  Twisted electromagnetic waves, of which the helical phase front is called
orbital angular momentum (OAM), have been recently explored for quantum
information, high speed communication and radar detections. In this context,
generation of high purity waves carrying OAM is of great significance and
challenge from low frequency band to optical area. Here, a novel strategy of
mode combination method is proposed to generate twisted waves with arbitrary
order of OAM index. The higher order mode of a circular horn antenna is used to
generate the twisted waves with quite high purity. The proposed strategy is
verified with theoretical analysis, numerical simulation and experiments. A
circular horn antenna operating at millimeter wave band is designed,
fabricated, and measured. Two twisted waves with OAM index of l=+1 and l=-1
with a mode purity as high as 87% are obtained. Compared with the other OAM
antennas, the antenna proposed here owns a high antenna gain (over 12 dBi) and
wide operating bandwidth (over 15%). The high mode purity, high antenna gain
and wide operating band make the antenna suitable for the twisted-wave
applications, not only in the microwave and millimeter wave band, but also in
the terahertz band.
