EFHW planner

End-Fed Half-Wave Antenna Calculator

Calculate end-fed half-wave radiator and counterpoise lengths, apply velocity factor, estimate a 49:1 transformer winding and review harmonic band coverage.

Lowest operating frequency (MHz)

Velocity factor (wire shortening)

Bare copper wire is typically 0.95. Use 0.97–0.99 for thin enamelled wire and 0.90–0.94 for insulated hookup wire.

Counterpoise fraction of radiator

5% (0.05) is a common starting point for a short counterpoise to tame common-mode current.

Target high-impedance load (Ω)

2450 Ω approximates a 49:1 match into 50 Ω. Adjust if you measure a different feedpoint impedance.

Half-wave reference:

The heart of an EFHW is the impedance transformer, typically a 49:1 UNUN. This is required to match the high impedance of the antenna's end (≈2450Ω) to your 50Ω coax. You can design this transformer using our .

This transformer is often wound on a ferrite core. Use the to help select the right core (like an FT-140-43) and determine the number of turns needed to achieve sufficient inductance at your lowest operating frequency, which you can verify on the .

Wire lengths

Radiator length (m) 20.057 m

Radiator length (ft) 65.80 ft

Counterpoise length (m) 1.003 m

Counterpoise length (ft) 3.29 ft

Autotransformer guide

High-impedance target 2,450 Ω

Impedance ratio (high:50 Ω) 49.0 :1

Achieved ratio 49.0 :1

Turns ratio (secondary:primary) 7.00 :1

Primary turns (50 Ω side) 2

Secondary turns (high-Z tap) 14

Assumes 2 turns on the primary. Scale both windings equally to maintain the ratio.

Harmonics covered by this radiator

Order	Frequency	Approx. band	Electrical length
1×	7.10 MHz	≈ 42.2 m	1 × ½ λ
2×	14.32 MHz	≈ 20.9 m	2 × ½ λ
3×	21.66 MHz	≈ 13.8 m	3 × ½ λ
4×	29.12 MHz	≈ 10.3 m	4 × ½ λ
5×	36.71 MHz	≈ 8.2 m	5 × ½ λ
6×	44.41 MHz	≈ 6.8 m	6 × ½ λ

Free-space estimates—nearby objects and loading coils will shift the resonant points slightly.