Microstrip Filling Factor versus Relative Dielectric Constant Given fixed Line Impedance

Research Links

• Microwaves101: Filling Factor

The question arose given a fixed microstrip transmission line characteristic impedance how does the percentage of the field in the substrate change? This is called the filling factor:

Keff=ER*FF+1x(1-FF)

It is the fraction of the field in the substrate.  I calculated the following values:

• FF=(Keff-1)/(ER-1)
• Dk=4 Z0=51 Deff = 3.07 which yields a filling factor of 0.69
• Dk=10 Z0=50.5 Deff = 6.72 which yields a filling factor of 0.635

Thus it appears that the filling factor goes down with Dk as Z0 is held constant.