End-Fed Half-Wave Antenna

The end-fed half-wave (EFHW) is a half-wavelength wire fed at one end through a 49:1 impedance transformer (unun). It is the default antenna in SimTX, configured as a 32.5 m wire at 10 m height, which is close to resonance at the default frequency of 4625 kHz.

Geometry in SimTX

A single wire hung as a catenary from the configured height, fed at one end. The 49:1 transformer is included in the electrical model. Default wire length 32.5 m.

Characteristics

• Typical gain around 2.1 dBi with a takeoff angle around 28 degrees at typical heights.
• Because the end of a half-wave wire presents a very high impedance, the 49:1 transformer is required to bring it near 50 ohms. The transformer costs a small insertion loss (about half a decibel, growing when the match is poor).
• Multi-band behaviour: like all end-fed half-waves it also presents usable matches near integer multiples of its fundamental, with slightly worse match and higher transformer loss on the harmonics.
• Practically convenient: one support, feedpoint at the end.

Sizing

The wire should be an electrical half-wave: roughly 143 to 150 divided by the frequency in MHz, in meters. The default 32.5 m corresponds to a fundamental near 4.6 MHz, which is why the application defaults to 4625 kHz. Operating the default antenna at a very different frequency (for example 7.1 MHz, between its harmonics) produces a severe mismatch and almost no radiated signal; either change the wire length or pick a frequency near a harmonic.

When to use

As a realistic general-purpose wire antenna, especially when simulating portable or space-constrained stations. For best single-band performance a resonant Dipole Antenna at good height still has the edge.