Ham Radio End-Fed Half-Wave Antennas — Complete Guide

The End-Fed Impedance Problem

A center-fed dipole has a feedpoint impedance of approximately 73Ω — close enough to 50Ω for direct coax connection with a simple balun. An end-fed half-wave antenna has its feedpoint at the wire tip rather than the center. At the end of a half-wave resonant antenna, the current is at its minimum and the voltage is at its maximum — producing a very high impedance at the feedpoint.

This end-of-wire impedance is typically 2,000–5,000Ω, varying with frequency, wire height, and surrounding environment. A 49:1 UNUN (unbalanced-to-unbalanced transformer) steps this down to approximately 50Ω for direct coax connection:

The 49:1 ratio is not a perfect match — the actual feedpoint impedance varies with wire length, height above ground, and frequency. A well-built UNUN with appropriate core material handles this variation while maintaining low SWR across the harmonic bands.

Harmonic Resonance — Why One Wire Covers Multiple Bands

An EFHW antenna resonates not only at its fundamental frequency but at every integer multiple of that frequency — all odd and even harmonics. This is because at each harmonic frequency, the wire is an exact multiple of half-wavelengths long, creating a standing wave pattern with a voltage maximum (high impedance) at the fed end.

A 40m EFHW wire (33 feet, fundamental at ~7.15 MHz) resonates on:

• 40m (7.15 MHz) — fundamental half-wave
• 20m (14.3 MHz) — 2nd harmonic, full wave
• 15m (21.45 MHz) — 3rd harmonic, 1.5 waves
• 10m (28.6 MHz) — 4th harmonic, 2 full waves
• These fall near amateur band centers with the 40m EFHW optimized for 7.15 MHz

On each harmonic band, the UNUN presents a good match to the coax without a tuner. The 49:1 ratio is designed to handle the impedance at all harmonic resonances. A short counterpoise wire (2–4 feet) attached to the UNUN chassis and run at right angles to the feedline helps stabilize the impedance and reduces common-mode current problems.

The 49:1 UNUN — Design and Winding

The UNUN is the critical component of any EFHW system. A poorly built or wrongly specified UNUN will produce high SWR, limit usable bands, and in extreme cases overheat and fail at power. The correct construction:

• Core material: FT-240-43 toroid (Fair-Rite type 43) for 3–30 MHz. This material has the right permeability and loss characteristics for HF EFHW operation. Type 31 or 61 core material does not work as well for this application.
• Wire turns: Primary = 2 turns, Secondary = 14 turns. Ratio = 14/2 = 7, impedance ratio = 7² = 49.
• Wire gauge: #24–26 AWG enameled (magnet) wire for low-power portable use; #18–20 AWG for 100W+ fixed installation.
• Winding method: Wind the primary (2 turns) first through the core. Wind the secondary (14 turns) through the same holes. Keep turns tight and consistent.
• Optional capacitor: A 100–150 pF capacitor across the high-impedance terminals can improve SWR on some bands — particularly 30m and 17m where the EFHW is not at exact harmonic resonance.

Common-Mode Current — The EFHW's Biggest Problem

Common-mode current is the primary technical challenge with EFHW antennas. Because the antenna is fed at a high-impedance end with an unbalanced UNUN, there is a strong tendency for RF current to flow back down the outside of the coax shield toward the radio — the coax acts as a counterpoise and becomes part of the radiating system.

Effects of common-mode current on an EFHW:

• SWR varies with coax length — the coax is electrically part of the antenna
• SWR changes when you touch the radio — your body detunes the antenna
• RF in the shack — hot mic, clicking audio, RF burns on hands
• The radiation pattern changes based on how the coax is routed
• Different SWR readings on different bands that don't follow the expected harmonic pattern

Solutions — apply all of them for best results:

• Add a short counterpoise wire (2–4 feet) at the UNUN feedpoint, at right angles to the coax
• Wind a current choke (6–8 turns on FT-240-31) immediately on the coax side of the UNUN
• Keep the coax run away from the antenna wire — perpendicular if possible
• On portable deployments, let the coax hang straight down from the UNUN before routing to the radio

Horizontal — Best for Most Installations

The wire runs horizontal from the UNUN at one end to a support at the far end. This is the most common EFHW configuration and produces the familiar figure-8 dipole-like radiation pattern. The UNUN end is typically at low height (5–8 feet) for easy access, with the far end at maximum available height.

• UNUN at low height — easy to connect coax and adjust counterpoise
• Raise the far end as high as possible for better DX radiation angle
• An unequal height installation (UNUN low, far end high) is fine — the asymmetry has minimal effect on performance
• On portable deployments, a 10m (33 ft) fishing pole at the far end supports the wire end well
• Polarization is horizontal; the pattern is broadside to the wire with nulls off the wire ends

Inverted-L — Better DX Angle

Running the wire vertically for part of its length then horizontal creates an inverted-L configuration that produces lower-angle radiation than a purely horizontal wire — useful for DX performance when a tall mast or tree is available.

• Mount the UNUN at the base of a tall mast or tree
• Run wire vertically up the mast to the maximum height
• Continue horizontally from the top of the mast to a far support point
• The vertical section radiates at low elevation angles; the horizontal section radiates broadside
• The combination produces a lower average takeoff angle than a purely horizontal wire at the same maximum height
• Total wire length follows the same formula — 468/f(MHz) for the fundamental band

Stealth Configurations for HOA Properties

The EFHW is one of the best stealth antenna options for restricted properties because it requires only one feedpoint, no center support, and the wire can follow irregular paths:

• Fence line — run insulated wire along the top of a wooden privacy fence. At 33 feet of fence, a 40m EFHW fits on most residential lots. Use small standoff insulators to keep the wire 2–3 inches from the fence.
• Roofline — run wire along the edge of the roof or up the roofline. Keep the wire 6+ inches from gutters (metal gutters detune the antenna and cause common-mode problems).
• Through foliage — run thin dark wire through dense shrubbery or along tree branches. #26 AWG enamel wire in dark green or brown is nearly invisible from street level.
• Attic — an EFHW can operate from an attic if roofing materials are not lossy. Tile and asphalt shingles have minimal effect; metal roofs and foil-backed insulation significantly reduce performance.

Random Wire Antenna vs True EFHW

A random wire antenna is any wire of non-specific length fed through a matching transformer and tuner. It differs fundamentally from an EFHW:

• EFHW: exact half-wave length at the fundamental frequency — resonant, low SWR on harmonic bands without tuner
• Random wire: non-resonant length — requires a tuner on every band, matching transformer (typically 9:1 or 16:1 UNUN) handles the variable impedance
• EFHW has predictable performance across harmonic bands; random wire performance varies unpredictably
• For dedicated multi-band portable operation, an EFHW is significantly more convenient — no tuner adjustments needed when changing bands
• Random wire is better for emergency and expedient use where the exact wire length cannot be controlled

When choosing wire length for a random wire antenna, avoid lengths that are near λ/2 on any amateur band — near-resonant lengths present very high or very low impedance that the tuner and UNUN may not handle well. Common "good" random wire lengths: 29 ft, 35.5 ft, 41 ft, 58 ft, 71 ft, 84 ft.