J-Pole Antenna Calculator

The J-pole antenna relies on a half-wave radiating element matched to a quarter-wave stub, forming a 'J' shape that provides a natural 50-ohm impedance match without requiring external coils or capacitors. The foundational calculation starts with finding the wavelength (lambda) using the speed of propagation and the operating frequency: wavelength = speed / freq. From this base wavelength, the electrical lengths are scaled by the velocity factor of your construction material. The long radiator section is set to 75% of the wavelength (long_section = velocity * wavelength * 0.75), while the short tuning stub is set to 25% (short_section = velocity * wavelength * 0.25). The feed point for your coaxial cable is typically located a short distance up the stub (feed_point = velocity * wavelength * 0.025), and element spacing is traditionally maintained at a fraction of the wavelength to ensure proper electromagnetic coupling.

Let us design a J-pole antenna operating at a frequency of 146 MHz (in the 2-meter ham band) using standard copper tubing in open air, where the propagation speed is approximately 299,792,458 m/s and the velocity factor is 1.0. First, calculate the wavelength: wavelength = 299,792,458 / 146,000,000 = 2.053 meters. Next, calculate the long radiator section: 1.0 * 2.053 * 0.75 = 1.540 meters (about 154 cm). For the short matching stub, multiply by 0.25: 1.0 * 2.053 * 0.25 = 0.513 meters (about 51.3 cm). The coaxial feed point attachment is located at 1.0 * 2.053 * 0.025 = 0.051 meters (5.1 cm) from the bottom short, and the parallel element spacing is calculated using (0.045 * 2.053) / 2 = 0.046 meters (4.6 cm). These precise physical measurements yield a resonant 146 MHz antenna with a low standing wave ratio (SWR).

When constructing your antenna, always account for the physical thickness of your tubing or wire, as thicker elements slightly shorten the required electrical length. If you are mounting your copper J-pole inside a PVC pipe for weather protection, remember that the dielectric constant of the plastic housing will alter the velocity factor, requiring a recalculation of your dimensions. Finally, always tune your feed point connections using an SWR meter or antenna analyzer to achieve the absolute lowest reflection coefficient before mounting the antenna at a high elevation.