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DavidJo

Ideal RF PCB stackup

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Hello,

 

I was reading this quote from jpnorair and checking Elecrow PCB possibilities and am wondering what the ideal PCB stackup would be with regards to low-ISM RF, printed antennas and EMI.

 

In systems with monopole antennas, it is important to have an uninterrupted ground layer.  It dipole systems, this is less important.  However, you also want a ground layer underneath the transceiver and front-end analog circuits. For 866 MHz you will want the spacing to be no more than 0.8mm.  For 433 you can get away with 1.6mm.  You also want to sink a lot of vias between ground layers.  This prevents ground loops.

 

Therefore, 2-layer is OK, but it is difficult to design a PCB that is good for RF with only 2-layers.

 

 

 

I also read this :

 

Planes, in multilayer PCB

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There are many ways to get a good result, but components cannot be inside the board, and vias add reactances, so we need to find ways to put the traces on the outer layers.  Fortunately, there are ways!  ST has a nice app note, I think it covers pretty much everything.  

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/DM00065838.pdf

 

The main thing you need to think about is the fact that the traces on the board can act as antennas if you do not design properly.  They can also cause impedance mismatch, causing less power to go to your antenna if you don't do them the right way.  This becomes a bigger and bigger problem as MHz gets higher.  For example, at 434 MHz, you don't run into too many issues.  900 MHz, still not too bad, but getting to the point transmission line behavior is important.  1575, 1850, 2450... things get serious.

 

To design the traces "the right way" you should design them as coplanar waveguides or microstrip transmission lines, tuned to 50 Ohms.  The problem with a 2-layer board is that the transmission line traces need to be really wide to allow 50 Ohm impedance.

 

I searched on google and I found this website with transmission line calculators for coplanar waveguide, microstrip, and slot (you probably don't need the slot part).  I can also tell you a "magic number" -- at 434 MHz on normal FR4, with 0.16mm spacing, with 4-layers, with a ground plane directly below, and with a ground plane around the trace -- the magic trace width is 0.47mm.

Microstrip Line Calculator

Coplanar Waveguide Calculator

Slot Line Calculator

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