How to Use J-Pole Antenna Calculator
The J-Pole Antenna Calculator produces all four key dimensions from a single frequency entry.
- Enter the Operating Frequency — Type the centre frequency in MHz (e.g. 146.52 for the 2-m simplex calling frequency).
- Choose the Velocity Factor — Select a preset (bare copper wire ≈ 0.95, coax ≈ 0.66) or enter a custom value. The J-Pole Antenna Calculator adjusts all dimensions accordingly.
- Read the Four Dimensions — The results panel shows the radiator length (3λ/4), stub length (λ/4), feed-point position, and gap width, all in both metres and inches.
- Build and Tune — Cut to the calculated dimensions, attach the feed line at the calculated feed-point height from the bottom of the stub, and fine-tune SWR by sliding the feed connection up or down.
Formula & Theory — J-Pole Antenna Calculator
The J-Pole Antenna Calculator derives all dimensions from the free-space wavelength, corrected by the velocity factor:
λ_free = 300 / f (MHz) [metres]
Radiator = 0.750 × λ_free × VF
Stub = 0.250 × λ_free × VF
Feed pt = 0.0455 × λ_free × VF (from short end of stub)
Gap = 0.013–0.025 × λ_free × VF| Symbol | Meaning | Unit |
|---|---|---|
| f | Operating frequency | MHz |
| λ_free | Free-space wavelength | m |
| VF | Velocity factor (0–1) | — |
| Radiator | Half-wave radiating element | m |
| Stub | Quarter-wave matching section | m |
| Feed pt | Distance from stub bottom | m |
| Gap | Separation between conductors | m |
Velocity Factor Reference
| Material | Typical VF |
|---|---|
| Bare copper wire | 0.95–0.97 |
| Twin-lead (300 Ω) | 0.82 |
| RG-58 coax | 0.66 |
| RG-8X coax | 0.78 |
Use Cases for J-Pole Antenna Calculator
The J-Pole Antenna Calculator is a practical tool for builders at all experience levels:
- Amateur radio (ham) — Hams use the J-Pole Antenna Calculator to build lightweight VHF/UHF antennas from copper pipe or twin-lead for home base stations.
- APRS and packet radio — Digipeater and IGate installations benefit from the J-pole’s omnidirectional pattern and no-ground-plane design on rooftops.
- Emergency communications (EMCOMM) — The J-pole is a popular field-deployable antenna for community emergency radio teams; the calculator lets volunteers build one quickly from available materials.
- Repeater installations — Repeater trustees use the J-Pole Antenna Calculator to size antennas for building rooftops where a ground plane cannot be added.
- NOAA weather radio reception — A correctly sized J-pole dramatically improves weather alert reception in fringe areas.
- Antenna theory education — Electronics instructors use the calculator to demonstrate wavelength, velocity factor, and impedance matching concepts.