What this calculator does
A J-pole is a half-wave radiator fed by a quarter-wave matching stub, which is what gives it the J shape. At 146 MHz with a 0.95 velocity factor, the long element is 1,464 mm and the stub 488 mm.
The stub is exactly a third of the radiator length, always, because three quarters of a wavelength divided by one quarter is three. That ratio is a useful check on any J-pole dimension table you come across: if the two elements are not in a 3:1 ratio, something is wrong.
The formula
The free-space wavelength is 300 divided by the frequency in megahertz, giving metres. The radiator is three quarters of a wavelength and the matching stub one quarter, both multiplied by the velocity factor to account for the conductor. The feedpoint sits a short distance up from the base and is found by adjustment rather than calculation.
| Term | Meaning |
|---|---|
| Velocity factor | How much slower than light the signal travels along the conductor, typically 0.95 for bare tube. |
| Matching stub | The quarter-wave section transforming the radiator impedance to match the feedline. |
| Feedpoint | Where the coax attaches to the stub, tuned for lowest SWR. |
| SWR | Standing wave ratio, the measure of how well the antenna matches the feedline. |
The inputs explained
| Field | What to enter |
|---|---|
| Centre frequency (MHz) | Centre frequency in megahertz. Use the middle of the band you want to cover. |
| Velocity factor | Velocity factor of the conductor. 0.95 suits bare copper pipe or aluminium tube. |
When to use it
Building a 2 metre J-pole
The classic amateur radio construction project, usually made from copper pipe.
Cutting elements for a new band
The same geometry scales to any frequency by changing the wavelength.
Checking a published design
Dimensions found online can be verified against the 3:1 element ratio.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What dimensions for each band?
J-pole dimensions across three amateur bands.
| Frequency | Long element (radiator) | Short element (matching stub) | Free-space wavelength |
|---|---|---|---|
| 52 MHz | 4,110.6 mm | 1,370.2 mm | 5.769 m |
| 146 MHz | 1,464.0 mm | 488.0 mm | 2.055 m |
| 446 MHz | 479.3 mm | 159.8 mm | 0.673 m |
Questions
Why is the J-pole fed at the bottom?
Because the quarter-wave stub acts as an impedance transformer. The half-wave radiator has a very high feedpoint impedance, which cannot be driven directly by 50 ohm coax. The stub transforms it down, and moving the tap point along the stub selects the match.
How do I tune it?
By moving the feedpoint along the stub for lowest SWR, not by trimming the elements. The element lengths set the frequency; the tap position sets the match. Adjusting the tap is straightforward and reversible, which is why it is the tuning control.
What velocity factor should I use?
About 0.95 for bare copper pipe or aluminium tube, which is the usual J-pole construction. Insulated wire is slower, often 0.90 or below, and needs a correspondingly shorter element. If in doubt, cut slightly long and trim.
Does a J-pole need a ground plane?
No, which is one of its advantages over a quarter-wave vertical. The stub provides the counterpoise, so the antenna is self-contained and can be mounted on a mast without radials. Some common-mode current can still flow on the feedline, so a choke is worth fitting.
For a simple dipole instead, see the dipole antenna length calculator. For signal timing, see the propagation delay calculator.