| Length | Type | VF | End-effect | Estimated Frequency | Typical Use |
|---|---|---|---|---|---|
| 0.50 m | Quarter-wave | 1.00 | 0.95 | ~142.4 MHz | VHF handheld whip segment |
| 1.00 m | Half-wave dipole | 1.00 | 0.95 | ~142.4 MHz | Simple center-fed VHF dipole |
| 2.05 m | Half-wave dipole | 1.00 | 0.95 | ~69.5 MHz | Lower VHF monitoring |
| 0.165 m | Quarter-wave | 1.00 | 0.95 | ~431.6 MHz | UHF portable antennas |
Values are approximate. Nearby metal, diameter, and loading coils shift resonance.
This calculator uses the wavelength relationship between frequency and the speed of light:
- λ = c / f
- L = fraction × λ × VF × K (K is end-effect)
Solving for frequency from a measured antenna length:
f = (c × fraction × VF × K) / L
- Enter the antenna’s physical length and choose its unit.
- Select the antenna type matching how you measured length.
- Set VF to 1.00 unless you have a known factor.
- Keep end-effect at 0.95 for common wire elements.
- Click Calculate to view frequency and reference lengths.
- Download CSV or PDF for documentation or sharing.
1) What this calculator estimates
This calculator converts a physical antenna length into an estimated operating frequency for radio planning. Use it when you have a cut length and want a starting band, or when you are designing an element and need a first target before tuning. The output is an estimate, not a lab measurement.
2) The speed-of-light relationship
At the core is the wave relationship between frequency and wavelength. In free space, waves propagate at the speed of light, c = 299,792,458 m/s, so wavelength λ equals c divided by frequency f. Solving for f answers: which frequency makes your chosen antenna fraction match the length?
3) Fraction-of-wavelength antenna styles
Common antennas are built as wavelength fractions. A quarter-wave element is 0.25λ, while a half-wave dipole is 0.50λ measured tip-to-tip. Designs like 5/8-wave and 3/4-wave can change radiation angle and matching behavior. Switch fractions to see how the same length maps to different bands. For dipoles, each leg is roughly one quarter-wave, plus trimming margin.
4) Velocity factor (VF) and materials
Velocity factor (VF) adjusts for how RF behaves along conductors. A bare element in open air is near 1.00, while insulation, loading, or nearby materials can reduce it. Because frequency scales with VF, moving from 1.00 to 0.95 shifts the estimate by about five percent.
5) End-effect factor and real-world trimming
End-effect factor models the antenna being electrically longer than its measured length. Diameter, end hardware, and surrounding objects add capacitance and shift resonance. Many builders start near 0.95 for wire and then trim. Installations near metal, rooftops, or indoor structures often need extra adjustment.
6) Quick reference data you can sanity-check
Use reference numbers to sanity-check inputs. With VF = 1.00 and end-effect = 0.95, a quarter-wave around 0.50 m is near 142 MHz, and a quarter-wave around 0.165 m is near 432 MHz. Doubling length roughly halves resonant frequency for the same fraction.
7) Interpreting the output and next steps
After calculation, review frequency, free-space wavelength, and the reference lengths for quarter, half, 5/8, and full-wave at that frequency. These references help you scale elements while staying in the same band. Export CSV for build logs and PDF for workshop notes. Always validate with an analyzer or SWR meter, and trim in steps to reach final resonance.
FAQs
FAQ 1: Why does my tuned frequency differ from the estimate?
Nearby metal, height above ground, element diameter, and feedpoint hardware shift resonance. Use the calculator as a starting point, then trim gradually and measure with an analyzer.
FAQ 2: What VF should I use for a simple wire element?
Start with 1.00 for bare wire in open air. If insulation, loading, or tight routing exists, try 0.95–0.99 and compare against measurements.
FAQ 3: What does the end-effect factor represent?
It approximates electrical lengthening at the element ends. Many builders begin with 0.95 and adjust after testing. Thicker conductors and nearby objects often require more correction.
FAQ 4: Is the “half-wave dipole” length one side or total?
In this tool, half-wave dipole length is tip-to-tip total length. Each leg is roughly half of that total, before trimming.
FAQ 5: Can I use this for loops?
Use “full-wave” as a rough loop circumference estimate, then refine. Square and circular loops vary slightly due to geometry and conductor thickness.
FAQ 6: Which speed-of-light option should I pick?
Choose the exact value for consistent engineering work. The approximate value is fine for quick field estimates. Custom values are useful for specialized propagation or instructional demonstrations.