Antenna Length Calculator
Cut lengths for half-wave dipoles, quarter-wave verticals, 5/8-wave whips and loops at any frequency, using the 468/f end effect and coax velocity factor.
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About this toolWhat is the Antenna Length Calculator?
A wire antenna works best when it is resonant: its length matches a set fraction of the wavelength at your operating frequency. This antenna length calculator turns a frequency into the cut length of the common types: a half-wave dipole (total and each leg), a quarter-wave vertical or ground-plane radial, a 5/8-wave vertical and a full-wave loop. Lengths show in feet and inches or in metres, next to the classic ham-radio rules such as 468 ÷ f for a dipole.
Two correction factors are adjustable. The end-effect factor K (0.95 by default) accounts for a real wire resonating shorter than half a free-space wavelength; it is where the 468 in 468/f comes from. The velocity factor of your feedline sets the physical length of coax or ladder-line sections such as quarter-wave stubs and matching sections. A reverse panel estimates the resonant frequency of a dipole you have already cut.
- Half-wave dipole with per-leg length, quarter-wave, 5/8-wave and full-wave loop
- Adjustable end-effect factor and the 468/f, 234/f, 585/f and 1005/f rules for comparison
- Velocity factor presets for coax, twin-lead and ladder line with 1/4, 1/2 and 1 λ sections
- Feet and inches or metres, band presets from 80 m to 2.4 GHz
- Dipole length to resonant frequency
- Runs in your browser, shareable link
How to use the Antenna Length Calculator
- Enter the frequency and pick kHz, MHz or GHz, or tap a band preset such as 40 m, 2 m or 433.92 MHz.
- Leave the end-effect factor at 0.95 for bare HF wire, or lower it a little for insulated wire or thick tubing.
- Set the feedline velocity factor from the preset list or your cable datasheet if you need stub lengths.
- Read the element lengths in feet and inches or metres. For a dipole, cut two legs of the length shown.
- Cut slightly long, raise the antenna, measure SWR and trim both ends equally until the minimum lands where you want it.
Worked examples
20 m dipole
Input
14.2 MHz, K 0.95
Output
10.028 m (32 ft 10.8 in) total, 5.014 m per leg
468 ÷ 14.2 gives 32.96 ft; the 0.2% difference is the rounding of 467.2 up to 468.
40 m dipole
Input
7.15 MHz, K 0.95
Output
19.92 m (65 ft 4.1 in) total
Half a free-space wavelength would be 20.96 m (68.8 ft); the end effect takes off about a metre.
2 m quarter-wave ground plane
Input
146 MHz
Output
Radiator 0.488 m (19.2 in)
The rule of thumb 234 ÷ 146 = 1.60 ft gives the same answer. Radials are usually cut the same length or a little longer.
Coax quarter-wave stub
Input
14.2 MHz, RG-58 (VF 0.66)
Output
3.48 m of cable
A free-space quarter wave is 5.28 m, but the wave travels at 66% of light speed in solid PE coax.
40 m full-wave loop
Input
7.15 MHz
Output
42.77 m (140 ft 3.8 in) around, 10.69 m per side if square
1005 ÷ 7.15 = 140.6 ft. The loop is longer than one wavelength, not shorter.
Background and insight
The wavelength in free space is λ = c ÷ f, with c = 299,792,458 m/s. At 14.2 MHz that is 21.11 m, so half a wavelength is 10.56 m or 34.6 ft. A dipole cut to that length resonates lower than 14.2 MHz, because the current does not drop to zero exactly at the wire ends and the ends add capacitance. Shortening the wire by about 5% compensates. Half a free-space wavelength in feet is 491.8 ÷ f(MHz), and 0.95 of it is 467.2 ÷ f, which the ARRL Antenna Book and every licensing manual round to 468 ÷ f. The quarter-wave rule 234 ÷ f and the 5/8-wave figure of about 585 ÷ f follow the same way.
A closed loop has no ends, so the end effect works the other way and a full-wave loop resonates a few percent longer than one wavelength. The ARRL figure is 1005 ÷ f(MHz) feet, about 1.02 λ, which the calculator uses for the loop row. The end-effect factor depends on the ratio of length to wire diameter: thin HF wire is close to 0.95 to 0.97, insulated wire a little lower because the insulation slows the wave, and fat VHF tubing lower still. That is why every formula gives a starting length to be trimmed, not a final one.
Inside a transmission line the wave travels slower than in air, by the velocity factor (VF). Solid polyethylene coax such as RG-58 and RG-213 has a VF of about 0.66, foam coax 0.78 to 0.85 (LMR-400 is 0.85), 300 Ω twin-lead about 0.80, 450 Ω window line about 0.91 and open-wire line about 0.97. A quarter-wave matching section or stub cut from coax is therefore only 0.66 of the free-space quarter wave. Velocity factor applies to feedline sections, not to the radiating wire.
Common use cases
Amateur radio
Cut dipoles, inverted Vs, ground planes and loops for HF, VHF and UHF bands.
CB and marine VHF
Size a quarter-wave or 5/8-wave whip for 27 MHz or 156.8 MHz.
IoT and RF modules
Make a quick wire antenna for 433 MHz, 868 MHz or 915 MHz LoRa and ISM radios.
Receiving antennas
Build an FM broadcast, ADS-B (1090 MHz) or weather-satellite antenna from wire or rod.
Tips and limitations
- Always cut long and trim. Each centimetre matters more at VHF than at HF.
- Height above ground, nearby trees, gutters and the feedline all move resonance by a few percent.
- An inverted V with a 90° to 120° apex usually needs to be a few percent shorter than a flat dipole.
- Insulated wire resonates lower than bare wire of the same length; start with a factor nearer 0.93 to 0.95.
- Use the velocity factor from your cable maker's datasheet. Foam coax varies more between makers than solid PE.
- A 5/8-wave vertical needs a matching coil at its base to present 50 Ω; the length here is the radiator only.
Frequently asked questions
How do you calculate dipole antenna length?
Divide 468 by the frequency in MHz for the total length in feet, or multiply half the free-space wavelength (c ÷ f ÷ 2) by about 0.95. Each leg is half of the total.
Where does the 468 in 468/f come from?
Half a wavelength in free space is 491.8 ÷ f(MHz) feet. Real wire resonates about 5% shorter because of end effect, and 0.95 × 491.8 = 467.2, rounded to 468.
How long is a quarter-wave antenna for 146 MHz?
About 19.2 in (0.488 m): 234 ÷ 146 = 1.60 ft. Trim it while checking SWR on your radio.
What is velocity factor in coax?
The speed of a signal in the cable as a fraction of the speed of light. Solid polyethylene coax is about 0.66, foam types 0.78 to 0.85. It sets the physical length of stubs and matching sections.
Why is a loop antenna longer than a wavelength?
A closed loop has no free ends, so there is no end effect shortening it. In practice a full-wave loop resonates at about 1.02 wavelengths, which is the ARRL 1005 ÷ f(MHz) feet.
Does the velocity factor apply to the antenna wire?
No. Velocity factor applies to transmission lines. The radiating wire uses the end-effect factor, which is a smaller correction of a few percent.
How long should a 5/8-wave antenna be?
About 585 ÷ f(MHz) feet, or 0.625 × the wavelength × the end-effect factor. At 146 MHz that is roughly 4.0 ft (1.22 m), plus a base matching coil.
Is there an in-depth guide to the Antenna Length Calculator?
Yes. “Dipole Antenna Length: Where 468/f Comes From and When to Trust It” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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