What is a J antenna?
A J antenna (often called a J-pole antenna) is a simple and popular end-fed vertical antenna made from two parallel conductors. One side is the long radiating section (roughly 3/4 wavelength), and the other is a shorter matching stub (roughly 1/4 wavelength). The feedpoint is tapped somewhere up from the bottom of the stub to match your coax impedance.
Builders like J antennas for VHF/UHF amateur radio because they are low-cost, easy to build from copper pipe or wire, and provide a useful low-angle radiation pattern for local and repeater work.
How this J antenna calculator works
The calculator starts with the wavelength at your selected frequency, then applies your chosen shortening factor. The shortening factor compensates for real-world effects such as conductor diameter, nearby materials, and end effects.
Core formulas used
- Wavelength: λ = c / f
- Long element: 0.75 × λ × factor
- Short element: 0.25 × λ × factor
- Starting feedpoint height: about 0.04 to 0.06 × λ × factor from the bottom short
- Conductor spacing: about 0.01 to 0.02 × λ × factor as a practical starting range
These are practical starting dimensions, not guaranteed final values. Every antenna installation is different.
Step-by-step: using the results
1) Build slightly long
Cut the radiating and stub sections a little longer than calculated (for example 1–3% extra), then trim during tuning. It is easier to remove material than add it back.
2) Set initial feedpoint
Start near the middle of the recommended feedpoint range. Measure SWR. Move the feedpoint up or down in small increments to improve match.
3) Fine-tune resonance
If the resonant point is too low in frequency, shorten the radiating section slightly. If resonance is too high, lengthen if possible. Repeat until the antenna is tuned where you want it.
Typical build materials
- Copper pipe and fittings (rigid, weather resistant)
- Aluminum rod or tubing
- Wire and non-conductive support mast
- 50-ohm coaxial cable (RG-58, RG-8X, LMR variants, etc.)
- Choke/balun at feedpoint to reduce common-mode current
Practical tips for better performance
- Mount as high and clear as possible from metal obstructions.
- Keep feedline routing clean; avoid tight coils near the feedpoint.
- Add a current choke (coax turns or ferrite beads) just below feedpoint.
- Weatherproof exposed joints and coax connectors.
- Verify tuning in final installation location, not only on the workbench.
Safety and compliance notes
Keep antennas and masts far from power lines. Follow local electrical and building codes. If transmitting, make sure your station complies with license rules, RF exposure limits, and band plans in your country.