I have a 4-layer PCB where
- L1 = Sig/Pwr
- L2 = GND
- L3 = GND
- L4 = Sig/Pwr.
I need to route a 50 Ω coplanar waveguide for an RF 4G-LTE antenna on L1. I know how to make the calculations for the impedance and all, but does it matter if I use L2 or L3 as GND plane for the waveguide?
If I use L2 the dielectric thickness would be 8 mils and I could design the waveguide with 12 mils trace thickness and 4 mils gap.
However, if I choose to use L3 as GND plane the distance to it becomes 32.8 mils and the RF trace will need to be 25 mils with 4 mils gap.
I would prefer to use L2 as GND plane because of the slimmer traces, but only if it doesn't degrade performance too much.