The J-pole antenna calculator above gives the four dimensions that define a J-pole: the total length of the long element, the length of the short parallel stub beside it, the spacing between the two conductors, and where on the stub the feedline attaches. It is one of the most widely built amateur antennas because it needs no radials, no balun in the conventional sense, and nothing more exotic than copper pipe and a couple of fittings.
Arb Digital publishes free engineering calculators that say plainly which parts of an answer are calculated and which must be found by adjustment. Three of the four dimensions here follow from the wavelength. The fourth, the feed tap, does not: it depends on conductor diameter, spacing and feedline in ways no closed-form expression captures well, and every serious J-pole build ends with someone sliding a clamp up and down while watching an analyser.
Transmitting Requires a Licence
Building an antenna is unrestricted almost everywhere. Connecting a transmitter to it is not. Deliberately radiating radio-frequency energy requires a licence in most jurisdictions, with allocated bands, power limits and technical conditions that vary by country. The two-metre and seventy-centimetre bands this antenna is usually built for are amateur allocations, and using them without a licence is an offence in most places. The ARRL guide to getting licensed describes the process for the amateur service in the United States, and every other country has an equivalent national regulator.
Power limits are almost always expressed as a radiated figure rather than transmitter output, so antenna gain and feeder loss both enter the calculation. Use the EIRP calculator to work out where your station actually sits against a limit once those are accounted for. There is also a separate radio-frequency exposure obligation in many jurisdictions, which is about distance from the radiating element rather than about licensing.
What This J-Pole Antenna Calculator Does
A J-pole is an end-fed half-wave radiator with a quarter-wave matching section built into its base. The half-wave radiator on its own has a very high feedpoint impedance at the end, in the thousands of ohms, which no coaxial cable can drive directly. The quarter-wave shorted stub underneath it is a transmission-line transformer: shorted at the bottom, it presents a very high impedance at the top, and tapping into it part way up finds a point where the impedance matches fifty ohms.
Geometrically that means one long conductor running the full three quarters of a wavelength, and one short conductor running a quarter wavelength beside it, joined by a shorting bar at the bottom. The tool computes each of those with the velocity factor applied, plus the spacing and a starting tap position.
The hero figure is the total length of the long element. The grid gives the stub, the radiator portion that projects above the stub top, the conductor spacing and the feed tap height.
How to Use It
- Enter the centre of your operating range. Cutting for a band edge wastes half the antenna's usable bandwidth on frequencies you never use.
- Set the velocity factor for your material. Thin wire sits near 0.98; half-inch copper pipe at 146 MHz is nearer 0.95. This factor moves the lengths by several centimetres at VHF.
- Choose a spacing you can actually build. Around one fiftieth of a wavelength is conventional, but a spacing that matches available fittings is more practical than one that matches a formula.
- Cut long and trim. Add a few per cent to the radiator, assemble, then shorten in small steps with an analyser connected until resonance lands where you want it.
- Find the tap by sliding, not by calculation. Use hose clamps rather than solder for the first fitting, and move the tap up and down for minimum reflected power before making anything permanent.
The Formula: How the Dimensions Are Derived
Free-space wavelength is λ = c ÷ f, using the exact NIST CODATA value for the speed of light in vacuum of 299,792,458 m/s. Every physical dimension is then a fraction of that wavelength multiplied by the velocity factor k, which accounts for the end effect and the finite conductor diameter.
The matching stub is a quarter wave: Lstub = 0.25λk. The radiator above the stub top is a half wave: Lrad = 0.5λk. The long element is the sum of the two, three quarters of a wavelength: Llong = 0.75λk. Spacing is your chosen fraction of the free-space wavelength, and the feed tap is your chosen percentage of the stub length measured up from the shorting bar.
Work the defaults through by hand at 146 MHz with k = 0.96. The free-space wavelength is 299,792,458 ÷ 146,000,000 = 2.0534 m. A quarter of that is 0.51334 m, and multiplying by 0.96 gives a stub of 0.4928 m, or 49.28 cm (19.40 in). The radiator is twice the stub, 98.56 cm (38.80 in), and the long element is three times it, 147.84 cm (58.21 in). Spacing at 0.02λ is 4.11 cm, and a tap at 10 per cent of the stub sits 4.93 cm above the shorting bar.
Where the J-Pole Differs From a Dipole
Both are half-wave radiators, and that is where the similarity ends. A dipole is fed at its centre, where the impedance is around seventy ohms and the structure is electrically balanced. The dipole antenna calculator covers that case: it computes a total element length and each leg, and it exists in a world where a balun is the standard answer to connecting unbalanced coax to a balanced antenna.
A J-pole is fed at the end, where the impedance is enormous, and the quarter-wave stub does the transformation that a balun and a matched feed would otherwise handle. That difference produces a genuinely different antenna. The J-pole is vertically polarised and omnidirectional in azimuth, which is what makes it a repeater and scanner favourite. It has no radials, so it mounts on a single mast without a ground plane. And it is a physically taller structure for the same band, because it is three quarters of a wavelength rather than a half.
Which one to build depends on the job. If you want a horizontally polarised antenna for weak-signal work, build a dipole. If you want a vertical for repeater work that survives weather and needs no radials, build a J-pole.
