Choose an amateur, CB, marine or broadcast band, or type any frequency, to get the cut length for a half-wave dipole, each dipole leg, a quarter-wave or 5/8-wave vertical, radials and a full-wave loop. Adjust the velocity or end-effect factor for your wire or tubing.
Last updated:
Start from
MHz
%
95% for wire dipoles and verticals; 100% = free-space length; use the cable VF for coax stubs
Enter your values to see the result.
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How to use the Antenna Length Calculator
Pick a band and where in the band you want the antenna resonant, or switch to a custom frequency.
Set the factor: 95% suits most wire antennas; use 100% for the free-space length.
Choose feet or meters for the results.
Read the dipole, vertical and loop lengths and the formula used.
Cut slightly long, then trim while checking SWR with an analyzer or meter.
Antenna length formulas
Antenna
Length (feet)
Length (meters)
Free-space wavelength
983.6 ÷ f
299.8 ÷ f
Half-wave dipole (total)
468 ÷ f
142.5 ÷ f
Dipole leg / quarter-wave vertical
234 ÷ f
71.3 ÷ f
5/8-wave vertical
584 ÷ f
178 ÷ f
Full-wave loop (perimeter)
1005 ÷ f
306.3 ÷ f
f is the frequency in MHz. The dipole and vertical rows include a 95% end-effect factor, which is what the calculator uses by default. Set the factor to 100% to see pure free-space fractions of a wavelength.
Typical dipole lengths
Band-center lengths with the default 95% factor (free-space wavelength × 0.475 for the total).
Band
Frequency
Dipole total
Each leg
80 m
3.75 MHz
124.6 ft (38.0 m)
62.3 ft
40 m
7.15 MHz
65.3 ft (19.9 m)
32.7 ft
20 m
14.175 MHz
33.0 ft (10.0 m)
16.5 ft
15 m
21.225 MHz
22.0 ft (6.7 m)
11.0 ft
10 m
28.85 MHz
16.2 ft (4.9 m)
8.1 ft
6 m
52 MHz
9.0 ft (2.7 m)
4.5 ft
Tips for building and tuning
Cut long, then trim. Removing wire raises the resonant frequency; adding it back is harder.
Height matters. A dipole less than about a half wavelength above ground shows a lower feed impedance and may resonate slightly lower in frequency.
Insulated wire usually needs a factor of about 93 to 95%, since the insulation slows the wave along the conductor.
Coax stubs and phasing lines use the cable's velocity factor instead (about 66% for solid polyethylene, 78 to 85% for foam dielectric).
Licensing: transmitting on amateur bands requires a license; receive-only antennas for broadcast, airband or weather radio do not.
Frequently asked questions
How do I calculate the length of a dipole antenna?
A half-wave dipole is about half a wavelength, shortened by about 5% for end effect. In feet the classic formula is 468 ÷ f(MHz); in meters it is about 142.5 ÷ f(MHz). For 14.2 MHz that gives about 32.96 ft in total, or two legs of about 16.5 ft each.
What is the formula for a quarter-wave vertical antenna?
A quarter-wave vertical is about 234 ÷ f(MHz) feet, or 71.3 ÷ f(MHz) meters, with the same 95% factor. For 146 MHz on the 2 m band that is about 1.6 ft (19.2 in).
What is the velocity or end-effect factor?
Radio waves travel along a wire slightly slower than in free space, and the capacitance at the wire ends makes an antenna resonate as if it were a little longer. Together these mean a practical antenna is cut about 2 to 5% shorter than the free-space length. Thicker elements and insulated wire need a lower factor.
Why is a 5/8-wave vertical popular?
A 5/8-wave vertical concentrates more energy toward the horizon than a quarter-wave, giving roughly 3 dB of gain over a quarter-wave ground plane at low radiation angles. It needs a matching coil or network at the base because its feed impedance is not close to 50 ohms.
How long should the radials be?
Elevated radials for a quarter-wave ground plane are normally a quarter wavelength each, about the same length as the vertical element. Three or four radials are typical for elevated antennas; ground-mounted verticals benefit from many more.
Why is a loop antenna longer than a wavelength?
A full-wave loop resonates when its perimeter is a little longer than one free-space wavelength, so it is usually cut to about 1005 ÷ f(MHz) feet, around 2% longer than the wavelength.
Should I cut the antenna to the exact calculated length?
Use the result as a starting point. Height above ground, nearby objects, wire insulation and the feed line all shift resonance, so cut the wire a few percent long and trim it while measuring SWR.
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