Ham Radio Antenna Calculator
Enter your operating frequency and pick an antenna type to get the exact element lengths to cut. Millimetres, centimetres, and feet/inches all at once, with a live diagram. Download a PDF for the workbench.
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Learn more: cutting antenna elements to length
Velocity factor shortens the cut length
Wavelength is the speed of light divided by frequency. At 14.175 MHz (the 20 m band) that is 299,792,458 / 14,175,000, about 21.15 m. A half-wave dipole is half of that, 10.575 m, but a real wire has to be cut shorter. With the calculator's bare-wire velocity factor of 0.975, the total cut length is 10.31 m, two legs of about 5.16 m.
Wikipedia's dipole antenna entry says the shortening factor runs from about 0.98 for thin wires to 0.94 for thick conductors, and gives the rule of thumb 143/f meters or 468/f feet, with f in MHz. For 14.175 MHz that rule gives 10.09 m, so the calculator's 10.31 m is deliberately a little long.
Impedance by antenna type
The same entry gives a half-wave dipole an impedance of about 73 ohms, and a quarter-wave monopole about 36 ohms, half as much. The calculator shows 73 and 35 ohms for those. Its five-eighths-wave vertical is shown as 50 ohms, but that figure assumes a base-loading coil or matching network, since the bare element needs one to suit 50 ohm coax.
The Yagi entry says the directors are slightly shorter than the driven element and the reflector slightly longer. The calculator sets the reflector at 1.05 times the half-wave length, the first director at 0.945 and the second at 0.905, with spacing of 0.2 wavelength. For a 3-element 20 m Yagi that gives a 10.83 m reflector, a 10.31 m driven element, a 9.74 m director and a boom of 8.46 m. The gain figures in the tool are rough estimates, and a real Yagi needs tuning.
Trim against an SWR meter
Insulation, nearby conductors and height above ground all move the resonant frequency. Cut a little long, check the standing wave ratio with an antenna analyzer or SWR meter, and trim in small steps until resonance lands where you want it.
FAQ
Why is coax's velocity factor so low?
The calculator uses 0.66 for coax with a solid polyethylene dielectric, against 0.975 for bare wire. The plastic between the conductors slows the wave down, so a length of coax cut for a given frequency is much shorter than bare wire.
Does antenna height matter more than type?
For HF it matters a great deal. Height above ground changes the radiation angle and so how well the antenna works for long-distance or regional contacts. Use the calculated length as a start and check the result in place.
What is the difference between NVIS and DX setups?
NVIS antennas are mounted low and send signals nearly straight up to return within a few hundred kilometers, which suits regional contacts. DX antennas are mounted higher to send signals out at a low angle for long distances.