Efficient End-Fed Half-Waves on 17 m, 15 m, 12 m, and 10 m

with LC Matching

The 49:1 transformer has become the default answer for almost every EFHW. On the upper HF bands, that convenience often hides a compromise: the antenna’s end impedance is not a fixed number, and it shifts with height, surroundings, wire routing, and how the feedline is managed. A 49:1 can be “usable,” but for a single-band 17 m, 15 m, 12 m, or 10 m half-wave, it is not automatically the right answer.

For these higher bands, a simple monoband LC matcher is often cleaner and more efficient: a series air-core inductor and a fixed shunt capacitor. The coil becomes your tuning knob (stretch/compress turns), while the capacitor stays fixed. The practical point is simple: for a monoband EFHW, it makes more sense to match the antenna you actually built than to force it through a generic broadband ratio.

The circuit

Coax center  ── L (air-core, adjustable) ──●── Radiator (½λ wire)
                                          |
                                          C (fixed)
                                          |
Coax shield ──────────────────────────────●── Return path (counterpoise / ground reference)
    

The feed arrangement is:

• Coax center conductor → series air-core inductor → radiator
• Fixed capacitor from the antenna side of the coil to the return node (shield/counterpoise)
• Coax shield to a defined return path: short counterpoise, or a short feedline section before a common-mode choke

Quick design math (good ballpark)

A practical starting assumption for a monoband EFHW is an end impedance in the low single-digit kilohms. If we pick R_p ≈ 3200 Ω as a “design center” and a 50 Ω radio, standard L-match relations give:

Ballpark values for 17 m, 15 m, 12 m, and 10 m

The table below assumes a 50 Ω radio, a target end impedance around 3.2 kΩ, and a half-wave wire cut slightly long (trim in the final stage). Wire length is based on a common starting formula 143 / f(MHz) in meters, padded by ~2% for trimming.

*Useful L range shown for a practical EFHW end-impedance span of roughly 2.5–4.0 kΩ. That span is realistic: EFHW end impedance varies strongly with geometry, height, and nearby conductors.

Choosing the “fixed” capacitor

On these bands, stray capacitance (box size, lead length, coil self-capacitance) matters. That’s why the capacitor values above are intentionally shown as “first try” ranges:

• 17 m: start with 22 pF
• 15 m: 18 pF is mathematically close, but 20 pF often lands well once stray C is included
• 12 m: 15–16 pF
• 10 m: 12 pF in compact layouts, 15 pF if the box/wiring adds more stray capacitance

Using coax as a rugged fixed capacitor

If you want a mechanically tough “fixed capacitor,” an open-ended piece of small coax can work well as a shunt capacitance element. Treat it like a component: it must be tuned in the same physical position it will occupy in the finished box.

Coil construction (the tuning knob)

On 17–10 m the inductor is pleasantly small, which is exactly why the LC approach shines here. A practical build is an air-core coil on a 20–25 mm former, using solid wire around #18 AWG (or similar).

• Wind for slightly too much inductance
• Reduce inductance by spreading turns (or compress to increase)
• Keep lead lengths short and mechanically stable

For quick intuition, the “zone” is: ~3.5 µH (17 m), ~3.0 µH (15 m), ~2.5 µH (12 m), ~2.2 µH (10 m). That’s close enough to get you on the air… the final setting is always the antenna, the box, the capacitor, and the installation working together.

Counterpoise and choking

Even with an EFHW, give the system a defined return path. A common starting value is around 0.05λ for the counterpoise (or the “free” coax section before the choke). That’s roughly:

• 17 m: ~0.83 m
• 15 m: ~0.71 m
• 12 m: ~0.60 m
• 10 m: ~0.53 m

After that, add a good common-mode choke so the feedline does not become part of the antenna. This is what makes the system more repeatable and keeps “mystery tuning” (and in-shack RF) under control.

Tuning procedure (coil first, wire last)

The cleanest tuning sequence is:

• Cut the radiator slightly long.
• Build the LC network with the chosen fixed capacitor and an air-core coil that is a bit high in inductance.
• Install the antenna with the intended counterpoise/choke arrangement.
• Adjust coil spacing first to bring the match dip where you want it.
• Trim the wire last, in very small steps, to land resonance precisely.

The order matters: the coil is your match control. The wire is your resonance trim. Mixing them randomly is how “it works but I don’t know why” installs are born.

Power and voltage warning

Do not let the small capacitance values fool you: the voltage at the high-impedance point of an EFHW can be substantial. Even modest power can create uncomfortable-to-dangerous RF voltages at the “hot end.”

Conclusion

For the upper HF bands, a monoband EFHW often deserves better than an automatic 49:1 reflex. A 49:1 may still “work,” but it’s a compromise ratio applied to a feedpoint impedance that is not fixed in real life. A simple LC matcher — one fixed capacitor and one tunable air-core coil — is often the more elegant answer for 17 m, 15 m, 12 m, and 10 m.

The beauty of this approach is that it is both efficient and honest: you’re no longer pretending every upper-band EFHW wants the same broadband transformer ratio… you’re building a match for the antenna that actually exists in front of you.

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