A dispersion formula satisfying recent requirements in microstrip CAD

A dispersion formula epsilon /sup *//sub eff/(f)= epsilon /sup */-( epsilon /sup */- epsilon /sup *//sub eff/(0))/(1+(f/f/sub 50/)/sup m/), for the effective relative permittivity epsilon /sup *//sub eff/(f) of an open microstrip line is derived for computer-aided design (CAD) use. The 50% dispersion point (the frequency f/sub 50/ at which epsilon /sup *//sub eff/(f/sub 50/)=( epsilon /sup */+ epsilon /sup *//sub eff/(0))/2)) is used a normalizing frequency in the proposed formula, and an expression for f/sub 50/ is derived. To obtain the best fit of epsilon /sup *//sub eff/(f) to the theoretical numerical model, the power m of the normalized frequency in the proposed formula is expressed as a function of width-to-height ratio w/h for w/h>or=0.7 and as a function of w/h, f/sub 50/, and f for w/h>