| Photonic Bandgap (PBG) materials are widely studied in the field of microwave recently. The Combinatorial Non-periodic Defected Ground Structure (CNPDGS) is developed at the base of PBG. The CNPDGS is capable of prohibiting the propagation of electromagnetic waves within a certain band of frequencies. This characteristic allows wide application in high-Q cavities, low-pass filters, power dividers and so on. The structure of CNPDGS is entirely different from the structure of periodic PBG. Compared with periodic PBG, the structure of the CNPDGS is simpler in structure, smaller in dimension and lower in insertion loss. The structure of CNPDGS is calculated with finite-difference time- domain (FDTD) method in this paper, through which we will obtain the electromagnetic waves of time-domain propagated in the FDTD domain. Then the frequency characteristic of the structures can be compute by Fourier transform. The effects of various kinds of shapes and their dimensions of CNPDGS on frequency characteristic of the circuits are studied in this paper. And some new structures of CNPDGS are studied also. Technique of the perfectly matched layer is used for the absorption of electromagnetic waves in FDTD. Comparing with traditional Mur boundary condition, it has many merits such as can be calculated with a simpler feed source, smaller memory storage, better convergence, and has invariable performance in spite of the incidence angle and frequency.The FDTD method is programmed with the blend of C++ and Matlab languages, which combines the merits of the two languages. So the program is both simple and fast computing, that make it a powerful tool of studying CNPDGS. A kind of new structure of CNPDGS is brought out through a lot of experiments and analysis. A low-pass filter with alterable pass-band and another filter with two stop-bands are designed at last. The measure results are accord well with the numerical results, which prove the validity of the FDTD program.
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