Quad-Core EFHW Inverted-L 160 m — 80 m — 40 m

Updated 17 November 2025

A short backstory: inverted-L isn’t new

Long before amateur radio adopted broadband EFHW transformers, inverted-L and T-top antennas were already the workhorses of early wireless, AM broadcast, naval and military HF systems. These are simply top-loaded monopoles: a vertical section providing most radiation, and a horizontal “flat-top” that raises radiation resistance and reduces required height.

Early stations and field units commonly fed them using open-wire coupled tuners or low-frequency base networks. Even the classic “Zepp” end-fed half-wave used a single-wire open-wire quarter-wave matching section — the direct ancestor of modern EFHW feeding concepts.

The RF.Guru approach (and why)

Each model uses a quad-core stack for thermal headroom, low magnetizing current and predictable performance without compensation tricks. They operate as dual-band antennas (full-wave on the second harmonic), but deliver peak efficiency on their half-wave fundamentals.

Why these ratios?

160 m (≈ 68:1) and 80 m (≈ 70:1)

The ≈68:1 and ≈70:1 ratios follow directly from the magnetizing reactance required on each band and the number of primary turns needed to achieve it — not from chasing a “textbook EFHW value.”

On 160 m, the transformer must present the highest magnetizing reactance because of the low operating frequency. A 4-turn primary on a quad-core LF stack produces the inductance needed to keep the magnetizing branch far above 50 Ω, even at high power. When that primary is scaled through the autotransformer geometry, the resulting transformation naturally lands at ≈ 68:1, which fits the real-world end impedance (~3.2–3.6 kΩ) of a 160 m inverted-L at 16–20 m height.

On 80 m, frequency doubles — meaning the required reactance doubles automatically. This allows dropping to a 3-turn primary while still keeping excellent reactance margins. Fewer turns slightly increase the effective turns ratio in the autotransformer structure, bringing the natural transformation close to ≈ 70:1. The reduced turn count also lowers parasitic capacitance and leakage inductance, improving high-band behaviour without a capacitor.

In short:
• 4-turn primary → ≈68:1 → optimum for 160 m
• 3-turn primary → ≈70:1 → optimum for 80 m
These values come directly from the physics of reactance and transformer geometry — a practical engineering result rather than a chosen “magic number.”

40 m (≈ 20:1)

A practical 40 m inverted-L includes a long vertical section strongly coupled to ground and the return path. End impedances usually fall in the ~0.8–1.3 kΩ range — far lower than “textbook” EFHW values for high horizontals. A ratio around 20:1 is therefore ideal.

High-frequency ferrite mixes keep loss low and temperature stable on both 40 m and 20 m operation.

Winding topology

To maximise coupling, control leakage inductance and withstand multi-kV end voltage, we vary the winding style along the autotransformer:

• Tri-filar at the cold/tap region → maximum coupling, lowest leakage where current is highest.
• Bi-filar in the mid-section → controlled capacitance with efficient coupling.
• Mono-filar toward the antenna terminal → higher creepage, reduced inter-turn capacitance.

Why we do NOT use a capacitor

Some EFHW transformers add a primary capacitor to flatten SWR on higher HF. We omit it because:

• The magnetizing branch is already far above 50 Ω on the intended bands.
• A capacitor shifts the apparent ratio with height and frequency, leading to narrow sweet-spots.
• It adds a highly stressed component in the hottest part of the transformer.
• Dual-band use suffers: one band improves while the other degrades.

Primary turns: 4× vs 3× vs 2×

• 160 m → 4-turn primary (low HF, highest reactance required).
• 80 m → 3-turn primary (reactance still high at doubled frequency).
• 40 m → 3-turn primary (significantly better reactance and flux margin than 2-turn).

Feedpoint height, counterpoise & choke placement

Heights

• Feedpoint: 1–3 m above ground.
• Horizontal wire: 16–20 m (160/80 m), 10–16 m (40 m).

Short counterpoise (non-resonant)

Start around 0.02 λ (acceptable 0.01–0.03 λ):

• 160 m → ~1.6–5 m
• 80 m → ~0.8–2.5 m
• 40 m → ~0.4–1.2 m
• 20 m → ~0.2–0.6 m

Choke placement

Use a high-Z CMC at 0.05–0.1 λ from the feedpoint, and another at the shack entry:

• 160 m → 8–16 m downline
• 80 m → 4–8 m
• 40 m → 2–4 m
• 20 m → 1–2 m

Dual-band behaviour (full-wave on the harmonic)

A full-wave EFHW often presents a higher end-impedance than the half-wave case. The 160/80 m boxes (≈ 68–70:1) remain matchable on their harmonics. The 40 m version (≈ 20:1) typically exhibits ~100–200 Ω on 20 m — tuner-easy or close to 50 Ω depending on height.

Quick spec cheat-sheet

• Feedpoint height: 1–3 m
• Horizontal height: 16–20 m (160/80), 10–16 m (40)
• Counterpoise: ~0.02 λ
• Chokes: 0.05–0.1 λ + shack entry
• Primary turns: 160→4T, 80→3T, 40→3T

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