Characteristics of Inhomogeneous Broadside-Coupled Striplines

A variational method for the analysis of inhomogeneous broadside-coupled striplines is described. The data for even- and odd-mode characteristic impedances, effective dielectric constants, and mode phase velocity ratios are presented. It is found that the phase velocity ratio may be varied over the range 1.14 ≤ ( V_e / V₀) ≤ 3.6 for broadside-coupled suspended microstrip lines (BSML) and 0.36 ≤ ( V_e / V₀) ≤ 0.93 for broadside-coupled inverted microstrip lines (BIML) using materials with dielectric constant less than 16 and S/b ≥ 0.05, W/b ≤ 2.0. The effect of nonzero strip thickness is also calculated. It is noticed that the effect of thickness is more pronounced for the odd-mode case than for the even mode. Losses are obtained using the incremental inductance rule of Wheeler. The odd-mode attenuation constant is always higher than the even-mode value.