$\begin{array}{cc}\mathrm{dB}=D\times {10}^{\sqrt{\frac{{\epsilon }_r}{138}}*z}& \left(3\right)\end{array}$

In the pass characteristic B, a stopband extending from the region in the vicinity of 5 (GHz) which is the pass band to substantially 30 (GHz) is formed, on the other hand, even in the case where each of the electrical lengths d1 and d2 is similarly set to substantially 30 degrees, a pass band can occur in the vicinity of a band in which the second harmonic with respect to 5 (GHz) is included, as shown in the pass characteristic F. Thus, the pass characteristic F is the result different from the circuit synthesis calculation result (1) shown in FIG. 7. This is because the spacing dB is 8.8 (mm) and hence is too wide for the rejection of the transmission of higher harmonics, so that harmonic components are transmitted. Therefore, it is necessary to determine the spacing dB by means of a simulation calculation in such a way that a stopband is formed in all frequency bands in which harmonic components up to a desired-order (e.g., fourth-order) harmonic can occur.