J-Pole Antenna Calculator

Your details

Center frequency you want the antenna tuned to. Common choices: 144-148 MHz (2 m amateur), 420-450 MHz (70 cm amateur), 155-158 MHz (public safety VHF).
MHz
The fraction of the speed of light at which the radio wave travels in the conductor. Bare copper or aluminium tubing: 0.95-0.97. Twin-lead or ladder line: 0.88-0.97 depending on type. Defaults to 0.95 for bare metal tubing.
Outer diameter of the rod or tube used for both the radiator and the matching stub. A larger diameter increases bandwidth. Common choices: 6 mm (1/4") for copper or aluminium tubing, 12 mm (1/2") for heavier builds.
mm
Center-to-center distance between the two parallel conductors that form the matching stub. Wider spacing raises characteristic impedance toward 50 ohms. Typical range: 15-50 mm (0.6-2 in).
mm
Total antenna height (A)High impedance - reduce element spacing
1,463

3/4-wavelength total height: radiator + stub together

Radiator length (B)975.4
Stub / matching section length (C)487.7
Feedpoint height above stub bottom (D)78
Element spacing (E)25
Stub characteristic impedance (Z0)254.1ohm
Free-space wavelength2,053.4
Estimated 2:1 SWR bandwidth1.34MHz
Radiator (B)975.4
Stub (C)487.7
Feedpoint (D)78

J-pole for 146.000 MHz (VHF band) - total height 1463.0 mm.

  • Radiator (B) is 975.4 mm and the matching stub (C) is 487.7 mm.
  • Connect the coax center conductor at 78.0 mm above the bottom of the stub for a 50-ohm feedpoint.
  • Stub impedance is 254 ohms - adjust the center-to-center spacing until Z0 is closer to 50 ohms for the best match.
  • Estimated 2:1 SWR bandwidth is about 1.34 MHz - use a larger conductor diameter to widen it.

Next stepAfter cutting, trim the radiator from the top in small increments and use an SWR meter or antenna analyzer to fine-tune the feedpoint height for the lowest SWR at your exact frequency.

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