J-Pole Antenna Calculator
Enter your target operating frequency to estimate the tube or wire lengths needed to build a J-pole antenna.
What This Calculator Does
A J-pole is a popular end-fed, omnidirectional antenna built from a half-wave radiator fed through a quarter-wave matching stub, giving it its distinctive ‘J’ shape. This tool estimates the key tube or wire lengths from your target frequency so you have a solid starting point for building or trimming one.
The Formula
The free-space wavelength (in inches) is found from 11811.03 ÷ frequency (MHz). The long radiating element is roughly three-quarters of a wavelength and the matching stub is roughly one-quarter, both scaled by a velocity factor (typically 0.94–0.97 for bare wire or tubing) that accounts for the antenna material’s effect on wave propagation. A small feed-point gap separates the two elements, and the coax tap point is measured up from the bottom of the stub to find the 50-ohm match point.
- Long element: the main radiating leg, roughly 3/4 wavelength.
- Stub: the shorter parallel matching section, roughly 1/4 wavelength.
- Tap point: where the coax connects; fine-tune with an SWR meter for best results.
These are standard reference dimensions for general planning and educational purposes — always trim and test with an antenna analyzer or SWR meter, since real-world element diameter and mounting hardware shift the exact resonant length.