Comparison of approximate formulas for the capacitance of microstrip line

In the development of CAD routines at rf or microwave frequencies, closed form models for the capacitance per unit length of microstrip interconnects are easily incorporated. Numerous formulas have been proposed based on analytical, numerical, and empirical approaches. When there are a host of formulas found in the literature, knowledge of their accuracy against measured values and a case study of comparison is all the more vital in the design practice. This paper considers 12 such closed form models published over the past four decades and makes a critical comparison and seeks a way of improving the accuracy. While new formulas are proposed that give rise to less % error compared to few others taken as reference, it is found that composite formulas inherit combined goodness from which they are derived. Our case study provides some interesting results.