J-Pole / Slim Jim Calculator
End-fed ½-wave with matching section. Separate starting dimensions for classic J-Pole and Slim Jim. No radials required. Always prune for lowest SWR.
How the J-Pole and Slim Jim are formed
The J-pole is an end-fed half-wave radiator matched through a quarter-wave parallel section. The diagram on this page shows the relationship that makes the antenna distinctive: a half-wave vertical element rises from a quarter-wave matching stub, and the coax feed connects at a selected point on that stub. The same general principle appears in the Slim Jim family, although the physical construction and current distribution differ.
The calculator estimates the major electrical lengths from frequency. Those lengths are starting values. The velocity factor of the matching section, conductor spacing, conductor diameter, feedpoint position and the surrounding environment affect the final impedance. A wire version, ladder-line version and tubular version can all require different physical dimensions for the same nominal electrical length. This is why a J-pole should be measured after construction rather than built directly to the calculated number without adjustment.
Keep the two conductors of the matching section mechanically stable and maintain their spacing over the full length. Small changes near the feedpoint can produce noticeable impedance changes, so use an adjustable or carefully measured feedpoint during initial tuning. Route the coax away from the lower section without allowing it to run parallel to the radiator for an excessive distance. A choke may be useful if the coax begins to participate in the radiation system. Tune at low power, locate the minimum SWR or desired impedance region, and then secure the feedpoint permanently. The image is a direct representation of the geometry behind the calculator, which is more useful to a builder than a generic photograph of an unrelated vertical antenna.
Dimensions
J-Pole vs Slim Jim
Classic J-Pole – Separate ½-wave radiator and ¼-wave matching stub, usually built from copper pipe or tubing with a fixed spacing. Feed point is adjusted a short distance up the stub for 50 Ω.
Slim Jim – Continuous folded element (often ladder line or twin-lead). The matching section is part of the continuous conductor. Radiation pattern is slightly different and the whole structure is often lighter.
These are starting dimensions only. Length correction of the conductor, end effects and mounting all shift resonance. Cut long and trim while measuring SWR.