Calculator
Example Data Table
| Frequency | Band | Approx full dipole | Approx each leg | Common use |
|---|---|---|---|---|
| 7.15 MHz | 40 m | 20.06 m | 10.03 m | HF wire antenna |
| 14.20 MHz | 20 m | 10.10 m | 5.05 m | Portable field dipole |
| 28.40 MHz | 10 m | 5.02 m | 2.51 m | Compact HF dipole |
| 146.00 MHz | 2 m | 0.98 m | 0.49 m | VHF simple antenna |
Formula Used
The calculator first converts the entered frequency to hertz.
Wavelength: wavelength = speed of light / frequency.
Electrical half wave: half wave = wavelength / 2.
Practical dipole length: full length = half wave × velocity factor × end effect factor.
Each leg: each leg = full length / 2.
Total cut wire: total cut wire = two × each leg plus trim allowance on both legs.
The default end effect factor is 0.95. It reflects the common shortening used for many practical wire dipoles.
How to Use This Calculator
Enter the target frequency and choose the matching unit. Select the antenna style for your record. Enter velocity factor if your conductor or layout needs adjustment. Keep 0.95 as the end effect factor for a normal first estimate. Add the center feed gap and trim allowance. Choose your output unit. Press calculate. Review the result above the form. Use the CSV or PDF buttons to save the calculation.
Article
Why Dipole Length Matters
A dipole is simple, but its length controls performance. When the wire is close to resonance, more transmitter power reaches the antenna. Less power returns through the feed line. The calculator starts with frequency because wavelength changes as frequency changes. A lower frequency needs a longer wire. A higher frequency needs a shorter wire.
Practical Design Factors
Real antennas are not built in perfect free space. Wire diameter, insulation, nearby metal, ground height, and end supports can shift resonance. That is why the tool includes velocity factor and end effect fields. The default end effect is a practical shortening factor. It gives a good first cut for many half wave wire dipoles. The trim allowance helps you cut each leg a little long. You can then fold, clip, or solder after testing.
Using Results Safely
The full dipole length is the total active wire. Each arm is one half of that value. The tip to tip span adds the feed gap. The total cut wire adds your chosen trim allowance. These separate values prevent common mistakes. Many builders confuse span with conductor length. They are close, but not always the same.
Tuning Guidance
Start with the calculator result. Install the antenna in its final position if possible. Measure resonance with an analyzer or radio meter at low power. If resonance is too low, shorten both legs equally. If resonance is too high, lengthen both legs if you left extra wire. Make small changes. Recheck after every cut.
Best Use Cases
This calculator supports quick planning for ham radio, shortwave listening, field antennas, and classroom physics work. It is useful for half wave dipoles, inverted V layouts, and basic portable wire antennas. It does not replace final measurement. It gives a clean starting point with clear unit conversions and exportable results.
Common Planning Checks
Check the available support distance before cutting wire. Keep the feed point strong and balanced. Use proper strain relief near the center insulator. Keep people away from transmitting elements. Weather, rain, and nearby objects can change readings. Record every cut in the notes field. Saved CSV and PDF files help compare tests after later adjustments. This makes future repairs easier and more accurate.
FAQs
1. What is a dipole antenna?
A dipole antenna has two conductive arms fed from the center. For a half wave dipole, both arms together are about one half wavelength long.
2. Why does frequency change antenna length?
Frequency sets wavelength. Lower frequencies have longer wavelengths, so they need longer antenna elements. Higher frequencies have shorter wavelengths and need shorter elements.
3. What does each leg length mean?
Each leg length is the recommended length for one side of the center-fed dipole. Cut two equal legs for a balanced basic design.
4. Should I cut exactly to the calculated length?
No. Cut slightly long first. Install the antenna, measure resonance, then trim both legs equally in small steps until the target point is reached.
5. What is velocity factor?
Velocity factor adjusts length for wave speed in or near the conductor system. For many bare wire dipoles, using 1 is a practical starting value.
6. What is the end effect factor?
End effect accounts for practical shortening caused by real antenna behavior. A value near 0.95 is often used for basic wire dipole estimates.
7. Can this calculator be used for an inverted V?
Yes. Use it as a starting estimate. An inverted V often needs final trimming because angle, height, and ground coupling affect resonance.
8. Why are final measurements still needed?
Nearby objects, height, wire coating, feed line, and weather can shift resonance. A meter or analyzer confirms the real installed antenna behavior.