Abstract: To address the robustness degradation caused by eigenbeam nulls in modal beamformers, this paper proposes a null-free modal beamforming method based on an arc array. The method constructs a set of null-free arc eigenbeams and proves that they form a complete and orthogonal basis, thereby establishing a theoretical framework for modal beamforming in the arc harmonic domain. Within this framework, the design methodologies for delay-and-sum, minimum variance distortionless response, phase mode, and multi-constraint beamformers are systematically derived. Simulation and lake trial results demonstrate that the proposed method effectively eliminates eigenbeam nulls, maintains high robustness under broadband conditions, and overcomes the bandwidth limitation imposed by the truncation order in conventional circular arrays. Notably, when the arc array spans a semicircle, the applicable bandwidth can be extended up to four times that of the circular array.