Antenna Length Calculator
Estimate physical antenna length from frequency, antenna type, velocity factor and shortening factor. Calculate quarter-wave monopoles, half-wave dipoles, five-eighths-wave radiators, full-wave references and custom wavelength fractions.
This is a geometry and first-cut tuning calculator. It does not predict impedance, gain, radiation pattern, bandwidth, ground-plane behavior, matching networks or final VSWR.
Engineering tool
Antenna Length Calculator
Estimate quarter-wave, dipole, 5/8-wave, full-wave and custom antenna lengths from frequency, velocity factor and shortening factor.
Result console
- Free-Space Wavelength
- 69.236133 cm
- Effective Wavelength
- 69.236133 cm
- Electrical Length
- 17.309033 cm
- Physical Length
- 16.443582 cm
- Physical Length mm
- 164.435817mm
- Physical Length inches
- 6.473851in
- Total Length
- 16.443582 cm
- Per-Element Length
- Not applicable
- Wavelength Fraction
- 0.25
- Electrical Degrees
- 90°
- Velocity Factor Used
- 1
- Shortening Factor Used
- 0.95
- Formula Used
- Lphysical = (c × VF / f) × K × Ks
Calculated antenna length is an ideal starting point. Verify and tune the final antenna with a VNA, SWR meter, or measured installation data.
A quarter-wave monopole usually needs a ground plane, radials, or equivalent return path.
Formula reference
Antenna Length Formulas
RF-006 reuses RF wavelength relationships and applies antenna-specific wavelength fractions and shortening factors.
λ0 = c / fλmedium = c × VF / fLelectrical = λmedium × KLphysical = λmedium × K × Ksf = c × VF × K × Ks / LphysicalHalf-wave dipole: Ltotal = λmedium × 1/2 × KsHalf-wave dipole: Lelement = Ltotal / 2θ = 360° × KVariable definitions
- λ0
- free-space wavelength
- λmedium
- wavelength adjusted by velocity factor
- c
- speed of light, 299,792,458 m/s
- f
- frequency in Hz
- VF
- velocity factor
- Ks
- shortening factor
- K
- wavelength fraction
- Lelectrical
- electrical antenna length before shortening
- Lphysical
- physical antenna length after shortening
- Ltotal
- total antenna length
- Lelement
- one dipole side element length
- θ
- equivalent electrical degrees
Worked Examples
433 MHz quarter-wave
With VF = 1 and Ks = 1, wavelength is 0.692361 m and quarter-wave length is 0.173090 m, or 173.09 mm.
433 MHz with shortening
With Ks = 0.95, the 433 MHz quarter-wave physical length becomes about 164.44 mm.
2.4 GHz quarter-wave
At 2.4 GHz, quarter-wave length is about 31.2284 mm before shortening.
100 MHz half-wave dipole
Total length is 1.498962 m, and each element is 0.749481 m.
100 MHz dipole with Ks = 0.95
Total physical length is 1.424014 m, and each element is 0.712007 m.
915 MHz five-eighths-wave
5λ/8 at 915 MHz is about 0.204776 m, or 204.78 mm.
Coaxial medium reference
At 100 MHz with VF = 0.66, quarter-wave in the medium is about 0.494658 m. This is not automatically an air-wire antenna recommendation.
Length to frequency
A 0.25 m quarter-wave with VF = 1 and Ks = 1 corresponds to about 299.792458 MHz.
Dipole element to frequency
One 0.75 m half-wave dipole element means 1.5 m total length and about 99.930819 MHz.
Custom three-quarter wave
At 150 MHz with K = 0.75, length is about 1.498962 m and electrical length is 270 degrees.
Engineering Notes
- Calculated antenna length is an ideal initial design value.
- End effect, conductor diameter, insulation, PCB ground, enclosure, battery, body proximity and metal structures can detune the antenna.
- A quarter-wave monopole usually needs a ground plane, radials or equivalent return path.
- A half-wave dipole total length is about λ/2; each side is about λ/4.
- Correct length does not guarantee 50 Ω impedance or low VSWR.
- Real antennas usually need measurement and tuning with a VNA or SWR meter.
- Shortening factor is structure-dependent and is not a universal fixed value.
- When trimming a physical antenna, it is usually safer to start slightly long and trim in small increments.
- Transmission-line velocity factor is not automatically the same as free-space radiator shortening factor.
- This calculator does not predict gain, radiation pattern, efficiency or bandwidth.
Common Mistakes
- Entering MHz as Hz.
- Using one dipole element length as the total dipole length.
- Calling the total half-wave dipole length λ/4.
- Confusing velocity factor with shortening factor.
- Using 0.95 as a universal shortening standard.
- Assuming correct length automatically means low VSWR or 50 Ω impedance.
- Ignoring ground plane and nearby enclosure effects.
- Cutting directly to theoretical length without leaving tuning margin.
- Using coax velocity factor for an air-wire antenna.
- Treating 5/8-wave length as a complete matched antenna design.
Support reference
FAQ
How do I calculate antenna length from frequency?
Find wavelength with λ = c × VF / f, multiply by the wavelength fraction, then apply the shortening factor: Lphysical = λ × K × Ks.
How long is a quarter-wave antenna?
An ideal quarter-wave antenna is λ/4. At 433 MHz in air with no shortening factor, that is about 173.09 mm.
How long is a half-wave dipole?
A half-wave dipole total tip-to-tip length is about λ/2. Each side is about λ/4 before practical trimming and end-effect correction.
Is a dipole element λ/4 or λ/2?
Each dipole element is approximately λ/4, while the complete two-sided dipole is approximately λ/2.
What is antenna shortening factor?
Shortening factor estimates how physical length differs from ideal electrical length because of end effects, conductor diameter, insulation and structure.
What is the difference between velocity factor and shortening factor?
Velocity factor changes wavelength because propagation speed changes. Shortening factor adjusts the physical radiator length after the electrical length is found.
Why is a real antenna shorter than the theoretical value?
End effects, conductor diameter, nearby dielectric material, enclosures, PCB ground, feed structure and loading can shift resonance.
Does a quarter-wave antenna need a ground plane?
Usually yes. A quarter-wave monopole normally needs a ground plane, radials, counterpoise or another return structure.
Why does my calculated antenna have high VSWR?
Correct length alone does not guarantee 50 ohm impedance or low VSWR. Feedpoint, ground plane, nearby objects and matching all matter.
Should I cut the antenna exactly to the calculated length?
Usually start slightly long, measure in the final installation, then trim in small steps. The correct trim direction and amount depend on the antenna.
Can I use coax velocity factor for an air-wire antenna?
No. Coax velocity factor describes waves inside the cable. It is not automatically the shortening factor for a free-space wire antenna.
How do I calculate frequency from antenna length?
Use f = c × VF × K × Ks / Lphysical. For a dipole element input, double the element length to get the total dipole length first.
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Future Engineering Guide Topics
How to Calculate Antenna Length
In_DevelopmentQuarter-Wave Antennas Explained
In_DevelopmentHalf-Wave Dipole Design
In_DevelopmentVelocity Factor vs Shortening Factor
In_DevelopmentHow to Tune an Antenna with a VNA
In_DevelopmentGround Planes for Monopole Antennas
In_DevelopmentWhy Nearby Objects Detune Antennas
In_DevelopmentDisclaimer
This antenna length calculator provides first-cut geometry estimates only. Final antenna designs should be measured and tuned in their intended installation with appropriate RF instruments.