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.<\/p>\n
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).<\/p>\n
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.<\/p>\n
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.<\/p>\n
More notes:
\nFR=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.)
\n“1oz” copper: 1.37 mils thickness (multiply-divide for other copper weights).
\nAn example calculator is here: http:\/\/chemandy.com\/calculators\/coplanar-waveguide-with-ground-calculator.htm<\/p>\n","protected":false},"excerpt":{"rendered":"
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 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_FSMCFIC_featured_image_caption":"","_FSMCFIC_featured_image_nocaption":"","_FSMCFIC_featured_image_hide":"","iawp_total_views":13,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1331","post","type-post","status-publish","format-standard","hentry","category-general"],"_links":{"self":[{"href":"https:\/\/tim.cexx.org\/index.php?rest_route=\/wp\/v2\/posts\/1331","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/tim.cexx.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/tim.cexx.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/tim.cexx.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tim.cexx.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1331"}],"version-history":[{"count":4,"href":"https:\/\/tim.cexx.org\/index.php?rest_route=\/wp\/v2\/posts\/1331\/revisions"}],"predecessor-version":[{"id":1335,"href":"https:\/\/tim.cexx.org\/index.php?rest_route=\/wp\/v2\/posts\/1331\/revisions\/1335"}],"wp:attachment":[{"href":"https:\/\/tim.cexx.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1331"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/tim.cexx.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1331"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/tim.cexx.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1331"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}