I am working on a SDP antenna. For that I need 50ohms coplanar stripline feed. Plz explain how to do so in CST
Hello!
For CPW ports in CST, you can simply draw the CPW geometry in the modeler and then use a "Waveguide port" defined around the cross-section. I have attached a model of a CPW short-circuited stub as an example.
As with microstrip, the trick is to make the waveguide port big enough to fully encompass the transverse fields so you get the right impedance, while not making it so big that the port appears overmoded. The details will depend a little bit on the geometry (e.g. how thick the substrate is relative to the trace and gap widths, etc.) but as a general rule you want it to be at least a few times wider/taller than the cross-section of the t-line, and at least the full substrate thickness (unless the substrate happens to be very much thicker than the width of the line). Basically, you want the port mode fields to be near-zero at the edges of the port window. Sometimes its worth changing the port dimensions and re-running the simulation to make sure you get the same result. If the characteristic impedance changes significantly, than your port window is not big enough.
You also have to take care that the two grounds are at the same potential. In the attached example, I did this by adding a magnetic symmetry plane down the center of the stub. If your circuit geometry does not allow that, you can connect the two grounds with an air bridge, or with vias to a back-side ground plane. Alternatively, you can increase the number of modes at the port to 2, one of which will be the common-mode (normally what you want when dealing with CPW) and the other will be the differential mode (which is more like a pair of couple slotlines).
Lastly, be sure there are a few mesh lines across the width of the trace and in the gaps.
I hope this is helpful. Best regards.
Dear Sudhanshu,
Smrity is right.
Please do follow the help/example files too given in CST
Best wishes
Dear Sudhanshu, Hello
It is easy we can check the CST help, it very helpful and have some good examples
and Smrity's explanation is very good.
CST help is very interesting, have a look if yu have some problems and for the 50 Ohm we can use the CST calculator From Macros- TL calculator - then select CPW with ground or without ground.
best wishes
Hany
To find out how big to make the port all you have to do is to pick the stripline metal then go to Macros, from here you select solver then port then discrete port. Fill in all parameters if they are not already filled in then Click calculate. You get the multiplying factor. This helps you work out the size of your port. Go back to your simulation clear the microstrip metal. Contruct the 4 points of your port with the data you have. Click waveguide under simulation. You will have a discrete waveguide port.
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