At what frequency does Alternating Current start to emit electromagnetic radiation?

Anything above zero. But since the principle radiation frequency will be at the AC frequency, there are practical limitations, since the wavelength is related to the frequency, and the efficiency of power radiated depends on the antenna.

 

11 hours ago, swansont said:

Anything above zero. But since the principle radiation frequency will be at the AC frequency, there are practical limitations, since the wavelength is related to the frequency, and the efficiency of power radiated depends on the antenna.

Can transmission lines act as an antenna for AC? 

35 minutes ago, John Harmonic said:

Can transmission lines act as an antenna for AC? 

Yes. You can run a loop of wire underneath and extract power from it (don’t do this, it is illegal). Or hold a fluorescent tube under it

1 hour ago, John Harmonic said:

Can transmission lines act as an antenna for AC? 

As Strange has noted, yes. This is why you tend to use wires twisted around each other for AC signals. In the far field you don't see a net current flow. The radiation intensity drops off more rapidly than if you had a single conductor.