J-Pole Antenna Calculator
A J-pole is a simple, omnidirectional, gain-free vertical antenna made from a half-wave radiating element fed by a quarter-wave matching stub. Enter your target frequency and conductor velocity factor to get all the physical dimensions ready for construction.
MHz
Output unit
Length of the main half-wave vertical radiating element
- 1
Free-space wavelength λ₀ = c ÷ f
299,792,458 m/s ÷ (146 × 10⁶ Hz) = 2.0534 m - 2
Velocity-corrected wavelength λ = λ₀ × vf
2.0534 m × 0.95 = 1.9507 mVelocity factor shortens the electrical wavelength of the conductor. - 3
Radiating element = λ ÷ 2
1.9507 m ÷ 2 = 97.5 cm
How does this calculator work?
λ = (c / f) × vf. Radiating element = λ/2, matching stub = λ/4, gap ≈ stub/12, feed tap ≈ 4% of stub from bottom. At 146 MHz with vf = 0.95: radiating element ≈ 97.5 cm, stub ≈ 48.7 cm. Trim to minimise SWR after construction.
Formula
How this is calculated
The J-pole (sometimes called a Zepp antenna because of its historical use on Zeppelin airships) consists of a half-wave vertical radiator fed through a quarter-wave shorted matching stub. The stub transforms the high impedance at the end of the half-wave element down to a low impedance at the 50 Ω coax feed point. A small air gap between the bottom of the stub and the bottom of the radiating element prevents the outer shield of the coax from radiating.
The velocity factor (vf) accounts for the fact that RF travels along a conductor slightly slower than in free space. Copper tubing has vf ≈ 0.95; 300 Ω TV twin-lead ≈ 0.91; 450 Ω ladder line ≈ 0.93. Multiplying the free-space wavelength by vf gives the physical cut length. The feed point tap (approximately 4% of the stub length from the shorted bottom) is moved up or down during trimming to find the best SWR.
All dimensions are computed from the standard formulas used in amateur radio literature (e.g. the ARRL Antenna Book). Real-world antenna lengths typically need 1–3% trimming once built because conductor diameter, end effects and installation height above ground all affect the exact resonant frequency.
Frequently asked questions
Yes — it directly scales every dimension. Copper tubing at 0.95 versus 300 Ω ribbon at 0.91 gives a 4% difference in element length, which shifts resonance by several MHz on VHF bands.
The feed-point impedance at the tap is approximately 50 Ω, close enough to coax for a direct connection. You tune by moving the tap up (lower impedance) or down (higher impedance) until the SWR is minimised. A choke balun at the feed point is recommended to prevent the coax braid from radiating.
A J-pole has approximately 2.1 dBi of gain (0 dBd), the same as any half-wave dipole — it is not a gain antenna. Its advantage is mechanical: it is self-supporting, easily built from copper plumbing or ribbon cable, and can be fed directly with 50 Ω coax.
Also known as
TG we-Calculate Editorial Team. (2026). J-Pole Antenna Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/j-pole-antenna-calculator
TG we-Calculate Editorial Team. "J-Pole Antenna Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/j-pole-antenna-calculator.
TG we-Calculate Editorial Team, "J-Pole Antenna Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/j-pole-antenna-calculator
@misc{wecalculate_j_pole_antenna_calculator, title = {J-Pole Antenna Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/j-pole-antenna-calculator}}, year = {2026}, note = {TG we-Calculate} }
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