  It has been known for a century that electromagnetic fields can transport not
only energy and linear momentum but also angular momentum. However, it was not
until twenty years ago, with the discovery in laser optics of experimental
techniques for the generation, detection and manipulation of photons in
well-defined, pure orbital angular momentum (OAM) states, that twisted light
and its pertinent optical vorticity and phase singularities began to come into
widespread use in science and technology. We have now shown experimentally how
OAM and vorticity can be readily imparted onto radio beams. Our results extend
those of earlier experiments on angular momentum and vorticity in radio in that
we used a single antenna and reflector to directly generate twisted radio beams
and verified that their topological properties agree with theoretical
predictions. This opens the possibility to work with photon OAM at frequencies
low enough to allow the use of antennas and digital signal processing, thus
enabling software controlled experimentation also with first-order quantities,
and not only second (and higher) order quantities as in optics-type
experiments. Since the OAM state space is infinite, our findings provide new
tools for achieving high efficiency in radio communications and radar
technology.
