Practically everywhere. Radio waves are produced simply when you pick up electrical charges and accelerate or brake them. As simple as that. Actually an antenna is a relatively easy thing, it's a piece of metal in which the electrons move from top to bottom, all this hustle makes this antenna emit radio waves.

Virtually wherever there are electrical charges and are moving there will be radio waves. An example could be a storm where many electrons move between the clouds and the ground and can be heard if a radio is turned on, a kind of creaking that occurs every time there is a lightning. These crunches are actually lightning that is happening in clouds that can be very far away and in fact you can study the storms using the radio at a distance, you don't need to be there inside.

Can you hear distant storms?

With a normal radio you can hear only nearby storms but if you have much better antennas you can hear storms even thousands of kilometers away, the radio waves bounce against the ground and the sky. The atmosphere has a layer that is above 100 kilometers called ionosphere.

In that layer, the Sun picks up electrons from the molecules of the air, so all that layer is filled with electrons, then when the radio waves get up there and they find these electrons try to move them but the electrons resist and expel them, returning them down. The ionosphere in practice is a kind of mirror for radio waves and radio waves can bounce in the ground and in the ionosphere traveling the Earth. This rebound effect makes it possible for us to listen to distant radio stations and is also useful for measuring radio waves of natural origin.