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I'm working on a RF Frequency board with up to 6GHz signals. To connect components on the board (Mixer, Attenuator, ...) I decided on using GCPW. I designed my GCPW Using AppCad calculator (free software from Broadcom) for 50Ohm on RO4003. You can see the parameter in the below.

Then I see via fence is common in boards and my components reference designs (ex HMC1119) but neither AppCad nor Allegro (my layout software) not mention anything about how should via stitching be done around the transmission line.

Now I'm wondering how via snitching change the characteristic impedance and how can I calculate for that? Do I need a simulation in 3D simulator software?

GCPW for 50Ohm on RO4003 in AppCad

Note1: I already found that Via fence should be around lambda/20 from each other nevertheless how far should via fence be from center of transmission line and how this would affect the RF performance?

Note2: Unfortunately my English is poor so please feel free to edit the post and fix my grammar mistakes. Thanks

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  • \$\begingroup\$ Interesting question. I think this is really hard to answer without a full 3D simulation. Of course, if the fence is too near, it will disrupt the waveguide. A fence too far from the trace is like no fence. So this is a optimization problem. Lambda/10 is 76mil, so I would place the via hole plating at least 4mil (2x G) and max 50mil from the edge of the plane. \$\endgroup\$ Oct 12, 2018 at 10:58
  • \$\begingroup\$ @StefanWyss Thanks Stefan for your suggestion. I'll be sure to follow your mentioned rules. Also hoping as you to someone with a simulated results show up and help us on this ambiguity :) \$\endgroup\$
    – pazel1374
    Oct 12, 2018 at 11:50

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Min common annular ring around for a via is 0.15mm. I used this distance (via hole distance from ground edge) for 30GHz coplanar line and it worked fine. It will work also at 6GHz.

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