J-Pole Antenna Calculator: 2m & 70cm Dimensions + Build Guide

⚡ Instant Calculator

J-Pole Radiator, Stub & Feedpoint

Radiator = 468/f, stub = 234/f — the same constants as a dipole and vertical, applied to one J-shaped conductor.

Frequency (MHz)

Conductor Material

Radiator Length

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Stub Length

—

Feedpoint (from shorted end)

—

Total Height

—

Radiator = 468/f × velocity factor, stub = 234/f × velocity factor, both in feet. Feedpoint distance is an empirical starting point (~20% up the stub from the shorted end) derived from many real builds, not a clean formula — always tune it by sliding the connection point for lowest SWR. Build slightly long and trim down.

📋 In This Guide

How the J-pole calculator works
Why a 2m J-pole won't work on 70cm
The common-mode current problem
Building & tuning your J-pole
J-pole vs. ground plane vs. dipole
Common mistakes
Other antenna tools on KE6MGB
Frequently asked questions

📐 How the J-Pole Calculator Works

A J-pole is an end-fed half-wave radiator — electrically the same wire length as a dipole — paired with a quarter-wave matching stub that transforms the radiator's very high end-fed impedance down to something close to 50Ω. That's why the math borrows directly from formulas we've already covered: the radiator uses 468 ÷ f (the same dipole constant), and the stub uses 234 ÷ f (the same quarter-wave vertical constant).

The one factor that matters more here than on our HF wire-antenna calculators is velocity factor. At VHF/UHF, the physical size of the conductor relative to the wavelength is large enough that the conductor material meaningfully changes the result — anywhere from 0.90 for large-diameter tubing to 0.98 for thin bare wire. Getting this wrong shifts your resonant frequency noticeably more than it would on an HF wire dipole.

468/f × VF

Radiator length (ft)

234/f × VF

Stub length (ft)

0.90–0.98

Velocity factor range

~50Ω

Target feed impedance

Worked example — 2m J-pole at 146 MHz, copper pipe (VF 0.95):
Radiator: 468 ÷ 146 × 0.95 ≈ 3.05 ft (≈ 36.6 in)
Stub: 234 ÷ 146 × 0.95 ≈ 1.52 ft (≈ 18.3 in)
Feedpoint: roughly 20% up the stub from the shorted end as a starting point, then tuned for lowest SWR

🔍 Why a 2m J-Pole Won't Work on 70cm

This trips up a lot of new builders, and it's worth understanding clearly. The 3rd harmonic of a 2m J-pole's design frequency (say, 146 × 3 = 438 MHz) does land inside the 70cm band (420–450 MHz) — so on paper it looks like one antenna should just work on both bands.

It doesn't, because the matching stub's impedance transformation is only correct at its designed fundamental frequency. At 3× that frequency, the stub presents a badly mismatched impedance to the radiator — commonly producing SWR above 10:1, even though the frequency itself is technically right where you'd want it. The frequency lines up; the matching network does not.

⚠️

True Dual-Band J-Poles Use a Second Element, Not a Harmonic

Real dual-band 2m/70cm J-pole designs add a separate second radiating element for the higher band rather than relying on the harmonic of the same stub. If you want genuine dual-band coverage, build (or buy) a design with two distinct radiating sections — don't expect a single-band J-pole to cover both just because the numbers seem to line up.

🛡️ The Common-Mode Current Problem

J-poles have a real, well-documented reputation for inconsistent SWR and pattern problems — and the cause is usually common-mode current, not a poorly built stub. Here's the part most builders get wrong: the matching stub itself does nothing to prevent this. Its only job is impedance transformation. Decoupling the feedline and any supporting mast from RF current is a completely separate problem that needs its own solution.

Two genuinely effective, well-documented fixes:

Use a non-conductive mast. Mounting the J-pole on fiberglass or PVC, rather than a metal mast, is the most reliable way to avoid the mast-current problems that give J-poles their inconsistent reputation. A J-pole bonded directly to a grounded metal mast is a recipe for SWR and pattern that change depending on mast length, grounding, even weather.
Choke the feedline. Several turns of coax through a ferrite toroid, or a string of ferrite beads, placed on the feedline near the feedpoint reduces common-mode current on the coax itself — a separate fix from the stub, addressing a separate problem.

How to know if you have this problem: if your SWR changes noticeably when you touch, move, or reposition the coax near the antenna, that's a classic common-mode symptom — not a sign your stub dimensions are wrong.

🛠️ Building & Tuning Your J-Pole (Quick Overview)

Here's the short version — for the full walkthrough with materials, soldering steps, and photos-worth of detail, see our complete J-pole build guide.

Cut the radiator and stub from the calculator's numbers

Copper pipe/tubing is the most common material — durable, self-supporting, and easy to source at hardware stores. Cut slightly long; you'll trim for resonance.

