J-Pole Antenna Length Calculator
Enter your operating frequency and velocity factor to estimate J-pole element lengths, feed-point starting location, and spacing guidelines.
What this J-pole calculator does
A J-pole antenna is a practical, no-radial vertical antenna often used for VHF and UHF ham radio work. This calculator gives you a fast, field-friendly starting point for cutting your elements before final tuning. It computes the key dimensions from your target frequency:
- Long element (radiator): approximately 3/4 wavelength
- Short element (matching stub): approximately 1/4 wavelength
- Suggested feed-point height: a starting range above the shorted end
- Suggested element spacing range: practical spacing to begin tuning
J-pole dimension basics
The classic J-pole is built from two parallel conductors connected together (shorted) at the bottom. The long side becomes the radiating element; the short side functions as the matching section. Because real conductors are not ideal and the surrounding structure affects resonance, exact dimensions always need trimming and feed-point adjustment.
Core formulas used
The calculator starts from wavelength:
λ (meters) = 299.792458 / frequency (MHz)
Then it applies your velocity factor to estimate physical conductor length:
- Short element = λ/4 × VF
- Long element = 3λ/4 × VF
- Feed-point start = 0.05λ × VF (with range 0.03λ to 0.07λ)
How to use these results in a real build
1) Cut slightly long
Cut both elements about 1-2% longer than the calculated value. It is safer to trim than to add metal back later.
2) Assemble with stable spacing
Keep the spacing between elements mechanically rigid and consistent. Variations in spacing can shift your resonance and feed impedance.
3) Start feed-point at calculated location
Attach the coax center conductor to the long element and coax shield to the short element at the suggested height above the shorted end. Then move the feed point up/down a little to improve match.
4) Tune for lowest SWR at operating frequency
After feed-point adjustment, trim the top of the long element in very small increments. Re-test after each trim. A little movement can make a large difference.
Typical pitfalls to avoid
- Ignoring velocity factor and using free-space lengths directly
- Using very long coax while tuning and misreading SWR behavior
- Mounting too close to metal masts, gutters, or wiring during initial tests
- Making large cuts too early instead of gradual trimming
Example (2 meter band)
For a design around 146.52 MHz with VF = 0.95, you can expect roughly:
- Short element near 19 inches
- Long element near 57 inches
- Feed-point initial position near 3.8 inches above the shorted base
From there, fine-tune for your actual installation environment and target segment of the band.
Final thoughts
A J-pole is popular because it is simple to build, rugged, and can perform very well when tuned carefully. Use this calculator to get dependable starting dimensions, then trust your measurement tools for final optimization. A thoughtful build and patient tuning process usually beats chasing “perfect” formula numbers on paper.