J-Pole Antenna Calculator
Enter your target frequency and conductor properties to get every dimension you need to build a J-pole antenna. The calculator gives you the radiator length, matching stub length, feedpoint position, estimated 50-ohm stub impedance, and 2:1 SWR bandwidth. Switch between millimetres and inches. Results update as you type.
Formula
Worked example
For 146 MHz with VF = 0.95 and 6 mm conductor: lambda_eff = (299,792 mm / 146) x 0.95 = 1,951 mm. Radiator B = 975 mm, Stub C = 488 mm, Total A = 1,463 mm. Feedpoint D = 0.04 x 1,951 = 78 mm above stub base. With 25 mm spacing, Z0 = 276 x log10(50/6) = 276 x 0.921 = 254 ohms - note: actual impedance at the feedpoint depends on position along the stub, not Z0 directly.
What is a J-pole antenna?
A J-pole antenna (more formally, the J-type end-fed half-wave, or EFHW) is a popular omnidirectional vertical antenna used widely in amateur radio, APRS, public safety, and other VHF/UHF applications. It consists of two sections: a half-wave radiating element and a quarter-wave open-wire matching stub that is shorted at the bottom. The stub acts as a quarter-wave transformer, stepping the high impedance at the end of the half-wave radiator down to 50 ohms at the feedpoint, so the antenna can be connected directly to standard coaxial cable without a separate matching unit. The resulting shape - a long element with a short parallel section at the base - resembles the letter J, which gives the antenna its name. Because the radiator is a full half-wave, the J-pole has approximately 2 dBd of gain over a quarter-wave vertical, and it can be mounted at the top of a mast without a ground plane.
How to use this calculator
Enter the center frequency of the band you want to cover, the velocity factor of the conductor material (0.95 is a good starting point for bare copper or aluminium tubing), the outer diameter of the conductor, and the center-to-center spacing between the two parallel stub elements. The calculator returns all five key dimensions: total antenna height A, radiator length B, stub length C, feedpoint height D, and element spacing E. It also shows the characteristic impedance Z0 of the parallel-wire stub and an estimate of the 2:1 SWR bandwidth. Use the chart to see how all three major dimensions change across a +/- 20% frequency sweep around your target. After building, use an SWR meter or antenna analyzer to trim the top of the radiator and slide the feedpoint up or down for the lowest SWR at your exact frequency.
Velocity factor and why it matters
Radio waves travel slightly slower in a physical conductor than in free space. The velocity factor (VF) is the ratio of that speed to the speed of light in vacuum, expressed as a number between 0 and 1. For bare aluminium or copper tubing, VF is typically 0.95 to 0.97, meaning the wave travels at 95-97% of c. Because the wavelength in the conductor is proportional to speed, a lower VF produces shorter physical dimensions for the same electrical length. This is why the formulas for B and C multiply the free-space wavelength by VF before dividing. For wire antennas (as opposed to tubing), the thinner conductor results in a slightly lower VF, around 0.93-0.95. For ladder line or 300-ohm twin-lead used as the matching stub, VF is typically 0.88-0.97 depending on the dielectric. Using the correct VF is important: a 2% error in VF shifts the resonant frequency by 2%, which at 146 MHz moves the SWR minimum by about 3 MHz.
Feedpoint position and impedance matching
The matching stub is a shorted quarter-wave transmission line. At the short-circuit end (bottom), impedance is zero; at the open end (where the radiator connects), impedance is theoretically infinite. At some point partway up the stub, the impedance passes through 50 ohms - this is the feedpoint. For a 50-ohm coaxial feedline, that point is typically about 4% of the effective wavelength above the bottom, or roughly 20-25% of the stub length. The exact position depends on the ratio of stub impedance to 50 ohms. The stub impedance Z0 = 276 x log10(2s/d), where s is center-to-center spacing and d is conductor diameter. If Z0 is close to 50 ohms, the feedpoint sits low on the stub (closer to the short); if Z0 is higher, the feedpoint moves up. In practice, the best technique is to cut the stub slightly long, set the feedpoint at the calculated position, and then slide the connection point up or down in 5 mm increments while watching the SWR meter until you find the minimum.
