Advanced J-Pole Calculator

Plan a J-Pole antenna using practical engineering inputs. Estimate radiator, stub, feedpoint, and spacing quickly. Review charts, exports, and examples for cleaner design decisions.

Build a practical J-Pole antenna design from frequency, material factor, feed estimate, conductor size, and spacing assumptions. Results appear below this header and above the form after submission.

Calculator Inputs

Operating Frequency

Frequency Unit

Material Factor

Desired Feed Impedance (Ω)

Feed Tap Percent of Stub

Conductor Diameter

Diameter Unit

Element Spacing Multiplier

Trim Allowance (%)

Output Unit

Graph Sweep (%)

Example Data Table

Band Example	Frequency (MHz)	Material Factor	Long Element (cm)	Short Element (cm)	Feed Tap Start (cm)
2 meter	144.0000	0.95	148.3318	49.4439	2.7194
2 meter	146.0000	0.95	146.2998	48.7666	2.6822
70 centimeter	446.0000	0.95	47.8604	15.9535	0.8774

Example feed tap starts assume a 5.5% stub position. Final tap and trim depend on real conductor geometry, mounting, nearby objects, and analyzer readings.

Formula Used

1) Free-space wavelength:
λ = c / f

2) Quarter-wave matching stub:
L_stub = (λ / 4) × material factor

3) Half-wave radiator section:
L_radiator = (λ / 2) × material factor

4) Long element total:
L_long = (3λ / 4) × material factor

5) Rough-cut lengths:
L_cut = corrected length × (1 + trim allowance / 100)

6) Feed tap distance:
Tap distance = corrected stub length × (feed tap percent / 100)

7) Element spacing estimate:
Spacing = conductor diameter × spacing multiplier

The material factor accounts for practical shortening during construction. The automatic feed tap is only a starting estimate for tuning.

How to Use This Calculator

Enter the operating frequency and choose its unit.
Set a material factor. A value near 0.95 is common for practical starting cuts.
Enter conductor diameter and choose the diameter unit.
Choose a spacing multiplier to estimate the gap between the two parallel elements.
Leave feed tap percent blank for an automatic starting point, or enter your own value.
Set the trim allowance so you can cut slightly long and trim during final tuning.
Submit the form to show results above the form and below the header.
Review the chart, compare dimensions, then export the table as CSV or PDF.

FAQs

1) What does a J-Pole calculator estimate?

It estimates practical J-Pole dimensions from operating frequency. You get wavelength, stub length, long element length, rough-cut values, spacing, and a feed tap starting point for tuning.

2) Why is the long element three quarters of a wavelength?

The classic J-Pole combines a quarter-wave matching stub with a half-wave radiating section. Together they form the longer conductor, which is approximately three quarters of a wavelength.

3) Why should I keep a trim allowance?

Real installations shift resonance. A trim allowance lets you cut slightly long, then shorten carefully while watching analyzer readings until resonance and feed behavior improve.

4) Is the automatic feed tap exact?

No. It is a practical starting estimate only. Final feedpoint height depends on conductor diameter, spacing, nearby objects, mounting hardware, and the target feed impedance.

5) Why does conductor diameter matter?

Conductor diameter affects spacing choices, mechanical layout, and tuning behavior. Thicker conductors can alter resonance and bandwidth, so geometry should stay consistent during the build.

6) Should I use free-space values exactly as cut lengths?

No. Free-space values are theoretical references. Practical antennas need shortening factors, trim margins, and measurement-based tuning after assembly and mounting.

7) Can I use this for VHF and UHF designs?

Yes. The calculator works across frequencies as long as your entered units are correct. Mechanical tolerances become more critical as the design frequency increases.

8) What is the best way to finalize the design?

Use these outputs for first cuts, build carefully, mount the antenna in its real location, and confirm resonance, impedance, and SWR with a suitable analyzer.