There are two unwanted propagation modes you want to avoid in a GCPW:
1. Slot line mode: This is avoided by keeping the grounds on the signal layer at the same potential by shorting them with an air bridge.
2. Parallel plate mode: This can be avoided by shorting the grounds on the signal layer to the substrate ground with the help of a via.
After that, it is advisable to keep the width of the ground line on the G-S-G layer at least 2.5 times the gap between the ground and signal lines to avoid return currents.
You may find more information on this at:
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