Language:

Main >> Applications >> Sample problems >> Microstrip transmission line simulation
A shielded microstrip transmission line consists of a substrate, a microstrip, and a shield.
Problem Type:
Plane problem of electrostatics.
Geometry:
The transmission line is directed along zaxis, its cross section is shown on the sketch. The rectangle ABCD is a section of the shield, the line EF represents a conductor strip.
Model depth l = 1 m.
Given:
Relative permittivity of air ε = 1;
Relative permittivity of substrate ε = 10.
Problem:
Determine the capacitance of a microstrip transmission line.
Solution:
There are several different approaches to calculate the capacitance of the line:
Both these approaches make use of the equation for capacitance:
C = q / U.
Other possible approaches are based on calculation of stored energy of electric field. When the voltage is known:
C = 2·W / U^{2},
and when the charge is known:
C = q ^{2} / 2·W
Experiment with this example shows that energybased approaches give little bit less accuracy than approaches based on charge and voltage only. The first approach needs to get the charge as a value of integral along some contour, and the second one uses only a local value of potential, this approach is the simplest and in many cases the most reliable.
Results:
Potential distribution in microstrip transmission line:
Theoretical result (model depth l = 1 m.) 
C = 178.1 pF. 
Approach 1 
C = 177.83 pF (99.8%) 
Approach 2 
C = 178.47 pF (100.2%) 
Approach 3 
C = 177.33 pF (99.6%) 
Approach 4 
C = 179.61 pF (100.8%) 
See the Elec1_1.pbm and Elec1_2.pbm problems for the 1,3 approaches and the 2,4 approaches respectively.
View simulation report in PDF.
Download simulation files (files may be viewed using any QuickField Edition).