Range–angle-dependent OAM beamforming with a concentric helical circular FDA

Secure and spatially selective wireless transmission requires orbital angular momentum (OAM) beams that are confined to a specific range and angle, rather than propagating indefinitely along the beam axis. In this paper, a concentric helical circular frequency diverse array (CHCFDA) is proposed to generate range–angle-dependent OAM beams without requiring external phase shifters. The helical element positioning inherently provides the necessary interelement phase distribution through physical step height, while logarithmically increasing frequency offsets are applied across concentric rings—and optionally across individual elements—to eliminate range periodicity and achieve a single, well-focused OAM beam exclusively at the target location. Both linear and logarithmic frequency offset schemes are analyzed and compared. Numerical results demonstrate that the proposed logarithmic CHCFDA achieves a half-power beamwidth in range (HPBR) of (16.6 m)—approximately six times narrower than conventional circular FDA structures—while maintaining (100%) mode purity at the target range (r = 40 m) and an OAM leakage level of (0%). Compared to existing array configurations, the proposed structure provides superior range selectivity, effective suppression of undesired radiation outside the focal region, and stable OAM mode purity, offering a robust solution for security in OAM-based wireless communications. A Monte Carlo robustness analysis further confirms that the structure maintains the mode purity above 90% for positioning tolerances up to σp = 0.05λ and is completely insensitive to frequency synthesizer errors up to σf = 1%.