Is it wise to have a ground conductor underneath the two traces, so as to provide reference, and hence maintain 50ohm impedance throughout the length of these tracks?
What do you mean with "two traces"?
Do you use a differential line pair for each RF signal?
A transmission line consists of signal and return path. The choice of microstrip (one signal line over ground plane) or differential line pair (signal and return side by side) or some other structure depends on your application.
As expected. In this case, you also don't actually need 50 ohms transmission lines. Instead the trace can be considered as lumped inductance and goes into the dimensioning of the matching network. In may be appropriate to design a low inductance connection anyway, so wide traces above a ground plane seem reasonable. Of course, you can design the connection as 50 line to simplify calculations, but it isn't actually required.We are using 13.56MHz RF, for reading tags.
... 50 ohm differential is a rather low impedance and can be best achieved with a ground plane and a thin substrate (differential microstrip). A differential pair without ground plane (e.g. differential coplanar waveguide) would result in rather extreme gap to trace width relations.
The same basically applies to 50 ohm single ended transmission lines. But if you intend to incorporate antennas with your PCB, you should primarly care for the antenna requirements regarding ground planes and PCB substrate. In any case, you can expect, that the individual antennas interact with each other and the common ground plane.
The term "antenna coil" is mostly used for D << λ inductive couplers, that don't have much antenna properties.
P.S.:
As expected. In this case, you also don't actually need 50 ohms transmission lines. Instead the trace can be considered as lumped inductance and considered in the dimensioning of the matching network. In may be appropriate to design a low inductance connection anyway, so wide traces above a ground plane seem reasonable. Of course, you can design the connection as 50 line to simplfy calculations, but it isn't actually required.
If we can get this transmission line between the switch IC and the antennas right, we are saving a lot of time, expenses and also avoiding potential board-to-board variations.
A differential pair without ground plane (e.g. differential coplanar waveguide) would result in rather extreme gap to trace width relations.
... so we need a different approach.
One possible solution is to use broadside coupled lines (i.e. line pair where lines are vertically stacked, instead of side by side on the same layer), but that depends on the thickness of the dielectric between the lines.
Do you intend 50 ohm or 50+50=100 ohm balanced line? 50 ohm balanced would be inapparoriate low impedance in my opinion.Yes, we are planning to run a balanced line
sounds interesting, but haven't explored this method yet. Will definitely give it a thought
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