J-Pole Antenna Calculator
Calculate element lengths, spacing, feed point, and EIRP for your J-pole antenna build.
Enter the operating frequency and transmit power to get all critical J-pole dimensions instantly.
J-Pole Antenna Calculator
Calculate element lengths, spacing, feed point, and EIRP for your J-pole antenna build.
About the J-Pole Antenna Calculator
The J-pole antenna is one of the most popular wire antennas among amateur radio operators and professional VHF/UHF users alike. Its name comes from its distinctive shape—resembling the letter J—formed by a half-wave radiating element joined to a quarter-wave matching stub. Unlike a simple dipole, the J-pole does not require a ground plane, making it ideal for portable, base, or repeater station installations.
The operating principle is straightforward. A half-wavelength radiating section sits atop a quarter-wavelength matching stub, with a parallel transmission-line feed at the bottom. The current distribution on the radiating element produces an omnidirectional radiation pattern in the azimuthal plane and a theoretical gain of approximately 2.15 dBd (or 4.3 dBi) over an isotropic radiator.
This calculator uses the standard J-pole design equations with a velocity factor of 0.95 applied to account for the physical shortening of conductors in practice. The wavelength λ is derived from λ = 300 / f (MHz). The long radiating element is 3/4 × λ × VF, and the short matching stub is 1/4 × λ × VF. The element spacing and feed-point height are set to industry-standard fractions of the wavelength.
Effective Isotropic Radiated Power (EIRP) combines the transmitter output power with the antenna gain in dBi to express the total radiated power referenced to an isotropic source. Higher EIRP means a stronger signal reaching your target area without increasing transmitter power.
The J-pole is widely used on the 2-meter (144–148 MHz) and 70-centimeter (420–450 MHz) amateur radio bands but can be scaled to any VHF or UHF frequency. Common construction materials include copper pipe for sturdy base installations or twin-lead feedline for lightweight portable designs.
Always verify your finished antenna with an SWR meter or antenna analyzer before applying full power, and ensure compliance with your local amateur radio licensing conditions.
Worked Examples
These examples demonstrate J-pole dimensions for common amateur radio frequencies.
| Input (Freq / Power) | Key Dimensions | Use Case |
|---|---|---|
| 146.52 MHz / 50 W | λ=2.0503 m, long=1.5377 m, short=0.5126 m, EIRP=134.7 W | 2m calling frequency |
| 446 MHz / 25 W | λ=0.6726 m, long=0.5045 m, short=0.1682 m, EIRP=67.3 W | 70cm PMR446 / UHF |
| 162.4 MHz / 5 W | λ=1.847 m, long=1.385 m, short=0.462 m, EIRP=13.5 W | NOAA Weather Radio |
How to Use This Calculator
- Enter your operating frequency in MHz into the Frequency field.
- Enter your transmitter output power in watts.
- Optionally fill in the coax length, type, and antenna height for reference.
- Click Calculate to see the full set of element dimensions and EIRP.
- Use the example buttons to pre-fill common amateur radio frequencies.
Frequently Asked Questions
What is a J-pole antenna?
A J-pole is an end-fed omnidirectional antenna consisting of a half-wave radiating element and a quarter-wave parallel matching stub. It requires no radials or ground plane, making it suitable for rooftop or portable use.
Why is a velocity factor of 0.95 used?
In practice, the speed of an electromagnetic wave along a conductor is slightly less than the speed of light in free space. A velocity factor of 0.95 accounts for this and results in the correct physical cut lengths for copper or aluminum elements.
What is EIRP and why does it matter?
EIRP (Effective Isotropic Radiated Power) is the total power the antenna would need to emit uniformly in all directions to produce the same signal strength at a given point. It combines transmitter power with antenna gain and is used by regulators and link-budget engineers to characterize transmitter systems.
Can I build a J-pole for frequencies other than 2m and 70cm?
Yes. This calculator works for any frequency where the physical element sizes are practical. J-poles are commonly built for 6m, 2m, 1.25m, 70cm, and 33cm amateur bands as well as commercial VHF/UHF channels.
How do I match the J-pole to 50-ohm coax?
Slide the coax tap point up or down the matching stub until the SWR is minimized. Typically the tap is placed about 5% of the wavelength from the bottom of the stub, but trimming on a real antenna with an SWR meter gives the best results.
Does the calculator account for coax losses?
The EIRP result is calculated from the transmitter power and antenna gain only. To account for coax losses, subtract the feedline loss (in dB) from the transmitter output before entering it, or reduce the calculated EIRP by the feedline loss factor.