End-Fed Half-Wave Antenna Calculator
Calculate the practical wire length and feed-system references for an end-fed half-wave (EFHW) antenna.
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Inputs
Live
Math
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Related
Enter parameters and click Calculate to view results
Formula & Theory
Ideal Half-Wave Length (m) = 150 / f(MHz), Cut Length (m) = Ideal Half-Wave Length × Shortening FactorThis formula is used to calculate antenna parameters for end-fed half-wave antenna calculator.
Overview
Use this end-fed half-wave antenna calculator to estimate a starting wire length for a chosen amateur-radio frequency. It applies a wire shortening factor to the free-space half wavelength, giving a cut length that should be measured and tuned after the antenna is installed.
Input Guide
Enter Resonant Frequency, Wire Shortening Factor exactly in the units shown by this end-fed half-wave antenna. Check the operating band, unit prefix, and decimal position before calculating; these are the inputs used by the formula.
- Resonant Frequency — use MHz.
- Wire Shortening Factor.
Output Guide
The results describe the calculated end-fed half-wave antenna values for the inputs you entered. Check each value against the available space, selected components, feed system, and operating conditions before making a final design decision.
How This Calculator Works
The End-Fed Half-Wave Antenna uses Ideal Half-Wave Length (m) = 150 / f(MHz), Cut Length (m) = Ideal Half-Wave Length × Shortening Factor. Supply Resonant Frequency (MHz), Wire Shortening Factor in the displayed units, then use the calculated values as the first engineering target for this wire antennas design or analysis.
Design Notes
An EFHW has a high feed-point impedance and typically uses a 49:1 transformer, though the required transformation ratio depends on the wire, counterpoise, band, and installation. Height, slope, nearby metal, insulation, and feed-line routing can all move the resonant frequency and affect common-mode current.
Build and Tuning Notes
Cut the radiator slightly longer than the calculated value, install the transformer and wire in their final positions, then measure the intended band. Trim or fold back a small amount of wire at a time, and use an effective common-mode choke where needed to keep the coax from becoming part of the antenna.
Frequently Asked Questions
Why does an end-fed half-wave antenna use a 49:1 transformer?
The voltage-fed end of a half-wave wire has a much higher impedance than 50-ohm coaxial cable. A 49:1 transformer provides an approximate impedance transformation to make matching practical, but the installed impedance still varies.
Should I cut an EFHW wire to the exact calculated length?
Use the calculated length as a starting point and leave a small trimming allowance. Wire insulation, end hardware, mounting height, and nearby objects affect the final resonant length.
Does an EFHW antenna need a counterpoise or choke?
Many EFHW installations benefit from a short counterpoise, a carefully managed feed line, and a common-mode choke. These reduce unwanted current on the coax and make antenna behaviour more repeatable.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.