📡 Antenna Length Calculator

How to Calculate Dipole Antenna Length

The basic formula for a half-wave dipole is Length (m) = 150 / Frequency (MHz) for the total length, or 75 / Frequency (MHz) for each leg. This gives the free-space length. In practice, wire antennas experience an "end effect" where the electrical length is slightly shorter than the physical length due to capacitive loading at the wire ends. A typical end-effect factor of 0.95 is applied for thin wire dipoles in free space.

For an Inverted-V, the included angle affects the impedance (typically ~50Ω at 90–120°) and the physical length is slightly longer than a flat dipole for the same frequency due to the bent configuration.

A quarter-wave vertical requires a ground plane (radials) for proper operation. The radiator length is 75 / Frequency (MHz) × VF where VF is the velocity factor of the radiator (usually 0.95 for thin wire). Radial length is typically the same as the radiator or slightly longer.

Velocity factor for insulated wire is lower than bare wire due to the dielectric effect of the insulation. Typical values: bare copper = 0.98–1.0, PVC insulated = 0.92–0.95, polyethylene = 0.88–0.92.

Amateur Radio Band Frequencies

• 160m: 1.8 – 2.0 MHz (80m total / 40m each leg)
• 80m: 3.5 – 4.0 MHz (40m total / 20m each leg)
• 40m: 7.0 – 7.3 MHz (20m total / 10m each leg)
• 30m: 10.1 – 10.15 MHz (14.8m total / 7.4m each leg)
• 20m: 14.0 – 14.35 MHz (10.6m total / 5.3m each leg)
• 17m: 18.068 – 18.168 MHz (8.3m total / 4.15m each leg)
• 15m: 21.0 – 21.45 MHz (7.1m total / 3.55m each leg)
• 12m: 24.89 – 24.99 MHz (6.0m total / 3.0m each leg)
• 10m: 28.0 – 29.7 MHz (5.3m total / 2.65m each leg)
• 6m: 50 – 54 MHz (2.9m total / 1.45m each leg)
• 2m: 144 – 148 MHz (1.0m total / 0.5m each leg)
• 70cm: 430 – 440 MHz (0.34m total / 0.17m each leg)

FAQ

Why is my dipole not resonant at the calculated frequency? The height above ground, nearby objects, wire gauge, and insulation all affect the resonant frequency. Always cut long and trim. The end-effect factor varies with wire diameter — thicker wire has a lower factor (closer to free space).

What is the best height for a dipole? For DX, a half-wave dipole works best at λ/2 or higher (approximately 10m for 20m band). For NVIS (local coverage), mount at 0.1–0.2 wavelengths (2–4m for 40m band).

Can I use a dipole on multiple bands? A half-wave dipole can be used on odd harmonics (e.g., a 40m dipole works on 15m with a higher impedance). For multiband operation, consider a fan dipole, trap dipole, or use an antenna tuner.