The Common Mode Problem Nobody Mentions
The J-pole's reputation for not needing a balun is misleading, and it is the most common reason a home-built one behaves badly. The structure is unbalanced and the feedline connects to a point on the stub, not to a balanced pair. Radio-frequency current can and does flow on the outside of the coax braid, which turns the feedline into part of the radiating system.
The consequences are practical rather than theoretical. The radiation pattern tilts and distorts, because the feedline is now an unintended radiator hanging below the antenna. The measured standing wave ratio changes when you move the coax or touch it, which is the diagnostic symptom. And radio-frequency energy comes back into the shack, sometimes upsetting the transmitter or nearby equipment.
The fix is a common-mode choke at the feedpoint: several turns of the coax through a suitable ferrite, or a coiled section of coax, placed right at the tap. It is the single most worthwhile addition to a J-pole and it is left out of most build descriptions. Mounting matters too: a metal mast running up close beside the stub couples into the antenna and detunes it, so keep the mast below the shorting bar or use a non-conductive section.
Tuning a J-Pole in Practice
Two adjustments do different jobs and it is worth keeping them separate. Trimming the radiator length moves the resonant frequency. Moving the feed tap changes the impedance match at that frequency. If you adjust both at once you will chase the antenna around for a long time.
The usual sequence is to set the tap somewhere sensible, find where the antenna is resonant, trim the radiator until resonance lands on your target frequency, then slide the tap for the lowest reflected power without touching the length again. Expect a small interaction and one more pass around the loop.
Do the work in the antenna's final position and orientation. A J-pole tuned on a workbench and then mounted on a metal mast against a wall will not be tuned any more. The ARRL's antenna reference material covers measurement technique for exactly this kind of adjustment.
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Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Cutting to the calculated length exactly — the velocity factor is an estimate for your specific tubing, so cut long and trim to resonance with an instrument connected.
- Omitting a common-mode choke — a J-pole is an unbalanced structure and feedline current distorts the pattern and changes the reading whenever the coax moves.
- Running a metal mast up beside the stub — the mast couples into the matching section and detunes the antenna; keep it below the shorting bar.
- Soldering the tap before tuning — the correct tap point depends on spacing, tubing diameter and feedline, so use clamps until the match is confirmed.
- Tuning indoors and mounting outdoors — nearby walls, wiring and metalwork all shift resonance, so final adjustment belongs in the installed position.
Related Free Tools From Arb Digital
Compare this against the dipole antenna calculator for the centre-fed balanced case, and use the EIRP calculator to check radiated power against a regulatory limit. The wavelength calculator gives the free-space wavelength directly for any frequency, the impedance matching calculator covers lumped-element matching where a stub is not practical, and the cable impedance calculator and insertion loss calculator cover the feedline. For the link itself, the free space path loss calculator estimates what survives the journey. Everything Arb Digital publishes is on the free online tools hub.
Frequently Asked Questions
It is an end-fed half-wave radiator with a quarter-wave shorted stub built into its base. The end of a half-wave element has a very high impedance that coax cannot drive. The shorted quarter-wave stub acts as a transmission-line transformer, and tapping into it a short distance above the short finds a point where the impedance is close to fifty ohms.
A dipole is fed at its centre where the impedance is around seventy ohms and the structure is balanced, so it normally needs a balun. A J-pole is fed at the end through a quarter-wave matching stub, is vertically polarised and omnidirectional, needs no radials, and stands three quarters of a wavelength tall rather than half.
It does need a common-mode choke, despite the common claim that it does not. The structure is unbalanced and current flows on the outside of the coax braid, which makes the feedline part of the radiating system. The symptom is a standing wave ratio that changes when you move or touch the coax. Several turns of coax through ferrite at the feedpoint fixes it.
Because the velocity factor depends on your specific conductor diameter, and because nearby metal detunes the antenna. Thick tubing has a lower factor than thin wire, and a mast, wall or roof close to the element shifts resonance further. This is why the standard advice is to cut long and trim in the final mounting position.
Published designs place it somewhere between about five and fifteen per cent of the stub length above the shorting bar, and the right point depends on spacing, tubing diameter and feedline. No formula predicts it reliably, so fit the connection with adjustable clamps, slide it while watching an analyser, and fix it permanently only once the reflected power is at a minimum.
A two-metre J-pole often works acceptably on seventy centimetres because the third harmonic falls in that band, and many commercial designs exploit this. The match is usually worse than a purpose-built antenna and the radiation pattern breaks into multiple lobes at the higher frequency, so treat dual-band operation as a useful bonus rather than a design goal.
To transmit, yes, in most jurisdictions. The bands these antennas are usually built for are amateur allocations with licensing requirements, power limits and technical conditions that differ by country. Receiving is generally unrestricted and building the antenna is unrestricted, but connecting a transmitter puts you inside the licensing rules. Check with your national regulator.
This tool is provided for educational and preliminary design use. Dimensions are first-order figures from published wavelength relations and will need trimming for your conductor diameter and mounting environment; the feed tap position in particular must be found by adjustment. Transmitting requires a licence in most jurisdictions, and radio-frequency exposure limits are a separate obligation — confirm both with your national regulator.
