Question

What source are radio waves emitted by?

Answer 1

Any accelerated charge produces EM waves.

Explanation:

Around a charge in motion there is an electromagnetic field but when the charge accelerates the field gets distorted and this distorting is the wave that propagates outwards.

As examples you can consider electrons moving up and down along an antenna (accelerating continuously) generating a radio wave or when electrons are decelerated abruptly as in X-ray generation.