Engineering the End-Fed Half-Wave (EFHW) Antenna: Multi-Band Theory & Matching
An in-depth analysis of high-impedance voltage feeding, toroidal impedance transformers, and harmonic behavior across the amateur HF spectrum.
The End-Fed Half-Wave antenna has emerged as one of the most effective and versatile aerial designs for modern amateur radio installations, particularly for operators facing HOA restrictions, portable SOTA/POTA activators, and space-constrained stations. Unlike traditional center-fed dipoles that require coaxial cable suspended in mid-span, an EFHW is fed at a single high-voltage boundary point, allowing direct feedline entry at the station wall, balcony, or mast base. Our precision EFHW antenna calculator provides exact radiator dimensions, harmonic expectations, and counterpoise specifications to guarantee peak radiation efficiency.
Voltage-Fed Impedance Transformation with a 49:1 UNUN
At the terminal end of an electrical half-wavelength conductor, the standing wave distribution exhibits a current minimum (node) and a voltage maximum (antinode). Because electrical impedance is the mathematical ratio of voltage to current (Z = V / I), the feedpoint impedance at the wire tip reaches extreme values, typically ranging between 2,400Ω and 3,300Ω depending on wire height above ground.
To match this high load impedance to standard 50-ohm coaxial transmission lines, a broadband impedance transformer is mandatory. A 49:1 UNUN (unbalanced-to-unbalanced) transforms 2,450Ω directly down to 50Ω (50 × 49 = 2450Ω), utilizing a 2-turn primary to 14-turn secondary winding on low-loss NiZn or MnZn ferrite toroids (such as Fair-Rite 43-mix or 52-mix cores). For antennas operating over low-conductivity soil where end impedances exceed 3,000Ω, a 64:1 UNUN (2 turns primary to 16 turns secondary) can be substituted to achieve a lower baseline SWR. A high-voltage 100 pF to 150 pF compensation capacitor across the primary winding neutralizes transformer leakage inductance on higher HF bands.
UNUN Transformation Ratios
Harmonic Resonance Across Multiple Amateur Bands
The crowning advantage of an end-fed half-wave radiator is multi-band capability without needing an antenna tuner. Because any integral multiple of a half-wavelength (n × λ/2) produces a high-voltage node at the wire termination, a single wire cut for a 40-meter fundamental frequency (~7.1 MHz) automatically supports natural harmonic resonance on 20 meters (2nd harmonic / 14.2 MHz), 15 meters (3rd harmonic / 21.3 MHz), and 10 meters (4th harmonic / 28.4 MHz).
However, due to end-effect capacitive fringing that remains relatively constant regardless of frequency, the higher harmonic resonances naturally shift upward by 1% to 2% relative to exact arithmetic multiples. When cutting your radiator to the calculated end-fed half wave length, tune the fundamental frequency toward the lower edge of the CW/Digital band allocation. This ensures that the 20m, 15m, and 10m harmonic frequencies fall squarely inside the active voice sub-bands.
Conductor Insulation, Wire Velocity Factor, and Counterpoise
When selecting wire for an outdoor installation, the dielectric permittivity of the insulating jacket plays a major role. Polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE), and nylon jacketed stranded copper wires slow RF wave propagation. This dielectric slows the current speed relative to free space, introducing a wire velocity factor typically between 0.95 and 0.97 for insulated conductors, compared to 0.98 for bare solid copper wire. Failing to account for this velocity factor will result in an antenna wire that is electrically 3 to 4 feet too long on lower bands.
Finally, although an EFHW operates with minimal feedpoint current, Kirchoff's current law dictates that a return path must exist. An effective installation requires a dedicated counterpoise wire approximately 0.05 wavelengths long (~6.5 feet on 40 meters) connected to the ground terminal of the transformer. Alternatively, you can permit 15 to 20 feet of the coaxial feedline braid to serve as the counterpoise return, terminating it with a high-choking-impedance 1:1 common-mode choke (ferrite sleeve isolator) before the cable enters the radio room. This suppresses stray RF feedback in the shack and stabilizes the receiver noise floor.