A simple and concise analysis of twisted radio waves

The use of spin and orbital angular momentum has come to the fore in optics and in atomic and molecular physics. Applications for the use of the orbital angular momentum of radio waves have been proposed, including information-rich radio astronomy and novel wireless communication concepts. In the literature different views can be found concerning the possibility of separating the angular momentum. Here I derive analytic expressions for the angular momentum density in terms of helicity (circular polarisation) and vorticity (phase singularities). I find that the angular momentum can be clearly separated into spin and orbital angular momentum, and I demonstrate that the angular momentum of radio waves is transported all the way out to infinity; a fact that is not always well appreciated.