Super J Pole Antenna Calculator

Calculator Inputs

Target frequency MHz

Radiator velocity factor

End effect correction %

Lower radiator factor

Upper radiator factor

Phasing section factor

Matching stub factor

Short stub ratio

Feed point % of stub

Stub spacing factor

Trim allowance %

Output unit

Formula Used

The calculator starts with the speed of light and the target frequency.

Free space wavelength: λ = c / f

Corrected wavelength: λc = λ × velocity factor × (1 − end effect % / 100)

Section length: length = λc × selected section factor

Short stub: short stub = matching stub × short stub ratio

Feed point: feed point = matching stub × feed point percent / 100

Stub spacing: spacing = λc × spacing factor

How to Use This Calculator

Enter the operating frequency in MHz.
Adjust velocity factor for your material and build style.
Keep a small end effect correction for open wire or tube ends.
Use the default factors for a first super J pole layout.
Choose the output unit used in your workshop.
Press calculate to show dimensions above the form.
Download a CSV or PDF report for cutting notes.
Build slightly long, then trim while measuring match quality.

Example Data Table

Frequency MHz	Lower radiator cm	Upper radiator cm	Matching stub cm	Feed point cm
50.125	275.5698	275.5698	137.7849	9.6449
144.39	95.6641	95.6641	47.832	3.3482
146.52	94.2734	94.2734	47.1367	3.2996
223.5	61.8029	61.8029	30.9014	2.1631
433.5	31.8638	31.8638	15.9319	1.1152

Super J Pole Planning Guide

A super J pole is a compact vertical antenna for VHF and UHF work. It joins two half wave sections with a phasing section. The lower section also uses a quarter wave matching stub. This shape gives more gain than a simple quarter wave whip. It can still be built from rod, tube, twin lead, or ladder line.

Why dimensions matter

Every part follows the operating wavelength. A small error shifts the best match away from the target frequency. The calculator starts with the free space wavelength. It then applies velocity factor and end effect correction. These settings make the first cut closer to a real antenna. They also help when different conductor types are used.

Main sections

The lower radiator is usually near one half wavelength. The upper radiator is also near one half wavelength. The phasing section is set by the selected fraction. Many builders begin near a quarter wavelength. The matching stub is normally a quarter wavelength. Its short side is slightly shorter when an open feed gap is used. The feed point is placed up from the shorted end. Moving it changes impedance.

Building notes

Use the calculated values as starting lengths. Leave extra trim allowance on radiators and stubs. Mount the antenna away from metal objects during tuning. Measure standing wave ratio at low, center, and high band points. If the best match is low in frequency, shorten the radiators a little. If it is high, lengthen where possible. Make small changes and recheck.

Field use

This tool is useful for repeaters, simplex channels, and portable stations. It gives lengths in several unit systems. The CSV file is handy for workshop sheets. The PDF file is better for quick printouts. Final performance depends on construction quality, feed line routing, weather sealing, and nearby supports. Always test before permanent mounting.

Safety and adjustment

Keep the driven area insulated from hands while transmitting. Use low power for first checks. Record each cut, reading, and weather condition. This habit prevents confusion during repeated tests. A current choke below the feed point can reduce common mode current. It often improves pattern shape and receiver noise. Good notes make future antenna changes faster and safer.

FAQs

1. Is this calculator a final construction plan?

No. It gives starting dimensions. Real antennas need trimming because tubing, insulation, nearby objects, feed line routing, and mounting height affect resonance.

2. What frequency should I enter?

Enter the center frequency you want to favor. For a repeater antenna, use the part of the band where you expect the best match.

3. What does velocity factor mean?

Velocity factor adjusts wavelength for real conductors and insulated materials. Bare metal is often near one, while twin lead or coax based builds may be lower.

4. Why include end effect correction?

Open conductor ends can make an antenna behave slightly longer. The correction shortens the first cut estimate and helps reduce large trimming changes.

5. Where should the feed point start?

The feed point begins a small distance above the shorted end of the stub. Slide it during testing to find the lowest reflected power.

6. Can I use inches instead of centimeters?

Yes. Select inches from the output unit menu. The same formulas run, and every result table length changes to the selected unit.

7. Why is trim allowance useful?

It adds extra material to starting cuts. This is safer because a long part can be shortened, but a short part is harder to repair.

8. Does conductor diameter affect performance?

Yes. Wider conductors can broaden bandwidth and change tuning slightly. Use the result as a start, then tune the finished antenna with proper test gear.