Mechanical properties of the radio frequency field emitted by an antenna   array
  Angular momentum densities of electromagnetic beams are connected to helicity (circular polarization) and topological charge (azimuthal phase shift and vorticity). Computing the electromagnetic fields emitted by a circular antenna array, analytic expressions are found for the densities of energy, linear and angular momentum in terms of helicity and vorticity. It is found that the angular momentum density can be separated into spin and orbital parts, a result that is known to be true in a beam geometry. The results are of importance for information-rich radio astronomy and space physics as well as novel radio, radar, and wireless communication concepts. 
H. Then and B. Thid\'e