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