J-Pole Antenna Calculator

J-Pole Antenna Dimensions

The J-pole antenna is a popular omnidirectional antenna for VHF and UHF amateur radio. It consists of a half-wave radiating section and a quarter-wave matching stub, both made from a single conductor. This calculator gives standard element lengths based on operating frequency.

Formulas

$$\lambda = \frac{c}{f}$$

Element lengths for a standard J-pole:

Long radiating element: $0.75\lambda$
Short element: $0.25\lambda$
Matching stub: $0.25\lambda$

$c$ is the speed of light (299,792,458 m/s), $f$ is frequency in hertz, and $\lambda$ is wavelength. Enter frequency in MHz for convenience.

Building Tips

Real J-poles are typically built from copper pipe or ladder line. Actual dimensions may need slight trimming for lowest SWR at the target frequency. Mount the antenna vertically with the feed point at the bottom of the matching stub for best omnidirectional pattern.

Frequently Asked Questions

What frequency bands work with a J-pole?

J-poles are commonly built for 2 m (144–148 MHz) and 70 cm (420–450 MHz) amateur bands. Enter your target center frequency in MHz.

Why 0.75λ for the long element?

The J-pole acts as an end-fed half-wave radiator combined with a quarter-wave matching section. The 0.75λ dimension accounts for the folded geometry of the long section.

Do I need to adjust for velocity factor?

These are free-space wavelengths. Conductors in air are close to free space, but trimming 2–5% shorter after initial construction is common for lowest SWR.

What feed impedance does a J-pole present?

A properly built J-pole presents approximately 50 Ω at the feed point, matching standard coaxial cable without a separate matching network.

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J-Pole Antenna Dimensions

Formulas

Building Tips

Frequently Asked Questions

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