In this paper, the characteristic impedance matching of the inner line and outer line of the multi-filar tape-helix Blumlein pulse forming line (BPFL) is analyzed in detail by dispersion theory of tape helix. Analysis of the spatial harmonics of multi-filar tape-helix BPFL shows that the integer harmonic numbers of the excited spatial harmonics are not continuous. In addition, the basic harmonic component still dominates the dispersion characteristics of the multi-filar tape-helix BPFL at low frequency band. The impedance mismatching phenomenon caused by the discontinuity of filling dielectrics in the inner line of BPFL is studied as an important issue. Effects of dielectric discontinuity on the coupled electromagnetic fields and the parameters of the outer line are also analyzed. The impedance matching conditions are both obtained under the situations of continuous filling dielectric and discontinuous dielectrics, respectively. Impedance characteristics of these two situations are analyzed by comparison, and effects of the thickness of support dielectric on the impedance are also presented. When the 6 mm-thickness nylon support of the multi-filar tape helix is used in the filling dielectric of de-ionized water, the characteristic impedances of the inner line and outer line of BPFL are 53 Ω and 14.7 Ω, respectively. After the improvement about substituting de-ionized water by castor oil, the relative permittivities of the support dielectric and filling dielectric are almost the same, and the impedances of the inner and outer line of BPFL become 80 Ω and 79 Ω, respectively. That is to say, the impedance mismatching problem caused by dielectric discontinuity is solved. Circuit simulation and experimental results basically correspond to the theoretical results, and the fact demonstrates that impedance analysis of the multi-filar tape-helix BPFL based on dispersion theory is correct.
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