MICROSTRIP

Suppose I design a Low Pass Filter for S band and use Microstrip for capacitors ,Will the length of the Microstrip play any role or only Thickness, Height and Permitivity is important...How will I determine the capacitance of the Microstrip?

At lower frequencies, you can just use a pad of any shape, and the capacitance will be the parallel plate capaciatance PLUS the fringing capacitance (off of sharp corners, edges, etc).

At higher frequencies, where any dimension is greater than say 0.1λ, you had better at least model the pad as a low impedance transmission line. So a series L, shunt C, lowpass filter... becomes a high impedance, low impedance...series of transmission lines, with a microstrip discontinuity model used where ever the width abruptly changes.

Hi,

for some simulators (Eagleware, MW Office, ADS) you have developed libraries of vendors RLCs on a substrate, so all the effects of the substrate and leeds are taken into account. You can then connect those with microstrip lines in the schematics to have 100% real simulations. Just look for Modelitics libraries.

flyhigh

Enclosed is the file calculating the length capacitance of microstrip line. Kindly refer the book Microwave Engineering by Pozar. It is taken from this book.