Antenna trace keepout area on internal layers

[Andrei]

Hello, i have a question about keepout area under antenna trace in multilayer pcb. I have a 4-layer pcb, but substrate height between top layer and first mid layer isn't enough for good 50-ohm trace design, so i decide to make cutout in first mid layer and use second mid layer as ground base. But i'm confused about keepout/cutout area size.

I found these guidelines:

1. According document dm00660594 from STM (http://www.st.com/resource/en/application_note/dm00660594-optimized-rf-board-layout-for-stm32wl-series-stmicroelectronics.pdf) on page 7 cutout width must be = trace width + 2x spacing.

2. Answer on DevZone (https://devzone.nordicsemi.com/f/nordic-q-a/38170/nrf52832-ciaa-hdi-pcb-layout-tips): "It should not be less narrow than the opening in the 'opening' for the coplanar center conductor in the top layer, width of trace + 2*spacing".

3. According document "Rf Layout" from Quectel (https://www.quectel.com/UploadImage/Downlad/Quectel_RF_Layout_Application_Note_V2.0.pdf) on page 8 cutout width must be = trace width x5.

4. Answer on DevZone (https://devzone.nordicsemi.com/f/nordic-q-a/26394/nrf52-layout-questions/103859#103859) also 5x width.

And my question is - what width must have cutout area on inner layers?

[Andreas]

Hi,

 

There is not one correct answer to this, but in general wider is better. If more narrow than the top layer opening then the middle layer board will disrupt how the electromagnetic field is distributed from the center conductor, and thus change the impedance. Infinitely wide does not mean infinitely good though, so no need to make it very much wider than the top layer opening, you typically want as large ground planes as possible too.

 

Best regards,

Andreas

[Andrei]

Thanks for your answer!