Understanding End-Fed Half Wave Length
An end-fed half wave antenna uses a wire near one half wavelength long. It is fed at a high impedance point. That point usually needs a matching transformer. The length sets the natural resonance. Small changes can move resonance across a band. This calculator gives a strong starting length, then adds trim guidance.
Why Frequency Matters
Frequency controls wavelength. Higher frequency means shorter wire. Lower frequency needs longer wire. The base physics is simple. Wavelength equals wave speed divided by frequency. A half wave is half of that value. Real wire is not perfect free space. Insulation, height, bends, nearby metal, and ground all change the final value.
Velocity and End Effects
The velocity factor adjusts the wire for slower wave travel. Bare wire often uses a value near 0.95. Insulated wire can be lower. End effect accounts for capacitance at the wire end. It can make the wire behave longer than its measured length. Use conservative settings when space is tight. Cut long when possible. Then trim slowly.
Advanced Planning
Many EFHW antennas work on harmonic bands. A 40 meter wire can often load on 20, 15, and 10 meters. The actual match depends on the transformer, installation, and tuner. Harmonic count in this tool estimates longer wires for multiple half waves. It is useful for special layouts, portable work, and experiments.
Installation Notes
Keep the transformer close to the feed point. Use a short counterpoise when needed. Route the wire clear of gutters, fences, and power lines. Height changes the feed impedance and pattern. Sloping wires can work well. Inverted L layouts are also common. Always test with low power first.
Trimming Method
Measure resonance with an analyzer. Start below the target frequency if the wire is long. Shorten the far end in small cuts. Recheck after each trim. Leave a little foldback for later adjustments. Weather can shift readings. Final tuning should happen in the real location. Record every cut and measurement.
Safety Checks
Never install near electrical conductors. Use strong supports and strain relief. Add lightning protection where appropriate. Label the feed system. Save exported results with site notes. Good records make future repairs faster and repeatable in any season.