What the calculator actually solves
NIST gives the exact vacuum speed of light as 299,792,458 m/s. Divide that by frequency in hertz for free-space wavelength. A half-wave dipole uses ½ λ total conductor, a quarter-wave vertical uses ¼ λ radiator, a five-eighth-wave vertical uses ⅝ λ radiator, and a full-wave loop uses 1 λ perimeter as electrical-size references. The entered element factor multiplies the chosen fraction to produce a physical starting length. It is not a universal material constant or a simulation of feedpoint impedance, insulation, tubing diameter, height, ground, objects, or matching.
ARRL’s teaching guide uses 468 ÷ f(MHz) feet for a half-wave dipole, a rough starting rule corresponding to about 95% of exact free-space half-wave length. ARRL builders describe cutting longer and fine-tuning before use. Enter 100% to see pure geometry, or adjust the factor from your own measured design or documented reference. Do not treat a factor as a promise of resonance.
Specific element dimensions
A center-fed half-wave dipole divides the planned total conductor length into two equal arms. The computed length excludes a center feed gap, insulators, attachment bends, and trimming reserve. A quarter-wave or five-eighth-wave vertical result is radiator length only; ground or counterpoise and feed matching must be designed separately. ARRL explains why verticals need an appropriate radial or counterpoise system.
A full-wave loop result is total conductor perimeter. A square with that perimeter has each side equal to one quarter of it; an ideal circle has diameter equal to perimeter ÷ π. These are layout conversions, not a guarantee that both shapes tune identically. Small magnetic loops, loaded antennas, multi-element Yagis, traps, and helices need other models.
Do not mix antenna and cable factors
Element shortening is an empirical construction choice. A transmission line’s velocity factor is the ratio of propagation speed in its dielectric to free-space speed and belongs only to the optional feedline calculation. Its quarter-wave physical length is λ ÷ 4 × entered VF. ARRL’s electrical question pool uses that relationship for a quarter-wave coax example. Check the particular cable and its connectors; nominal VF can vary.
Entering a measured conductor length algebraically reverses the same simplified element equation. That “reference frequency” is not a VNA/SWR reading and does not reveal bandwidth or safe power handling. Use a suitable analyzer and a safe, repeatable adjustment process on the completed installed antenna.
Frequently asked questions
Why does 468 ÷ MHz differ from exact half a wavelength?
468 ÷ MHz feet is an empirical wire-dipole starting convention. Exact free-space half-wavelength is about 491.8 ÷ MHz feet. Their ratio is about 95.16%; actual resonance depends on wire, ends, height, surroundings, and feed arrangement.
Is each dipole arm half of the displayed planned length?
Yes. Planned length for a center-fed dipole is both equal conductors combined; each arm is half. Center hardware and any physical gap add to tip-to-tip spacing but are not included as conductor length.
Do lower and upper frequencies tell me my antenna bandwidth?
No. They show how the same formula changes cut length with frequency. A single fixed antenna’s impedance and usable bandwidth require measurements or a more complete model.
Can I use coax VF as an antenna wire factor?
No. Coax VF describes propagation in that cable dielectric. The editable element factor is a construction assumption for the antenna geometry and is independent of cable VF.
Is the calculated quarter-wave vertical ready to transmit?
No. Radials or a suitable counterpoise, matching, installation, RF exposure, grounding, and the applicable rules must be handled separately. Keep all installation work clear of power lines; ARRL highlights overhead-line hazards.