In order to locate the faint sources in the lightning discharge, an improved broadband VHF observation system is developed. Both the system configuration and locating algorithm are specially designed to locate weak radiation sources from background noise. The proposed system consists of seven broadband omnidirectional antennas in “L”shape and runs in continuous acquisition mode. An electromagnetic time reversal method designed in frequency domain is applied to locate the radiation sources. Two metrics have been introduced to distinguish between actual lightning sources and noise event. Numerical simulations and field calibration are carried out to test the system's performance in locating weak sources. Moreover, the performance on spatial resolution, robustness, and the ability of multi-source localization are investigated. Finally, this system is applied to map the propagation of the upward positive leader (UPL) from a classical triggered lightning with time resolution 0.128 μs. The results prove that the proposed system could well depict the whole progression of UPL until its extinction, and the advantages of the proposed seven antennas array in retrieving weak radiations are presented in detail.

In this paper, the conventional rigorous coupled-wave analysis (RCWA) method is improved by using the Cayley-Hamilton theorem. Using the proposed method, the forward and backward diffraction efficiencies of each order are solved directly from the eigenmatrix. At the same time, the calculation results of the conventional method, the proposed method, and the CST simulations are in good agreement with each other. The results show that the proposed method has obvious advantages in computational efficiency.

Based on the first-order Taylor expansion, an efficient Rigorous Coupled-Wave Analysis (RCWA) for one-dimensional ultrathin periodic structures is proposed in this paper. The derivation of the ultrathin form RCWA method is completed by using the first-order Taylor expansion to rearrange the matrix in the equations of boundary conditions. Then, the reliability of the proposed algorithm is verified by two examples. Finally, it is concluded that the proposed algorithm can reduce the CPU time of TE polarization and TM polarization by more than 50%. Meanwhile, compared with the conventional algorithm, the proposed algorithm also needs less memory.

An improved very high frequency location system, multiple antenna radiation continuous observation system (MARCOS), is combined with high‐speed video observation to explore radiation source evolution during return stroke (RS) stages in triggered lightning. A three‐dimensional (3‐D) channel reconstruction is performed for speed measurement of the RS processes. The 3‐D average speeds for the two selected strokes (RS1 and RS3) over the bottom channel within 1 km are 9.24 × 10⁷ m/s and 1.07 × 10⁸ m/s, respectively. The high temporal‐spatial resolution of MARCOS permits thorough examination of the speed variation. Our observation shows that the 3‐D RS speeds of both RS1 and RS3 are increased initially with height for the first several hundred meters, maximized, and then decreased with height. To the best of our knowledge, this is the first evidence regarding nonmonotonic 3‐D RS speed variation in triggered lightning.

Induced currents on an overhead line due to a natural lightning with a strike distance of 20 m and a triggered lightning with a strike distance of 8 m were analyzed in time domain, frequency domain, and time–frequency domain, respectively. The results indicated that the energy peak of most currents lagged far behind its time-domain peak. The arrival time of different frequency components of the induced currents varies nonmonotonically with respect to frequency, especially that of the currents caused by the natural lightning. The energy of the highest frequency component is not necessarily the strongest. When it occurs earlier than the time-domain current peak, the protection measures designed by referring the time-domain threshold voltage may not start as expected, and accidental damage that may be incurred owing to that energy in the higher frequency band is more easily coupled to the terminal. Compared with the natural lightning, the smaller time delay between current peak and its energy maxima is yielded in the triggered lightning, as well as the shorter duration time of both time-domain current profile and energy distribution. Additionally, the consistency among currents induced by the triggered lightning is better than that excited by the natural lightning.