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I'm currently design a 4layers pcb with 50r trace for an Ceramic chip antenna (on top layer). Due of a lot of trace, my trace antenna is a coplanar. Trace on top layer with polygon pour around AND internal_plane_1 (just below) connected to gnd.

The calculation: [https://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm[1 With E = 4 / S = 0.37 / W = 0.154 / h = 0.2. Impedance >> 50.0r

My question. We see often impedance controled track with Top and Bot layer, rarely with top and internal plane. is there a contraindication ?

joseph4008
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  • " ...impedance controled track with Top and Bot layer...". Do you mean running a trace on the top or bottom layer (1 or 4 in your case), with a reference plane internal to the PCB (layer 2 or 3)? I'm confused about the last question. Also, a sketch or picture would help. – SteveSh Feb 04 '20 at 11:38
  • generally, the track is on top with polygon pour around, nothing inside (no internal plane) and polygon pour on bottom. In my project, i would design my circuit as : Track with polygon pour around, and polygon pour in first internal plane – joseph4008 Feb 04 '20 at 12:19
  • This will increase the capacitive loading due to the close proximity. Different matching component values will be needed at least. – rdtsc Feb 04 '20 at 12:33

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The ground/reference plane to either microstrip or coplanar waveguide doesn't have to be on the bottom layer of a 4 layer PCB. If you are looking to reduce the width of your impedance controlled traces, you can certainly use either layer 2 or 3 for the reference plane to reduce the height.

BEE
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  • thanks for your opinion. Altium have published a document about permitivity (Core versus Prepreg) : https://resources.altium.com/pcb-design-blog/using-prepreg-vs-core-for-controlled-impedance-routing?utm_content=buffer25b6b&utm_medium=social&utm_source=facebook.com&utm_campaign=buffer&fbclid=IwAR01dKdckLU_Giiw3cwKy0arJFx5Rx1CY_ErdbR0oSCQ9C_qs1I34ORHfSk – joseph4008 Feb 05 '20 at 10:03
  • Yes, the key point that I take away from this article is to work with the board fabrication shop to settle on readily available board material/s (call this out specifically in your fab notes), and stackup. Once you have these settled, you can finalize your design. – BEE Feb 05 '20 at 16:17