This manuscript proposes the design of a 28 GHz antenna system that is capable of generating at the same time five orbital angular momentum (OAM) modes with aligned far-field antenna patterns. The antenna system includes a radial line slot antenna (RLSA), based on radial waveguide technology, which is fed by two Butler networks: one is $8 \times 8$ and the other is $16 \times 16$ . The Butler matrix networks are able to generate various progressive phase states for the RLSA to enable simultaneous generation of five concentric OAM beams with right hand circular polarization (RHCP). The proposed design could be utilized for increasing spectral efficiency by multiplexing a separate data stream onto each transmitted OAM beam and performing partial-aperture OAM reception in line-of-sight (LOS) communications in 6G. The experimental results performed on the antenna prototype reveal the effective performance of the proposed antenna system.