Join the radiator and stub at the top

The radiator and the longer stub leg are electrically connected at the top — this is what makes the "J" shape, with the stub running parallel to the bottom portion of the radiator.

Short the bottom of the stub

The two stub legs are electrically connected (shorted) at the very bottom — this shorted end is the reference point the feedpoint distance is measured from.

Attach the coax at the calculated feedpoint

Connect the coax center conductor to one stub leg and the shield to the other, at roughly the calculated feedpoint distance up from the shorted end. Leave yourself room to slide this connection point during tuning.

Add a choke near the feedpoint

Before final tuning, add your common-mode choke (ferrite toroid or beads on the coax) — tuning with the choke already in place gives you a more accurate final result.

Sweep with a NanoVNA and adjust the feedpoint

Unlike a dipole where you trim wire length, a J-pole is mainly tuned by sliding the feedpoint connection up or down the stub. Small movements make a real difference — go slowly. If you can't reach a good match by sliding the feedpoint alone, small stub length trims can help too.

Mount on a non-conductive support

Fiberglass or PVC mast, as covered above — this single choice prevents most of the common complaints associated with this antenna design.

⚖️ J-Pole vs. Ground Plane vs. Dipole

Feature	J-Pole	1/4-Wave Ground Plane	Center-Fed Dipole
Radials needed	No	Yes	No
Typical gain	~2.2 dBi (~0.1 dBd)	~0 dBd (reference)	~2.15 dBi (0 dBd)
Common-mode risk	Higher — well documented issue	Lower with proper radials	Lower — naturally balanced
Build difficulty	Moderate — feedpoint tuning takes patience	Easy	Easy
Best for	No-radial vertical install, roof/attic mount	Simple, well-understood base station	Cleanest RF, easiest tuning

Gain-wise, a well-built J-pole and a dipole are nearly identical — the real-world differences people report usually trace back to common-mode current, not some inherent gain advantage. Choose a J-pole specifically because you don't want to deal with radials, not because it's dramatically better on paper.

❌ Common Mistakes

Expecting one J-pole to cover 2m and 70cm: The 3rd harmonic frequency lines up, but the stub's matching doesn't — you'll see SWR above 10:1 without a dedicated dual-band design.

Assuming the stub handles common-mode current: It only handles impedance matching. Mast isolation and a feedline choke are separate, necessary steps.

Bonding the radiator directly to a grounded metal mast: This is the single most common cause of the "my J-pole's SWR keeps changing" complaint.

Trying to tune by trimming wire instead of sliding the feedpoint: The feedpoint position along the stub is the primary tuning control on a J-pole — length trims are a secondary adjustment, not the first one to reach for.

Ignoring velocity factor: At VHF/UHF, skipping this correction shifts resonance meaningfully more than it would on an HF wire antenna.

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❓ Frequently Asked Questions

What is the formula for a J-pole antenna?

The radiator length uses 468 divided by frequency in MHz for feet, the same constant used for a half-wave dipole. The matching stub length uses 234 divided by frequency in MHz, the same constant used for a quarter-wave vertical. Both lengths should be multiplied by the velocity factor of the conductor material, which ranges from roughly 0.90 for large diameter tubing to 0.98 for thin bare wire, since electrical signals travel slightly slower on a real conductor than in free space.

Will a 2 meter J-pole work on 70 centimeters too?

Not without modification, even though the third harmonic of 2 meters lands inside the 70cm band by frequency alone. The matching stub's impedance transformation only works correctly at its designed fundamental frequency; at the third harmonic the stub presents a badly mismatched impedance, commonly producing SWR above 10:1 even though the frequency itself is technically in range. True dual-band J-pole designs use a separate second radiating element rather than relying on harmonics of the same stub.

Does the matching stub on a J-pole prevent common mode current?

No, and this is one of the most common misconceptions about J-poles. The quarter-wave matching stub's job is purely impedance transformation, not decoupling the feedline or mast from RF current. A J-pole mounted on a conductive mast, or fed with an unchoked feedline, can develop real common-mode current that distorts the radiation pattern and causes SWR to shift depending on how the mast is grounded or how the feedline is routed, completely independent of how well the stub itself is built and tuned.

Should I ground a J-pole to its mast?

For best electrical performance, the J-pole should be RF-isolated from any conductive mast, most easily achieved by mounting it on a non-conductive support like fiberglass or PVC. If a metal mast must be used and a DC safety ground is wanted for lightning protection, that can be added without necessarily compromising RF performance, but the implementation matters a great deal — simply bonding the antenna directly to a grounded metal mast commonly causes the pattern and SWR instability J-poles are known for.

Why does my J-pole's SWR change when I touch the coax?

This is a classic symptom of common-mode current on the feedline, a well-documented issue with J-poles because the design feeds a balanced quarter-wave stub from unbalanced coax. A choke — several turns of the feedline through a ferrite toroid, or a string of ferrite beads on the coax near the feedpoint — is the standard fix, separate from and in addition to the matching stub itself.