end fed antenna calculator

What this end fed antenna calculator does

This calculator gives quick starting lengths for end-fed wires, especially the popular end-fed half-wave (EFHW) antenna. It uses frequency and velocity factor to estimate a practical wire length in both feet and meters. It also shows related values (quarter-wave and full-wave) and the expected odd-harmonic resonance points often used in multi-band operation.

The numbers are intended for initial cutting. Real-world tuning is always required because nearby trees, roofs, mast height, ground conductivity, feedline routing, and transformer design all shift the resonant frequency.

Core formulas used

Length equations

• Free-space wavelength (feet): 983.6 / f(MHz)
• Half-wave practical wire estimate (feet): 468 / f(MHz) × velocity factor
• Quarter-wave estimate (feet): (468 / f) × VF ÷ 2
• Full-wave estimate (feet): (468 / f) × VF × 2

Why velocity factor matters

Insulation and conductor construction make electromagnetic waves travel slightly slower in wire than in free space. That’s why two antennas cut to the same geometric length can tune differently. A velocity factor (VF) of 0.95 means the electrically equivalent length is reached with a slightly shorter physical wire than VF 1.00.

How to use the calculator effectively

• Pick your target center frequency for the band segment you use most.
• Select a wire preset or enter your own measured/known velocity factor.
• Use a 1% to 3% start-long margin so you can trim down in small steps.
• Install at intended height before final tuning; bench tuning is usually misleading.
• Trim symmetrically only where practical, and re-check SWR after each cut.

Example: 40m end-fed half-wave

Suppose you target 7.1 MHz with insulated wire around VF 0.95. The calculator will give a starting point close to the commonly used EFHW lengths for 40m operation. If you use a 2% start-long margin, you begin slightly longer and trim toward resonance, which is safer than cutting too short.

Transformer and station setup notes

Matching network

Most EFHW systems use a 49:1 transformer for half-wave wire operation. Correct core material, winding ratio, and build quality affect heat handling and bandwidth. Keep power level and duty cycle in mind, especially for digital modes.

Common-mode control

• Use a choke current balun on the coax to reduce feedline radiation.
• Consider a short counterpoise if your setup needs improved stability.
• Route coax away from the radiating section when possible.

Installation and tuning checklist

• Mount as high and clear as practical.
• Avoid running wire directly parallel to conductive gutters or fences.
• Measure with an antenna analyzer at the feedpoint if possible.
• Tune in small increments; write down every change.
• Re-check after weather changes, especially with portable wire antennas.

Safety and legal reminder

• Keep antenna wire and supports far from overhead power lines.
• Follow your local regulations and band plan rules.
• Use proper strain relief and weatherproofing at feedpoint connections.
• Ground and protect equipment according to accepted RF safety practices.

With a good estimate and careful trimming, an end-fed antenna can be compact, effective, and easy to deploy for home stations, field operation, and portable setups.