Long Wire Antenna Length Calculator
Calculate a practical cut length for an end-fed long wire antenna using frequency, electrical length, and wire velocity factor.
What This Calculator Does
A long wire antenna is usually an end-fed wire that is at least one wavelength long at your target frequency. This calculator estimates the physical wire length you should cut so your antenna starts close to resonance. It also gives you quick reference lengths for quarter-wave, half-wave, full-wave, and 1.5-wave sections at the same frequency.
Formula Used
The calculation is based on the wavelength relationship:
Wavelength (meters) = c / f
where c is the speed of light (299,792,458 m/s) and f is frequency in Hz. The final cut length is then adjusted by:
- Electrical length (λ): how many wavelengths of wire you want.
- Velocity factor: compensates for slower wave propagation in real wire.
- End-effect trim: optional shortening percentage for installation/tuning margin.
How to Choose Electrical Length
1.0 λ (one wavelength)
A classic long wire starting point. Radiation lobes begin to show directional behavior compared to shorter wires.
1.5 λ to 2.0 λ
Longer wires can provide gain in certain directions, but feed impedance and lobe patterns become more complex. These often work best with a tuner and a matching transformer (for example, a 9:1 unun in non-resonant setups).
Less than 1.0 λ
Still useful, but technically closer to random wire or end-fed wire operation than a “true” long wire on that band.
Practical Installation Tips
- Cut slightly long, then trim to tune—never cut exact final length on the first try.
- Keep the feedpoint and wire away from metal gutters, power lines, and utility wiring.
- Use a proper RF ground, counterpoise, or matching network as required by your system.
- Measure with an antenna analyzer at installed height, not on the ground.
- Weatherproof feedpoint and strain-relief connections for long-term reliability.
Safety Reminder
Antennas can contact dangerous voltages from nearby lines and can develop high RF voltages during transmit. Install only where legal and safe, follow local regulations, and keep clear of overhead power infrastructure at all times.
Quick FAQ
Does velocity factor really matter for wire antennas?
Yes. It can shift resonance enough to notice, especially on higher frequencies and with insulated wire.
Can I use this for multi-band operation?
Yes, as a starting point. Multi-band end-fed and random wire systems usually rely on a tuner, transformer, and careful feedline management.
Is this a replacement for field testing?
No. Use this calculator for initial cut length, then verify and refine with real measurements at your installed location.