3" Ugly Balun Coil Form with Retaining Flanges

What is an Ugly Balun? An "Ugly Balun" (also called a choke balun or coax choke) is one of the simplest and most effective baluns a ham radio operator can build. It works by coiling a length of coax cable — typically RG-8 or RG-8X — around a cylindrical form, creating a common-mode choke that blocks RF from traveling back down the outside of the feedline. No cores, no windings, no complicated assembly. Just coax on a tube. Ugly by name, elegant by function. About This Print This is a purpose-designed coil form for building your own Ugly Balun. The dimensions were chosen to give you an effective choke across HF bands when wound with RG-8 or RG-8X coax. Specs: Outer Diameter: 76.2 mm (3.0") Inner Diameter: 69.85 mm (2.75") Height: 127 mm (5.0") Wall Thickness: 3.175 mm (1/8") Retaining Flanges: 6.35 mm tall at each end to keep your coax coils from slipping off SO-239 mounting holes on both end walls AND both caps — standard 15.875 mm (5/8") center hole with four 3.81 mm bolt holes on a 17.98 mm bolt circle Parts to Print: 1× Cylinder body 2× End caps (same STL, print twice) The caps feature a press-fit lip that seats snugly inside the cylinder bore. No hardware needed to hold them together — just press fit, though a dab of CA glue or a couple of small screws will make it permanent. Assembly: Mount an SO-239 connector to each end cap (or directly to the cylinder end walls if you prefer) Press fit or glue the bottom cap in place Run coax through the body as needed and wind RG-8 or RG-8X tightly around the outside between the retaining flanges Press fit the top cap Connect SO-239s to your antenna feedline and your transceiver Print Settings: Material: PETG or ASA recommended (UV and heat resistant for outdoor use) — PLA is fine for indoor or temporary use Layer Height: 0.2 mm Infill: 30–40% (the wall thickness carries the structural load) Supports: Used Tree for the upper flange Perimeters/Walls: 3 minimum for the cylinder wall Notes: If the cap press fit is too tight or too loose for your printer, adjust the LIP_CLEAR variable in the OpenSCAD source (included). Default is 0.4 mm diametral clearance. All dimensions are parametric — open the .scad file and adjust OD, HEIGHT, WALL, or flange size to suit your needs. This design follows the guidelines commonly cited by ARRL and summarized by W1HIS and others for effective common-mode choke construction on HF.