Common frequency bands and typical J-pole dimensions
| Band | Frequency (MHz) | Radiator B (mm) | Stub C (mm) | Total A (mm) |
|---|---|---|---|---|
| 6 m amateur | 50.125 | 2,846 | 1,423 | 4,269 |
| 2 m amateur | 144.200 | 988 | 494 | 1,482 |
| 2 m APRS | 144.390 | 987 | 494 | 1,481 |
| VHF public safety | 155.340 | 916 | 458 | 1,374 |
| MURS (1-5) | 151.820 | 936 | 468 | 1,404 |
| 70 cm amateur | 432.100 | 329 | 165 | 494 |
| 70 cm simplex | 446.000 | 319 | 160 | 479 |
| UHF public safety | 460.000 | 309 | 155 | 464 |
| 900 MHz ISM | 915.000 | 155 | 78 | 233 |
Approximate dimensions for bare aluminium or copper tubing (VF = 0.95, 6 mm / 0.25 in conductor, 25 mm / 1 in spacing). Cut to these lengths and fine-tune with an SWR meter.
Frequently asked questions
What conductor diameter should I use for a 2-meter J-pole?
The most common choice for a homebrew 2-meter (144-148 MHz) J-pole is 1/2-inch (12.7 mm) or 3/4-inch (19 mm) copper or aluminium tubing for indoor mounts, and 3/4-inch to 1-inch tubing for outdoor use. A larger diameter increases the 2:1 SWR bandwidth - 1/2-inch tubing typically gives 3-5 MHz of usable bandwidth, which easily covers the entire 2 m amateur band. Smaller diameters such as 3/16-inch (4.7 mm) copper pipe or even coat-hanger wire work but produce a narrower bandwidth and are harder to tune.
How do I find the feedpoint height in practice?
Start at the calculated feedpoint height D and connect a coaxial cable - braid to one element, center to the other. Add a 1:1 choke balun or a few ferrite beads on the coax just below the feedpoint to prevent common-mode current from running back down the cable. Then use an SWR meter or antenna analyzer to check SWR at your target frequency. Slide the feedpoint up or down in 5-10 mm steps and remeasure until you reach the lowest SWR. A well-built J-pole should achieve SWR below 1.5:1 at the target frequency.
Does the J-pole need a ground plane?
No - that is one of its key advantages. Because the radiating element is a center-fed half-wave section (electrically speaking) with the matching stub providing the RF ground reference, the J-pole does not require a counterpoise or ground plane. This makes it ideal for mounting on the top of a non-conductive mast, the peak of a roof, or inside an attic. By contrast, a quarter-wave vertical or a rubber-duck antenna rely on a ground plane or the radio chassis to form the other half of the antenna.
Why is the stub impedance output important?
The characteristic impedance Z0 of the parallel-wire stub determines where along the stub the impedance equals 50 ohms. If Z0 is very low (close to 50 ohms), the feedpoint is near the bottom of the stub. If Z0 is much higher (150-300 ohms, which is common for widely spaced elements), the feedpoint is higher up. Knowing Z0 helps you predict the starting feedpoint height more accurately, reducing the number of trial-and-error adjustments needed on the bench.
What is the gain of a J-pole antenna?
A J-pole has approximately 2.1 dBi (0 dBd) of gain, similar to a half-wave dipole. The gain comes from the current distribution in a half-wave element, which concentrates radiation toward the horizon and reduces it at high angles (the wanted pattern for terrestrial communications). The matching stub contributes negligible radiation because the opposing currents in its two conductors cancel. Do not expect J-pole gain figures above 3 dBi - claims of 5-7 dBi from some commercial listings are marketing exaggerations for this antenna type.
Can I use twin-lead or ladder line as the matching stub?
Yes, and many builders do. 300-ohm TV twin-lead and 450-ohm open-wire ladder line have been used as the matching stub since the antenna was first described. The main adjustment is velocity factor: 300-ohm twin-lead is typically 0.82-0.88 and 450-ohm ladder line is 0.91-0.97 depending on construction. Plug the correct VF into this calculator to get the right stub length. The main practical advantage of ribbon line is that the spacing is fixed and uniform along the entire length, which makes construction easier. The downside is lower power handling and weathering issues unless the ribbon is protected.
How do I convert the dimensions to inches if I am working in fractions?
Switch the unit selector to imperial to get all dimensions in decimal inches. To convert to a fraction, multiply the decimal part by 16 (for sixteenths) or 32 (for thirty-seconds) and round to the nearest whole number. For example, 38.4 inches = 38 and 6/16 inches, which is 38-3/8 inches. For accurate metalwork, it is more practical to use a millimetre rule or a digital caliper in decimal-inch mode rather than trying to work in common fractions.