Chip antenna and PCB StackUp

Question

I'm working on a ISM (868Mhz) weather station for my home and i'm trying to implement a Ceramic antenna directly on PCB. After reading the guidelines, i have questions again.

One of then is what about 4layers circuits... Chip antenna reference : M620720 from AVX (link)

Actually, my pcb stackUp is like that : 1. Top (Signal). 2. internal_plane_1 (GND). 3. internal_plane_2 (different VCC). 4. Bot (signal).

On page 5, the constructor propose that :

My question, We see a big gnd copper planar (Top + internal_1 OR Top+Bot)(with correct impedance). if i use the first solution (Top+internal_1), what about internal_plane_2. i have to remove it or not ?

tell me more if you have opinion

Answer 1

Remove any metal underneath the antenna. The cut-out you see on the datasheet's "Bottom Metal" should apply to ALL internal layers regardless of how many layers you have.

A metal layer underneath will shield emission in that direction. Anything you do which is not in accordance to the datasheet will tune the chip antenna to another frequency, they are quite sensitive. This translates directly to range in the different directions.

Answer 2

You can certainly have your RF trace on the top layer, and the reference ground plane on layer 2 (or internal_1 as you call it) of a 4 layer board. You would just need to ensure the transmission line width is sized appropriately for the height between the two layers and the substrate material. You don't have to do anything special on the other two layers. You can freely use layer 3 and 4 for planes or signal routing.