Antenna Length Calculator
Calculate practical antenna lengths from operating frequency. Choose quarter-wave, half-wave, full-wave or another wavelength fraction and apply a velocity factor for a more realistic physical antenna estimate.
Antenna Length Calculator
Enter the operating frequency and select the desired antenna electrical length.
Antenna Length Formula
Antenna dimensions are commonly related to wavelength. The wavelength of a signal is determined by its propagation speed and frequency. A practical antenna can then be designed as a fraction of that wavelength.
Understanding Antenna Length
The operating frequency determines the wavelength of an RF signal. Because antennas interact strongly with the electromagnetic field around them, useful antenna dimensions are often expressed as fractions of a wavelength.
Quarter-Wave
A quarter-wave antenna has an electrical length of approximately λ/4. It is widely used for monopole antennas and practical vertical antennas, often with a ground plane or other return path.
Half-Wave
A half-wave antenna is approximately λ/2 long. A half-wave dipole consists of two approximately quarter-wave conductors and is a common RF reference antenna.
Full-Wave
A full-wave antenna has an electrical length of approximately one wavelength. Different full-wave antenna geometries can have very different impedance and radiation characteristics.
Velocity Factor
The ideal wavelength uses the propagation speed in free space. Real antenna structures can have an effective electrical length that differs from the simple free-space calculation. A velocity factor can therefore be used as a practical correction.
Ideal Calculation
With a velocity factor of 1.000, the calculator uses the selected wave speed directly. For electromagnetic waves in vacuum, the speed of light is approximately 299,792,458 m/s.
Practical Correction
Conductors, insulation, antenna diameter, nearby materials, feed structures, end effects and ground planes can change the effective electrical length. The final physical antenna may therefore need tuning.
Common Antenna Lengths
The following examples use the speed of light in vacuum and a velocity factor of 1.000. Practical antennas can require shorter or otherwise adjusted dimensions.
| Frequency | Wavelength | Quarter-Wave | Half-Wave | Full-Wave |
|---|---|---|---|---|
| 100 MHz | 2.998 m | 749.5 mm | 1.499 m | 2.998 m |
| 433 MHz | 692.4 mm | 173.2 mm | 346.4 mm | 692.4 mm |
| 868 MHz | 345.4 mm | 86.4 mm | 172.8 mm | 345.4 mm |
| 915 MHz | 327.6 mm | 81.9 mm | 163.8 mm | 327.6 mm |
| 1.0 GHz | 299.8 mm | 74.95 mm | 149.9 mm | 299.8 mm |
| 2.4 GHz | 124.9 mm | 31.22 mm | 62.45 mm | 124.9 mm |
| 5.8 GHz | 51.69 mm | 12.92 mm | 25.85 mm | 51.69 mm |
| 10 GHz | 29.98 mm | 7.495 mm | 14.99 mm | 29.98 mm |
Worked Antenna Length Examples
These examples demonstrate how frequency translates into wavelength and then into a useful antenna starting dimension.
2.4 GHz Quarter-Wave
For a 2.4 GHz signal using the speed of light:
λ/4 ≈ 31.23 mm
This is an ideal starting point. A practical 2.4 GHz antenna can require adjustment based on its construction and surrounding materials.
433 MHz Half-Wave
For a 433 MHz signal:
λ/2 ≈ 346.18 mm
A real dipole or other antenna structure can use a somewhat different physical dimension after accounting for conductor geometry and end effects.
868 MHz Quarter-Wave
At 868 MHz:
λ/4 ≈ 86.35 mm
A velocity factor below 1 can reduce the estimated physical length.
915 MHz Quarter-Wave
At 915 MHz:
λ/4 ≈ 81.91 mm
This calculation is useful as an initial dimension before measuring or tuning the completed antenna.
Antenna Length and RF Design
Frequency
Higher frequency means a shorter wavelength. This is why antennas for GHz systems are physically much smaller than antennas for lower-frequency radio systems.
Ground Plane
Quarter-wave monopoles commonly depend on a ground plane or another suitable return path. The ground structure can affect the electrical behavior and practical dimensions.
Feed Point
Antenna length alone does not determine impedance. Feed position, geometry and surrounding conductive structures also affect the input impedance.
Conductor Diameter
Conductor diameter can influence bandwidth, resonance and the exact physical length required to reach a target frequency.
Nearby Materials
Plastic enclosures, batteries, circuit boards, cables, metal surfaces and other objects can change antenna behavior, especially at higher frequencies.
Tuning
The calculated dimension is normally a starting point. Final tuning can be performed using an antenna analyzer, VNA or appropriate RF measurement equipment.
Custom Wavelength Fractions
Not every antenna uses exactly a quarter, half or full wavelength. The calculator therefore supports a custom wavelength fraction.
λ / 2
Half-wave reference length.
λ / 4
Quarter-wave reference length.
λ / 8
Shorter wavelength fraction.
k × λ
General wavelength-based dimension.
Antenna Length Calculator Applications
Wi-Fi
Estimate wavelength and basic antenna dimensions for 2.4 GHz and 5 GHz RF systems.
LoRa & ISM
Calculate starting dimensions for common 433 MHz, 868 MHz and 915 MHz systems.
RF Prototyping
Quickly estimate antenna dimensions during early electronics and PCB development.
Radio Projects
Calculate quarter-wave and half-wave reference dimensions for amateur and experimental radio work.
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Antenna Length Calculator FAQ
What is an antenna length calculator?
An antenna length calculator estimates the physical length of an antenna from its operating frequency, wave speed, antenna fraction such as quarter-wave or half-wave, and velocity factor.
What is the formula for antenna length?
The basic wavelength is λ = v/f. A quarter-wave antenna has an approximate length of λ/4, while a half-wave antenna has an approximate length of λ/2. The practical length can be adjusted using a velocity factor.
What is a quarter-wave antenna?
A quarter-wave antenna has an electrical length of approximately one quarter of a wavelength. It is commonly used in monopole antennas and many practical RF systems.
What is a half-wave antenna?
A half-wave antenna has an electrical length of approximately one half of a wavelength. The half-wave dipole is one of the most common reference antennas in radio-frequency engineering.
What is velocity factor?
Velocity factor is the ratio between the propagation speed in a practical antenna or transmission structure and the speed of light in vacuum. It can be used as a practical correction factor when estimating physical antenna length.
What velocity factor should I use?
A value of 1.00 represents the ideal free-space wavelength calculation. Real antennas can require a lower effective velocity factor because of conductor geometry, insulation, nearby materials, end effects and construction. The appropriate value depends on the antenna design.
What is the antenna length for 2.4 GHz?
At 2.4 GHz in free space, the wavelength is approximately 124.9 mm. A quarter-wave length is approximately 31.2 mm and a half-wave length is approximately 62.5 mm before practical correction factors.
Does antenna length have to be exact?
Antenna dimensions affect resonance and impedance, but the simple wavelength formulas are starting-point calculations. Real antenna performance also depends on conductor diameter, geometry, feed point, ground plane, nearby objects, materials and installation.