Update README.md

cad/README.md

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 # FreeCAD tracker models
 
+Rubber duck antenna:
 - `antenna_base` - base for rubber duck antenna, to be printed with PETG filament.
 - `antenna_cover` - cover for rubber duck antenna, to printed with TPU filament.
+
+Rubber duck antenna spiral widing was done with 1.3mm copper wire on 8mm aluminium scaffolding tube.
+
+Tracker case:
 - `aprs_tracker_e22_v2` - base for tracker case.
 - `aprs_tracker_e22_cover_v22` - upper cover for tracker case.