Antenna Length Calculator
Calculates wavelength and physical antenna length for quarter-wave, half-wave, full-wave, and 5/8-wave antenna types from operating frequency, with velocity factor adjustment and common RF band presets.
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How to Use
- Enter the operating frequency (in MHz), or click a preset button for a common band: 433MHz ISM, 868MHz, 915MHz, 2.4GHz WiFi, 5GHz WiFi, GPS (1575.42MHz), or FM broadcast (98MHz).
- Select the antenna type: quarter-wave, half-wave, full-wave, or 5/8-wave.
- Enter the velocity factor (typical: 0.95 for bare wire, 0.66 for coaxial cable), default 0.95.
- Click "Calculate" (or edit any field to trigger automatic recalculation).
- Results show the wavelength λ (in meters) and the corresponding antenna length in meters, centimeters, and inches, plus the formula used.
- A reference table below lists quarter-wave and half-wave lengths (in cm, at velocity factor 0.95) for common bands: 433MHz, 868MHz, 915MHz, 2.4GHz, 5GHz, GPS, and FM broadcast, for quick lookup.
- Click "Load Sample Data" to restore the default: 2.4GHz WiFi, quarter-wave, velocity factor 0.95.
Features
- Calculates the free-space wavelength for a given frequency using λ=c/f (speed of light taken as 3×10⁸ m/s).
- Supports four standard antenna length types: quarter-wave (factor 0.25), half-wave (factor 0.5), full-wave (factor 1.0), and 5/8-wave (factor 0.625), commonly used for monopole, dipole, and other antenna designs.
- Supports a velocity factor parameter to correct for electromagnetic wave propagation speed in real conductors/dielectrics being lower than free-space light speed (approximately 0.95 for bare wire, 0.66 for coaxial cable).
- Includes 7 built-in quick-select buttons for common RF bands: 433MHz ISM, 868MHz, 915MHz, 2.4GHz WiFi, 5GHz WiFi, GPS L1 (1575.42MHz), and FM broadcast (98MHz).
- Results display antenna length in meters, centimeters, and inches simultaneously, for easy use across different fabrication contexts.
- Provides a reference table listing quarter-wave and half-wave lengths for all preset bands under the standard velocity factor, without needing to calculate each one manually.
Use Cases
Antenna design for ISM-band wireless modules
Design a quarter-wave monopole antenna for 433MHz/868MHz/915MHz LoRa or Sub-GHz wireless modules, quickly determining the physical length needed for bare wire or PCB trace antennas.
WiFi/Bluetooth 2.4GHz antenna length verification
When designing an external antenna for a 2.4GHz WiFi module or Bluetooth device, compute the theoretical quarter-wave length as a reference for a PCB trace antenna or spring antenna design.
GPS receiver antenna size reference
Look up the wavelength and antenna length for the GPS L1 band (1575.42MHz) to inform external GPS antenna selection or custom antenna design.
Calculating electrical length for coaxial feedline segments
Adjust the velocity factor to a coax cable's typical value (~0.66) to calculate the physical cable length needed for a specific electrical length (e.g., a quarter-wave impedance transformer section) in an RF circuit.
FAQ
What exactly is the velocity factor and how do I choose it?
The velocity factor reflects the ratio of electromagnetic wave propagation speed in a real conductor/dielectric to the free-space speed of light. For bare wire (like an antenna element itself), affected by surroundings and wire gauge, a typical value is about 0.95. For coaxial cable, the dielectric filling slows propagation, with typical values around 0.66 (check the specific cable's datasheet, as it varies by model). The tool defaults to 0.95, suitable for bare-wire antenna length calculations; if you're calculating electrical length inside a cable, switch to that cable's velocity factor.
Why does my calculated 2.4GHz antenna length not exactly match some other references?
Two common reasons: different velocity factor assumptions (references may use values anywhere from 0.9 to 1.0), or different antenna types (a quarter-wave monopole is half the length of a half-wave dipole). Confirm the reference's velocity factor and antenna type match this tool's settings before comparing.
Is the calculated length theoretical, or does a real antenna need further tuning?
Yes, this is a theoretical estimate based on ideal free-space wavelength and a fixed velocity factor. Real antennas are affected by nearby metal structures, PCB copper pour, enclosure material, and feed method, causing impedance detuning. Fine-tuning with a vector network analyzer (VNA) after measuring the actual resonant point (commonly called "trim tuning") is usually needed; the theoretical value serves as a starting point.
What is a 5/8-wave antenna used for?
A 5/8-wave (factor 0.625) antenna offers higher gain and a lower radiation take-off angle than a quarter-wave antenna, commonly used in walkie-talkies and mobile base stations that need longer horizontal transmission range. It's a common compromise design between quarter-wave and full-wave, offered as one of the four selectable types in this tool.