J-Pole Antenna Length Calculator
Use this tool to estimate J-pole antenna dimensions from your target frequency. Results are shown in meters, centimeters, and inches.
What this J-pole calculator does
A J-pole antenna is a popular end-fed vertical antenna that combines a half-wave radiating section with a quarter-wave matching stub. This calculator gives fast starting dimensions for both sections based on your frequency in MHz and your selected velocity factor.
The output is designed for practical construction: long element length (3/4 wave), short element length (1/4 wave), recommended element spacing, and a feedpoint adjustment range measured up from the shorted end.
Core formulas used
Electrical wavelength
- Wavelength (meters) = 300 / Frequency (MHz)
J-pole element lengths
- Long element = 0.75 × wavelength × velocity factor
- Short element = 0.25 × wavelength × velocity factor
- Suggested spacing = spacing% × wavelength × velocity factor
- Feedpoint range = selected % of wavelength × velocity factor
How to use the calculator
Step 1: Enter frequency
Input your target operating frequency in MHz. For example, enter 146.52 for the 2-meter calling frequency, or 446.00 for a 70cm design.
Step 2: Choose velocity factor
Velocity factor compensates for practical construction effects. Real antennas are often a little shorter than ideal free-space math. A value around 0.95 is a common starting point for many builds.
Step 3: Start with suggested feedpoint and tune
Build slightly long, then trim during tuning. Move the feedpoint up and down the matching section to refine SWR and impedance match.
Example (146.52 MHz)
At 146.52 MHz with velocity factor 0.95, the calculator gives a long element around 1.459 m and a short element around 0.486 m. That is a solid first-cut geometry for a 2-meter J-pole before final analyzer-based tuning.
Practical tuning tips
Build safety margin into cuts
- Cut both elements about 1–2% longer than calculated.
- Trim gradually while checking SWR.
- Keep a record of each trim step and measured results.
Feedpoint optimization
- Slide the feedpoint upward for higher impedance; downward for lower impedance.
- Small feedpoint moves can make a big SWR difference.
- Finalize feedpoint after element trimming is close.
Installation matters
- Nearby metal, mast coupling, and coax routing will shift resonance.
- Test in final mounting position when possible.
- Use a current choke/balun at the feedline to reduce common-mode effects.
Frequently asked questions
Is this calculator exact?
No. It provides high-quality starting dimensions. Final values should always be confirmed with an antenna analyzer or VNA.
Can I use this for copper pipe, wire, or ladder line?
Yes. The geometry works across common construction methods. Just adjust velocity factor and expect final tuning differences based on material diameter and spacing.
Can I design for non-ham frequencies?
Yes. Enter any frequency in MHz and the tool will calculate corresponding J-pole lengths. Be sure to follow local laws and licensing rules before transmitting.