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I am designing a PCB which has a chip antenna with a coplanar wave guide. Having a 4-layer PCB (stackup layer: rf signal, GND, POWER, signal) can I run traces across the antenna transmission line on the farthest bottom layer?

here a picture a CPW just for reference https://i.stack.imgur.com/PhAtu.png

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  • \$\begingroup\$ What angle are the traces crossing the CPW at? Also, are the vias full-depth or blinded (stopping at the GND plane)? \$\endgroup\$ – ThreePhaseEel Dec 17 '17 at 23:36
  • \$\begingroup\$ They are crossing it at 90°. Vias are full-depth. Is there any difference in term signal reception if fence vias are full depth or blind? \$\endgroup\$ – Nihilo Dec 18 '17 at 11:36
  • \$\begingroup\$ Now I have one doubt crossing my mind. It's better to use blind vias for fencing \$\endgroup\$ – Nihilo Dec 19 '17 at 10:40
  • \$\begingroup\$ ..? If we have full depth fence vias connected from top to ground plane (2nd layer below the top layer) would them act as antennas since they go all the way to bottom layer? The downside of blind vias is the increase of PCB cost. \$\endgroup\$ – Nihilo Dec 19 '17 at 11:04
  • \$\begingroup\$ ask a new question for that please so it gets the attention it deserves \$\endgroup\$ – ThreePhaseEel Dec 19 '17 at 12:38
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Yes, you can run a trace on layer 4 in this scenario. The ground layer on layer 2 means that all field lines from the transmission line are terminated on layer 2, and anything that happens on layer 4 will have practically no effect on the transmission line.

If you are spacing your fencing vias as close as shown in the diagram, you might end up having to skip a via to make room for the layer 4 trace to pass. If you're working below 10 GHz, I don't expect this will create a significant problem. Even at higher frequencies, a single skipped via in the via fence is not likely to cause a noticeable problem (but of course you could do an EM simulation to be sure if your application would be sensitive to a tiny increase in loss or degradation in return loss)

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  • \$\begingroup\$ Many thanks. I am working around 900Mhz. So definitely shouldn't be an issue skipping some vias. I was more concerned the traces running across that could inhibit a proper functioning. \$\endgroup\$ – Nihilo Dec 18 '17 at 10:07

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