Notes To Myself: Cheap Feedlines for Cheap Boards

Goal: Produce reasonable impedance-matched (usually 50-ohms) RF feedlines for hobby-grade radio PCBs. Rather than get a PhD in RF engineering for a one-off project, use an online calculator and some rules of thumb to get a “good enough” first prototype.

Problem: Most RF boards and stripline calculators assume or drive toward 4-layer boards. In hobby quantities, 4-layer boards are much more expensive and have longer leadtimes. If using EAGLE, can no longer use the free/noncommercial or Lite editions (they only allow 2 layers).

The main driver of feedline impedance is its geometry and dielectric “distance” from the groundplane. The aforementioned stripline/microstrip/etc. calculators often assume there is nothing on the top (feedline) layer in its vicinity, there is just a groundplane on another PCB layer beneath it, and all proximity to the groundplane is through the FR-4 between the layers. For bog-standard 2-layer boards, that’s ~.062″ of material, which yields unacceptably wide traces (>100 mils) that cannot be cleanly terminated to most antennas or connectors, let alone a surface-mount IC pad.

Solution: Forget plain microstrip stuff, look up a “coplanar waveguide with ground” calculator instead. This takes into account a groundplane on the same layer, surrounding the feedline, in addition to a groundplane on a lower layer. Now the clearance between the feedline and coplanar goundplane can be tweaked to get a sane trace width for various copper weights, board thicknesses or other factors less easily in your direct control.

More notes:
FR=4 relative dialectric constant: 4.2 (in reality, it can vary quite a bit, and there are about a million material variants called “FR-4” and used interchangeably by board houses, but if you can’t be a chooser, this is probably as good an approximation as you get.)
“1oz” copper: 1.37 mils thickness (multiply-divide for other copper weights).
An example calculator is here:

One Response to “Notes To Myself: Cheap Feedlines for Cheap Boards”

  1. Martin says:

    I posted some microstrip and coplanar waveguide examples for OSH Park, last year:


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