More unusual features of the microwave vortex

Abstract

The electromagnetic vortex was reported by R.A. Speciale [1]-[2]. The solution of the vector wave equation for the free space propagation of unguided, collimated Bessel beams is reexamined and new features are described. First, the set of modes is doubly indexed, nondenumerable, and overcomplete. Second, the phenomenon of mode cutoff frequency is absent. Third, axial phase velocity is superluminal, i.e. exceeds c, while energy or group velocity is subluminal, i.e. less than c. Yet unlike waveguide modes, the geometric mean of the phase and group velocities does not equal c. Einstein's constant c loses significance as "the" speed of light, and instead is an upper bound on the speed of light in free space. To the two known methods for slowing light -media and guiding structures -must be added a third, orbital angular momentum (OAM). In view of these results, certain statements in classical books on electromagnetics require emendation.

Target scattering characteristics for OAM-based radar

“…For comparison, we define the concept of OAM-based radar cross section (ORCS) for OAM-based radar, in this paper. Currently, several open articles have discussed the unique features of the vortex EM waves, [20][21][22] e.g., the phase velocity in free space is greater than c and the energy velocity is less than c (c is the light speed in the vacuum). The properties of the EM scattering of a Bessel vortex beam by the water sphere were analyzed in Ref.…”

“…To illustrate the operating range of the OAM-based radar as a function of the radar characteristics (especially the ORCS), the propagation characteristics of the vortex EM waves should be analyzed and the OAM-based radar equation should be defined. In the cylindrical coordinate (ρ, φ, z), for the TE case and wave traveling in the z direction, the analytical expressions of the vortex EM field can be given by 21 FIG. 1.…”