About this tool
Resonant antennas have lengths that follow from the wavelength: λ = c / f, roughly 300 / f in MHz metres. A half-wave dipole is about λ/2 long, a ground plane has a λ/4 radiator and a full-wave loop has a circumference of about λ. The antenna length calculator shows all of these for one frequency at a glance.
Because charge builds up at the end of the wire (end effect), a real wire antenna is slightly shorter than the theoretical length. By default the calculator uses a shortening factor of 0.95, which matches the well-known rule of thumb dipole ≈ 143 / f(MHz) metres. For λ/4 and λ/2 coax lines it uses the cable’s velocity factor (0.66 for solid PE).
The quick picks jump to typical frequencies for amateur radio, CB, PMR446, LoRa, ADS-B and Wi-Fi; both factors can be changed in the advanced parameters. Check how well the finished antenna is matched with the SWR Calculator. Everything runs in your browser.
How it works
- Enter the frequency in MHz or pick a band from the quick picks.
- Read the lengths for dipole, ground plane, 5/8 radiator, loop and coax lines.
- Adjust the shortening factor of the wire and the velocity factor of the coax in the advanced parameters if needed.
Frequently asked questions
How long is a dipole for 2 m (145 MHz)?
The wavelength at 145 MHz is 2.07 m, half of it 1.03 m. With a shortening factor of 0.95 the dipole is about 98 cm in total, each leg about 49 cm. A quarter-wave ground plane for 145 MHz also has a radiator of about 49 cm.
What is the shortening factor?
With wire antennas the end effect moves the resonance to a slightly shorter length than λ/2 – about 95 % for thin wire, less for thick tubes or insulated wire. In coax cable the wave travels slower than in free space; the velocity factor is 0.66 for solid PE and about 0.8 to 0.85 for foam PE.
Why is the calculated length not exact?
The resonance also depends on the height above ground, nearby objects, wire thickness, insulation and the feed point. Cut the wire a few percent longer and trim it step by step until the SWR is lowest at the desired frequency.
What are quarter-wave coax lines used for?
A quarter-wave piece of coax transforms impedances: 75 Ω cable becomes a matching section between 50 Ω and about 112 Ω, e.g. for combining antennas. Shorted quarter-wave stubs act as a trap, half-wave lines repeat the impedance at their end. The length has to be calculated with the cable’s velocity factor.