chip antenna feed with reference plane and without reference plane

[electronx]

I am trying to design a pcb using a chip antenna. I see that in some reference layout topologies a reference plane passes under the feed trace and in some there is no reference plane. What is the difference between these?



Do I understand correctly?

[selcuk]

That transmission line is a coplanar waveguide. Check the link below:
https://en.wikipedia.org/wiki/Coplanar_waveguide

You can do it either with or without a ground plane. But it is a better wave guide with a ground plane. Using a ground plane is better for reducing cross talk issues. I think the manufacturers made the calculations about the track width, spacing and dielectric heights with respect to that ground plane so they suggest using it. If you don't want to use a plane, you need to recalculate the track parameters. Additionally, if you want to use a different layer stack (different thickness and Dk of dielectric etc.) you need to recalculate as well.

But do as suggested for the ground plane right under and around the chip antenna.

[Salitronic]

They a different variants of the coplanar waveguide. Generally it is more common to use a coplanar waveguide with ground plane (GCPW), particularly if you have a long feedline, bends or enclosure wall very close to the board. The ground plane in a GCPW serves not only for impedance matching but also for the return current, for a long CPW feedline (without ground) this return current needs to be handled correctly.

Note that the 2 chip antennae that you listed are quite different. The first one is an unusual chip antenna that is designed to be mounted directly over a ground plane. In this case the obvious choice is a GCPW feedline. This antenna is meant for very compact designs in which not needing metal clearance under the antenna is a significant convenience. There is a catch however as the efficiency of that antenna is quite low, only 38% on a typical 1.5mm board.

The other antenna is a more common type of chip antenna requiring metal clearance underneath it and has better efficiency. In this case you can choose either CPW or GCPW. GCPW is generally preferred.

Note that the dimensions and spacing calculations for the feedline will be different for CPW and GCPW.