conductor will do as well to detect uV/cm to mV/cm E fields.

Touch a 10Meg probe and get 60Vpp from 120Vrms 60 Hz. Go near ground and it reduces & touching ground suppresses it.

• There are no FETs in this circuit. Dec 20, 2023 at 17:09