To realize the electrical beam scanning of the phased-array radar, it is necessary to implement true-time delay compensation for the transmitted and received signals in each transmission channel according to the beam direction. Utilizing microwave photonic technology, the microwave signal can be modulated to the optical carrier for transmission and processing. Compared to the frequency of the laser carrier, the relative bandwidth of the microwave signal is extremely narrow. The true-time delay lines based on optical fibers or on-chip waveguides have a large microwave bandwidth, small in-band amplitude and phase fluctuations, and small propagating loss and are immune to electromagnetic interference. The increment of optical lengths of different delay states can be precisely controlled to be far less than a full microwave wavelength. Thus, broadband beamforming can be realized using only subwavelength stepped optical delay lines, which can greatly reduce the beam directional dispersion of broadband microwave signals. The discrete delay values of stepped delay lines lead to discrete directional directions of antenna beams, which results in a certain deviation between the actual and designed directions of the beam. The influence of the minimum delay change on the equivalent phase distribution of the microwave front is analyzed, and a theoretical model of the relationship between delay steps and directional deviations of radar beams is established. The theoretical analysis of beam scanning based on subwavelength stepped optical delay lines shows that the beam directional deviation is proportional to the minimum delay step and inversely proportional to the array element spacing, the square of the number of elements, and the cosine of the beam direction. Through numerical simulation for the X-band wideband radar, the beam direction at each frequency point is almost the same in the frequency range of 8~12 GHz, which indicates that the directional dispersion has been effectively suppressed. It can be observed that some singularities will appear at specific azimuths and delay steps, where the directional deviation will reach extreme values due to the discrete increment of delay. The azimuthal deviation of the singularity gradually increases as the azimuth and the delay step increase. When the delay step is not larger than 3 ps, the azimuthal deviation does not exceed ±0.13°, which is less than ±1/35 of the narrowest beam width and thus can be almost neglected. Peakpower loss and sidelobe suppression are also simulated. When the delay step is less than 3 ps, the beam peak power drops no more than 0.051 dB, and the in-band fluctuation is less than 0.028 dB. The maximum broadband relative sidelobe power is less than -12.5 dB, and the maximum fluctuation is less than 0.24 dB. Based on the scheme of a subwavelength stepped optical delay line without an electrical phase shifter, 9-bit optical delay lines with a minimum delay step of 3 ps are prepared, and the maximum delay exceeds 1.53 ns. The optical delay lines are manufactured by cascaded 2×2 magneto-optical switches and optical fibers. The distributions of the spatial electric field are measured on the nearfield platform and converted to farfield patterns by spherical wave compensation at different designed azimuths and frequencies. When the designed beams point at 0°, 30° and 60°, the measured maximum directional deviations are 0.24°, 0.28° and 0.77°, and the in-band directional dispersions are 0.21°, 0.28° and 0.98°, respectively. Compared to the beam squint based on the scheme of the wavelength stepped optical delay line and electrical phase shifter, the beam directional dispersion is effectively suppressed. Furthermore, under the microwave frequency range of 8~12 GHz and the azimuthal scanning range of ±60°, the experimental results demonstrate that the peak power loss can be reduced to less than 0.89 dB and that the sidelobe suppression ratio can exceed 11.06.