Calculate rough starting lengths for a J-Pole (Zepp) antenna for VHF/UHF radio.This tool provides radiating element and stub lengths only. Real builds also need element spacing, feedpoint tap position, velocity factor adjustment, and mechanical support design. Use as a starting point; validate against published designs for your exact frequency.
Last updated: April 2026 | RF & Amateur Radio Tool
Enter frequency in MHz (e.g., 146 for 2m band)
The J-Pole (also called a Zepp antenna) is an end-fed, half-wave radiator with a quarter-wave matching stub, creating an omnidirectional radiation pattern ideal for VHF/UHF communications. Unlike dipole antennas that require a feedline connection at the center, the J-Pole is fed at one end through a parallel transmission line that doubles as the matching stub. This design eliminates the need for a ground plane and provides 50Ω impedance matching without requiring an external tuner.
Design Principles: The J-Pole consists of two vertical conductors: a radiating element (half wavelength long) and a matching stub (quarter wavelength long), placed parallel and separated by a distance equal to the feedline width. The feedline connects near the bottom of the stub, where impedance is lowest. Current flows up the stub and across a short connection point to the radiator, then back down. This geometry naturally transforms the feedpoint impedance to approximately 50Ω, making it ideal for direct connection to standard coaxial cables without matching networks.
Advantages: J-Poles are simple to build from copper tubing or wire, require no ground plane (good for rooftop or portable use), provide omnidirectional coverage in the horizontal plane with favorable vertical radiation pattern, and achieve good efficiency without complex tuning. They're popular in amateur radio, weather station antennas, and VHF/UHF band applications. A single J-Pole can achieve 2-3 dB gain over a simple dipole due to end-fire coupling effects.
Use this calculator to find the radiating element (1/2 λ) and matching stub (1/4 λ) lengths for your operating frequency. Add 5% for velocity factor if using coaxial cable.
Obtain two pieces of 1/2" or 5/8" copper tubing (or 1/4" aluminum rod), copper wire for connections, PVC pipe for support, and 50Ω coaxial cable with connector (SO-239 or N-type recommended).
Cut two pieces to calculated lengths. Position them parallel and separated by 1-2 inches (feedline width). Secure with PVC spacers every 6-12 inches. The separation and spacing significantly affect impedance and radiation pattern.
Connect coax at approximately 6 inches from the bottom of the J-Pole, between the stub and radiator. The short connection point above feedpoint should be ~1-2 inches long. Solder connections securely; poor connections increase SWR and losses.
Use an SWR meter to check impedance matching. Target <1.5:1 SWR. If SWR is high, adjust feedpoint position slightly up or down, or adjust spacing between elements. Retest after each adjustment.
Build a J-Pole for 146 MHz (2m amateur band):
The antenna's shape resembles the letter 'J' when viewed from the side: a long vertical radiator (top of J) with a quarter-wave stub extending vertically below it (hook of J). The shape is distinctive and helped give the antenna its memorable name in amateur radio circles.
A dipole is fed at the center and requires two identical radiating elements. A J-Pole is fed at one end and includes a matching stub. J-Poles need no ground plane, offer better matching, and have omnidirectional patterns. Dipoles require mechanical balance and often need feedline tuners.
No, one major advantage of J-Poles. Unlike Yagis or verticals, J-Poles work well on rooftops, masts, or even portable tripods without ground plane elements. This makes them ideal for mobile and field operations.
Use larger diameter tubing (1/2" to 5/8") for lower losses, ensure tight solder joints at the feedpoint, keep the matching stub as straight as possible, and position higher for better horizon coverage. Adding a reflector element behind creates directivity.
Yes. Small adjustments to feedpoint position (up/down ~1 inch) or element spacing can tune impedance. For precise tuning, add small capacitive stubs near the feedpoint. Most J-Poles achieve good SWR on first build if dimensions are accurate.
Copper and aluminum are naturally weatherproof in most climates. Use UV-stable PVC spacers, ensure all solder joints are sealed (silicone caulk optional), and secure coaxial connections with waterproof tape. J-Poles typically last 10+ years outdoors with minimal maintenance.
Absolutely. Scale the dimensions for any frequency: 70cm, 33cm, 23cm, or higher bands. The formula remains 1/2 λ for radiator and 1/4 λ for stub. Smaller versions can be built from wire for portable HF use.
Well-designed J-Poles present 50-70Ω impedance at the feedpoint, giving 1-1.5:1 SWR without tuning. The matching stub's geometry determines final impedance. Wider spacing = higher impedance; narrower spacing = lower impedance. Optimize during construction.
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