Millimeter-Wave Dual-Mode Low Divergence Angle OAM Antennas With High Mode Purity in Wide Bandwidth

This communication introduces a new design of millimeter-wave (mmWave) dual-mode, low divergence angle orbital angular momentum (OAM) antennas with high mode purity over a wide bandwidth. Conventional feeding networks for OAM antennas rely on varying transmission line lengths realizing phase shift, which struggles to maintain high mode purity over a wide bandwidth. Therefore, an innovative feeding network scheme is proposed, utilizing a wideband 90° cascaded hybrid coupler and reversible feeding direction of the microstrip-to-slot transition, achieving wideband 90° and 180° phase shifts, respectively, then ensuring wideband high mode purity. Then, ±1 and ±2 mode OAM antennas are designed, where the proposed feeding network excites a lens-loaded uniform circular array (UCA). To ensure high E-field rotational symmetry and thus high OAM mode purity, the aperture of the ±1 mode UCA is shaped by loading an overmoded circular waveguide with notches. Measurement shows that the proposed OAM antennas support two simultaneous modes, with mode purity exceeding 90% and divergence angles of about ±4°/±6° from 25 to 28 GHz (11.3%). Compared to other OAM antennas, these designs excel in terms of high mode purity over a wide bandwidth in the mmWave band, making them ideal for mmWave OAM measurements and wireless communication systems.