Fig. 1. Mathematical model of the circular loop current of I0e−jlϕ. I0 represents the constant magnitude of either an electric current or a magnetic current.

Fig. 2. Evolution of the polarization states for the twisted radio waves with increasing radius of θ ranging from 0° to 90°. Reprinted with permission from [20]. All rights reserved.

Fig. 3. Link budgets of |l|=1, 2, 3, 4, and 5, respectively. D∈[10λ,100000λ]; f=10  GHz; and a=5λ.

Fig. 4. (a) Radiator with a ring-slot loaded on its narrow wall. The angle between two feeding ports (EPA and EPB) is γ. The outer radius and the inner radius of the radiator are a and b, respectively. (b) Explosion view of the metal resonator based parabolic antenna. Reprinted with permission from [9]. All rights reserved.

Fig. 5. (a) Simulated four-OAM-antenna model. (b) Partially enlarged sectional view of the dual concentrically stacked radiators where P1,2 and P3,4 are two pairs of feeding ports for generating l=±2 and l=±3, respectively, and the angle, γ, between the two feeding ports is 135° and 90° for l=±2 and l=±3, respectively.

Fig. 6. S-parameters of the four-OAM-mode antenna. Sii in (a) and Sji in (b) indicate the reflection and isolation, respectively.

Fig. 7. Simulated near-field radiation patterns for OAM modes of l=−2,+2,−3, and +3, respectively, from left to right. (a)–(d) Simulated transverse phase patterns. (e)–(h) Simulated transverse intensity patterns.

Fig. 8. Normalized radiation pattern of l=+5 as a function of different outer radius of a (mm) at an operating frequency of 10 GHz.

Fig. 9. Schematics (cross section of x−z plane) of the two cavities. (a) Air-filled cavity. (b) Dielectric-filled cavity.

Fig. 10. Far-field radiation patterns of the two radiators. (a) Air-filled radiator. (b) Dielectric-filled radiator.

Fig. 11. Electric fields (real part) of the generated vortices waves with OAM mode of l=+5 at x−y plane. (a) Air-filled radiator. (b) Dielectric-filled radiator.