Abstract-This paper presented a novel unconditional stable FDTD (US-FDTD) algorithm for solving the transient response of uniform or nonuniform multiconductor transmission line with arbitrary coupling status. Analytical proof of unconditional stability and detailed analysis of numerical dispersion are presented. The precise split-time-step scheme has been introduced to eliminate the restriction of the CourantFriedrich-Levy (CFL) condition. Compared to the conventional US-FDTD methods, the proposed approach generally achieves lower phase velocity error for coarse temporal resolution. So larger time scales can be chosen for the transient simulation to achieve accurate results efficiently. Several examples of coupled uniform and nonuniform lines are presented to demonstrate the accuracy, stability, and efficiency of the proposed model.

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A high-efficiency reconfigurable element for dynamic metasurface antennas (DMA), consisting of a complimentary electric-LC (CELC) resonator fed through the cavity from a waveguide slot, is proposed in this paper, with positive-intrinsic-negative (PIN) diodes loaded to realize the coding performance. To begin with, the low efficiency problem of a traditional CELC element is demonstrated before the introduction of the proposed element. Subsequently, an equivalent circuit is illustrated to analyse the resonant performance, with extensive parametric study for verification. The proposed element has a low ohmic loss when the diode is switched on, 25 times less than the traditional element. Furthermore, the radiation efficiency of a DMA with 10 proposed elements is compared with a traditional design, showing a significant efficiency improvement from 25% to 75% when half diodes are switched on. Finally, measurements are conducted to verify the feasibility of the dynamic element to achieve radiation ON and OFF states, showing a 12 dB radiation gain difference between two states. INDEX TERMS Dynamic metasurface antenna, high efficiency, metasurface, PIN diode, reconfigurable.

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