J Pole Antenna - Element Length and Stub Spacing

A j pole antenna calculator turns an operating frequency into the long element length, short element length, matching-stub spacing, and feed-point height of a half-wave end-fed antenna. The same 3/4 wave plus 1/4 wave matching stub is used on the 2 m, 70 cm, and CB bands and gives a builder the four dimensions needed to cut copper pipe and pick the feed-point before soldering.

• • 2 m Amateur Build: Compute element lengths and spacing for a 144 to 148 MHz J-pole on the 2 m amateur radio band.
• • 70 cm Scanner Antenna: Build a cut list for a 446 MHz J-pole used as a public-safety or scanner antenna.
• • Cross-Band Reference: Use the same wavelength formulas to scale a J-pole to 27 MHz CB, 446 MHz, or any other RF band.

The J-pole is one of the simplest antennas to homebrew because it uses rigid copper pipe and a soldered feed-point. The j pole antenna calculator handles the wavelength math once and reports every dimension in metric and imperial.

Because every J-pole result is a frequency-to-wavelength conversion, the Wave Speed Calculator shows the same v = f × lambda step behind the element-length math.

The calculator reads the operating frequency, applies a velocity / end-effect K factor to account for the finite-diameter copper pipe, and returns the long element, short element, stub spacing, and feed-point height in one pass.

• lambda: Free-space wavelength in metres, equal to the speed of light c divided by the operating frequency f in hertz.
• K: Velocity / end-effect factor for finite-diameter conductor. Typical values are 0.93 to 0.97 for 12 to 25 mm copper pipe.
• long: Long element length in metres, equal to 0.75 times K times lambda. This is the 3/4 wave radiating section.
• short: Short element length in metres, equal to 0.25 times K times lambda. This is the matching stub that brings the feed impedance down to roughly 50 ohms.
• spacing: Spacing between elements in metres, the larger of 5.08 cm (2 in) or 5 percent of the wavelength.

When the frequency selector is set to MHz the calculator multiplies the value by one million before applying c divided by f, so the wavelength always comes out in metres. K is clamped between 0.85 and 1.00 because values outside that range are not realistic for a passive antenna element.

According to Wikipedia: J-pole antenna, the J-pole consists of two parallel straight metal conductors, one 3/4 of a wavelength and the other 1/4 of a wavelength long at the operating frequency, shorted together at the bottom, which is the same 3/4 plus 1/4 wave matching stub the calculator reports after the velocity / end-effect K factor shortens each element

According to BIPM SI Brochure, the speed of light in vacuum is exactly 299,792,458 metres per second, which is the constant used to convert an RF frequency into a free-space wavelength

For the copper pipe used to build the matching stub and the long element, the Copper Wire Weight Calculator covers the diameter and mass of the same half-inch type-L tubing most homebrew J-poles are cut from.

Four ideas make every J-pole result easier to interpret: the wavelength, the half-wave end-fed topology, the velocity / end-effect K factor, and the matching-stub geometry that sets the feed impedance.

These ideas work together: lambda fixes the scale, the topology fixes the form factor, K adjusts the scale for finite-diameter pipe, and the matching-stub spacing sets the impedance transformation. The elements are measured as fractions of lambda, so two pi never appears in the formula.

When the J-pole is later modelled as a standing-wave problem, the Harmonic Wave Equation Calculator covers the harmonic-wave equation that describes the same current distribution.

Pick the operating frequency, choose a frequency unit, leave the velocity factor at 0.95 for half-inch copper pipe, and read the long element, short element, stub spacing, and feed-point height in one pass.

1. 1 Enter the operating frequency: Type the design-centre frequency and use the Frequency Unit selector to pick Hz, kHz, MHz, or GHz.
2. 2 Set the velocity / end-effect K: Leave K at 0.95 for 12 to 25 mm copper pipe, lower it to 0.93 for thick tubing, or raise it toward 0.97 for thin wire.
3. 3 Read the long and short elements: The result panel reports the long element (3/4 lambda) and short element (1/4 lambda) lengths in metres and feet for metric or imperial copper pipe.
4. 4 Set the stub spacing: Use the reported Stub Spacing row as the gap between elements. The calculator falls back to 5.08 cm (2 in) when 5 percent of lambda is too small to build.
5. 5 Mark the feed-point height: Drill the coax feed-point at the Feed-Point Height row above the stub bottom, where a 50 ohm coax cable sees the cleanest SWR after the matching stub is tuned.

For a 2 m J-pole, enter 145 with the unit selector at MHz, leave K at 0.95, and read long element = 1.473 m, short element = 0.491 m, stub spacing = 0.103 m, and feed-point = 5 cm above the stub bottom.

For the practical build side of the project, the Beat Frequency Calculator covers the same difference-between-two-frequencies math that shows up when you sweep the feed-point and watch the SWR dip fall out.

The j pole antenna calculator returns every dimension a build needs from one frequency input, with metric and imperial columns so the cut list works for any copper the workshop has on hand.

• • Single-input build list: One operating frequency returns the long element, short element, stub spacing, and feed-point height so the cut list is complete.
• • Metric and imperial output: Every length is reported in metres and feet (or inches for spacing and feed-point) so the same result fits type-L copper or imperial pipe.
• • Adjustable velocity factor: The K input dials the end-effect correction for the actual pipe diameter, which matters most at UHF.
• • Mechanical spacing fallback: On UHF and above the spacing falls back to 5.08 cm (2 in) so the matching stub stays buildable with a standard bracket.
• • Cross-band coverage: The same calculator covers HF CB, VHF 2 m, UHF 70 cm, and microwave J-poles without redoing the cut list logic.

The j pole antenna calculator is most useful when the builder is starting from a frequency rather than a target physical length.

If the project moves from a single J-pole design to a multi-band setup, the Frequency Calculator handles the cycles-per-second and wavelength conversion on its own.

The wavelength math is exact, so most build differences come from the velocity / end-effect K, the matching-stub spacing, and the choice of operating frequency within the band.

• • The calculator assumes the antenna is mounted in free space. Metal masts or roof lines within a fraction of a wavelength detune the long element and shift the SWR dip below the design centre.
• • K is clamped between 0.85 and 1.00 because the end-effect correction cannot realistically exceed 1 or drop below the thin-wire limit for any practical copper-pipe J-pole.

When a built J-pole does not match the calculator output, the first check is whether K matches the pipe on the bench. Speed of light c is an exact SI constant, so once the frequency and K are right the wavelength is fixed.

According to ARRL Band Plan, the 2 metre amateur band runs from 144 to 148 MHz, with 145 MHz commonly used as the J-pole design centre

When the J-pole design calls for matching the SWR to a different RF band, the dBm to Watts Calculator translates the conducted RF power readings that ride alongside SWR and antenna gain on a typical antenna worksheet.