Antenna Length Calculator (λ/4 · λ/2 · 5/8λ)
Compute free-space wavelength and the practical size of monopole, dipole, whip and loop antennas, including the velocity factor — before you cut wire or draw a PCB antenna.
Key points
Antenna Length Calculator (λ/4 · λ/2 · 5/8λ): Compute free-space wavelength and the practical size of monopole, dipole, whip and loop antennas, including the velocity factor — before you cut wire or draw a PCB antenna. Formula: λ = 300/f(MHz) m ｜ monopole ≈ λ/4 × velocity factor. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. The velocity factor accounts for end effects; 0.95 is a good starting point for thin wire. Trim with a VNA — cutting is easy, lengthening is not. Keep a copper keep-out around PCB antennas and stay clear of batteries and metal; a nearby hand shifts resonance, so leave room for tuning.
Inputs
λ = 300/f(MHz) m ｜ monopole ≈ λ/4 × velocity factorCalculation model (updates with parameters)
| Free-space wavelength λ0 | 69.14 cm | |
| Wavelength in medium λ (with factor) | 65.68 cm | ×0.95 |
| λ/4 monopole length | 16.42 cm | Needs a ground plane; theoretical gain 5.15dBi |
| λ/2 dipole total (each arm λ/4) | 32.84 cm ｜ 每臂 16.42 cm | Theoretical 2.15dBi |
| 5/8λ whip length | 41.05 cm | Slightly higher gain but needs matching (not purely resistive) |
| Loop circumference / diameter | 65.68 cm / 20.91 cm | Square or circular loop of about 1λ circumference |
| PCB antenna reference size | 164.2 mm | An inverted-F antenna is about λ/4 long and needs a ground plane ≥ λ/4 |
Engineering notes
- The velocity factor accounts for end effects; 0.95 is a good starting point for thin wire. Trim with a VNA — cutting is easy, lengthening is not.
- Keep a copper keep-out around PCB antennas and stay clear of batteries and metal; a nearby hand shifts resonance, so leave room for tuning.
FAQ
- What is Antenna Length Calculator (λ/4 · λ/2 · 5/8λ) for?
- Compute free-space wavelength and the practical size of monopole, dipole, whip and loop antennas, including the velocity factor — before you cut wire or draw a PCB antenna.
- What is the formula for Antenna Length Calculator (λ/4 · λ/2 · 5/8λ)?
- Core formula: λ = 300/f(MHz) m ｜ monopole ≈ λ/4 × velocity factor. The tool returns results instantly using this formula and the selected parameters.
- What engineering notes should I keep in mind when using Antenna Length Calculator (λ/4 · λ/2 · 5/8λ)?
- The velocity factor accounts for end effects; 0.95 is a good starting point for thin wire. Trim with a VNA — cutting is easy, lengthening is not. Keep a copper keep-out around PCB antennas and stay clear of batteries and metal; a nearby hand shifts resonance, so leave room for tuning.