Antenna Length Calculator
Calculate dipole, monopole, and 5/8 wave antenna lengths for any frequency.
Enter Frequency
Results
Common Antenna Frequencies
| Application | Frequency | Wavelength (λ) | Dipole | Monopole |
|---|---|---|---|---|
| CB Radio | 27 MHz | 11.10 m | 5.55 m | 2.78 m |
| FM Radio | 100 MHz | 3.00 m | 1.50 m | 74.9 cm |
| VHF Amateur | 145 MHz | 2.07 m | 1.03 m | 51.7 cm |
| ISM 433 | 433 MHz | 69.2 cm | 34.6 cm | 17.3 cm |
| LoRa (EU) | 868 MHz | 34.5 cm | 17.3 cm | 8.6 cm |
| LoRa (US) | 915 MHz | 32.8 cm | 16.4 cm | 8.2 cm |
| WiFi 2.4 GHz | 2.4 GHz | 12.5 cm | 6.2 cm | 3.1 cm |
| Bluetooth | 2.4 GHz | 12.5 cm | 6.2 cm | 3.1 cm |
| Zigbee | 2.4 GHz | 12.5 cm | 6.2 cm | 3.1 cm |
| WiFi 5 GHz | 5 GHz | 6.0 cm | 3.0 cm | 1.5 cm |
Antenna Length and RF Design
Antenna length is directly related to the operating frequency through the wavelength. The wavelength λ = c/f, where c is the speed of light (299,792,458 m/s) and f is the frequency in hertz. A half-wave dipole antenna is λ/2 long, and a quarter-wave monopole is λ/4 long.
The half-wave dipole is the most fundamental antenna type. It has a feed-point impedance of approximately 73 ohms, making it a good match for 75-ohm coaxial cable. Each arm of the dipole is λ/4 long, and the radiation pattern is omnidirectional in the plane perpendicular to the antenna.
A quarter-wave monopole antenna is half the length of a dipole and requires a ground plane to work. The ground plane acts as a mirror, creating a virtual image of the antenna. Its feed-point impedance is approximately 36 ohms. This is the most common antenna type for handheld radios, WiFi routers, and IoT devices.
The 5/8 wave antenna provides about 3-4 dB gain over a quarter-wave monopole by concentrating radiation at lower angles toward the horizon. It requires a matching network (usually a coil at the base) because its natural impedance is not 50 ohms. It is widely used in VHF/UHF mobile and base station applications.
Antenna Length Formula
λ = c / f Dipole = λ/2 Monopole = λ/4Key Points
- λ = c/f — wavelength is inversely proportional to frequency
- Dipole (λ/2) — ~73Ω impedance, omnidirectional pattern
- Monopole (λ/4) — ~36Ω impedance, requires ground plane
- 5/8 wave — ~3 dB gain over λ/4, needs matching network
- Real antennas are ~5% shorter due to end effects (velocity factor)
Applications
- WiFi and Bluetooth antenna design
- LoRa and IoT node antenna sizing
- Amateur radio antenna construction
- RF system design and link budgets
Antenna length formulas
Half-wave dipole: L = λ/2 = c/(2f) × 0.95 (×0.95 velocity factor)
Quarter-wave monopole: L = λ/4 = c/(4f) × 0.95
Full-wave loop: L = λ = c/f × 0.98
c = 299,792,458 m/s (speed of light)
Practical dipole shortening factor: 0.93–0.97 (end effect)Ham radio band reference
| Band | Frequency | Half-dipole | Quarter monopole | Notes |
|---|---|---|---|---|
| 80m | 3.7 MHz | 38.6 m | 19.3 m | Nighttime DX |
| 40m | 7.1 MHz | 20.1 m | 10.1 m | Most popular HF |
| 20m | 14.2 MHz | 10.1 m | 5.0 m | DX workhorse |
| 17m | 18.1 MHz | 7.9 m | 3.9 m | WARC band |
| 15m | 21.2 MHz | 6.7 m | 3.4 m | Solar cycle dep. |
| 10m | 28.5 MHz | 5.0 m | 2.5 m | Best for DX at peak |
| 2m | 145 MHz | 0.98 m | 0.49 m | Local VHF FM |
| 70cm | 433 MHz | 33 cm | 16.5 cm | LoRa, ISM band |
| WiFi | 2450 MHz | 5.8 cm | 2.9 cm | 2.4GHz PCB antenna |
| 5GHz | 5800 MHz | 2.5 cm | 1.2 cm | WiFi 5GHz |
Practical examples
Quarter-wave: L = (299,792,458 / 433,000,000) / 4 × 0.95 = 16.4 cm
Use 1.6mm copper wire, bend 5mm at bottom for feed. SWR < 1.5.
Quarter-wave: L = (0.122m) / 4 × 0.64 = 19.5mm (0.64 = PCB velocity factor)
Use 50Ω microstrip (1.6mm FR4: width ≈ 3mm). Keep ground clearance > 3mm.
Design tip: Real antennas are shorter than the theoretical λ/2 due to end effects. Use 0.95× multiplier for wire antennas in free space. PCB antennas: velocity factor ≈ 0.60–0.70 (depends on substrate εᵣ). Feed impedance: dipole ≈ 73Ω, monopole over ground ≈ 36Ω.
Did you know? A dipole antenna half-wavelength long at 2.4 GHz (Wi-Fi) is only ~6 cm. GPS satellites broadcast at 1.575 GHz with a signal power of about 20–50 W, yet after travelling 20,200 km the power received by your phone antenna is less than 10⁻¹⁵ W.