Vortex Electromagnetic Waves Generated by Using a Laddered Spiral Phase Plate and a Microstrip Antenna

A laddered spiral phase plate excited by a microwave antenna to generate a vortex electromagnetic wave is studied. By replacing the continuous spiral phase plate with several planar metal plates, a ladder spiral phase plate that is easily built is designed. When the spiral phase plate is slightly tilted away from the xoy-plane to approximate the parabola plate, and it is further excited by a microstrip patch antenna, different orbital angular momentums are obtained. The presented scheme provides a convenient and simple approach to generate a vortex wave at microwave frequencies.