This example of a stripline transmission line contains a short description of the theory, detailed information on how to construct the model and the fully constructed model ready for you to download.
When working through the example, you may notice some small differences in your model compared to ours – this is usually simply due to the fact that you are using a different software version.
A stripline is a type of transmission line based on planar microwave circuitry. It typically consists of a conducting strip surrounded by dielectric material and sandwiched between two conducting ground planes. The dielectric material may be of different materials and thicknesses above and below the conductor leading to inhomogeneous or asymmetrical stripline. Because of the insulation provided by the dielectric casing, the stripline can be easily miniaturized and provides enhanced noise immunity in microwave circuits.
A basic schematic is shown in Fig. 1.
Figure 1: A Stripline transmission line displaying TEM mode of propagation. The green lines represent the E-field and purple lines the H-field.
A stripline consists of a conductor of width W, centered in a dielectric material of thickness b and permittivity εr. Two ground planes separated by a distance b are placed above and below the stripline. The presence of the homogeneous dielectric between the conducting ground planes supports the TEM mode of propagation. Similar to coaxial lines, the stripline is also capable of supporting higher order modes of propagation, is non-dispersive and has no cutoff frequency.
The phase velocity and propagation constant is given by:
Striplines have higher effective dielectric constant, εe when compared to microstrip lines which contribute to lower propagation speeds. The more complex formulas for characteristic impedance, and attenuation on the line are given in .
Shown here is a two-port stripline transmission line constructed using copper conductor in CST Studio Suite. The model is simulated in the Time Domain Solver in the frequency range of 5 – 15 GHz.
Figure 2: Model of a stripline transmission line in CST Studio Suite.
The stripline is designed for a 50 Ω copper conductor at 10 GHz of electrical length of 1λ and with E-field and H-field monitors set up at 10 GHz frequency. The boundary conditions in Y-directions are set to “electric” to represent the top and bottom ground planes. Waveguide ports are added to the model to provide excitation and calculate propagation constant and wave mode information. The following parameters are of interest to this design.
Width of 50Ω copper stripline conductor
Thickness of copper stripline conductor
Dielectric permittivity of substrate
Loss tangent of substrate at 10 GHz
Separation between ground planes
Length of stripline corresponding to 1λ at 10 GHz
Width of the substrate
Discussion of Results
In Fig. 3 we see the electric and magnetic fields display a TEM mode of propagation constituting a non-dispersive medium of operation in the circuit.
Figure 3: E- and H-field patterns displaying TEM mode of propagation.
The non-dispersive nature of TEM wave results in a near constant characteristic line impedance over the frequency range of 5 – 15 GHz as seen in Fig. 4. This can be observed by selecting the ‘Line impedance’ option available in 1D Results folder under Port Information.
Figure 4: Characteristic line impedance of the stripline.
The Port Mode Information at the operating frequency can also be found by right-clicking the port in the dialog tree and selecting “Object Information.” Fig. 5 displays the wave information for the TEM mode of operation.
Figure 5: Wave information calculated at 10 GHz for TEM mode of propagation.
Inhomogeneous or asymmetric striplines can affect the TEM fields in the circuit. To see this effect, try creating a stripline with air dielectric above the conductor while keeping the same substrate material below the conductor. Check to see the wave type. Is it still TEM?
In order to avoid higher order mode propagation, the spacing between ground planes is restricted to less than λd/2. Increase the separation b to a value higher than λd/2 and re-adjust the waveguide ports to obtain the new simulation results. What changes are observed in the port information?
 D.M. Pozar, Microwave Engineering, 4th Edition, John Wiley & Sons, pp. 141-147