The EFHW Shunt Capacitor: A Double-Edged Sword

Updated: November 2025

End-fed half-wave (EFHW) antennas are popular because they are mechanically simple, multiband-capable, and can be fed with a compact impedance transformer (commonly 49:1 or 64:1) into coaxial cable. A frequent “extra” found in many EFHW matching boxes is a shunt capacitor — a capacitor placed across part of the transformer, usually on the low-impedance side.

That capacitor can make the SWR look better and can even pull the SWR dip into a band. But that convenience is exactly why it is a two-edged sword: it can move the apparent resonance away from the antenna’s true resonance, hide the real resonant points you should be cutting and tuning for, and in many cases reduce overall efficiency — even though the meter says things improved.

Resonance vs. “the SWR dip you see”

What real resonance means for an EFHW wire

For a half-wave radiator, resonance is fundamentally about the standing-wave current and voltage distribution along the wire:

• The electrical length is close to λ/2 on that band.
• The current distribution follows the expected half-wave shape (current maximum near the center, minima at the ends).
• The feedpoint impedance at the end is very high (often kilo-ohms) and ideally mostly resistive.

Exact values depend strongly on height, nearby objects, ground coupling, and feedline routing.

What the SWR meter is actually telling you

An SWR meter does not measure antenna resonance. It measures how close the entire system looks to 50 Ω resistive at the measurement point.

That “system” includes:

• the transformer,
• the shunt capacitor,
• parasitic inductance and capacitance in the box,
• feedline effects,
• common-mode currents,
• and the antenna itself.

A clean SWR dip can therefore be caused by something other than the wire being at true resonance.

What a shunt capacitor really does in an EFHW box

A shunt capacitor adds frequency-dependent susceptance. In simple RF terms:

• capacitive reactance decreases as frequency increases,
• in shunt, it acts as a frequency-dependent “leak” that alters the impedance seen by the radio.

If the EFHW system without the capacitor appears inductive at the 50 Ω port (very common), the shunt capacitor can cancel that inductive reactance at some frequency and make the port look more resistive.

The critical point: that cancellation can occur at a frequency where the wire itself is not resonant.

Why the capacitor shifts the apparent resonant point

The wire and transformer produce an input impedance that varies with frequency. Adding a shunt capacitor places another frequency-dependent element in parallel.

In admittance terms:

The apparent resonance occurs where the total susceptance is zero — not necessarily where the antenna’s own reactance crosses zero.

Once the capacitor is present, those two conditions no longer coincide. The SWR dip moves.

How the shunt capacitor hides the real resonant points

EFHW antennas already exhibit multiple resonances (fundamental and harmonics), with large impedance swings. Transformers are also non-ideal, with magnetizing inductance, leakage inductance, winding capacitance, and core loss.

Adding a shunt capacitor creates a new resonant interaction between:

• transformer inductances and parasitics,
• the shunt capacitor,
• and the antenna impedance.

The result can be a deep SWR dip dominated by the LC behavior of the matching unit, while the wire’s natural resonance sits elsewhere. If you trim the wire to chase that dip, you are tuning the box — not the antenna.

Why tuning to the “wrong dip” reduces efficiency

Low SWR does not guarantee high radiation efficiency. A dummy load proves that daily.

1) Increased circulating reactive current

When the capacitor resonates with transformer inductance, circulating current between L and C can be far higher than line current. Those currents do not radiate — they create heat through winding loss, ferrite loss, capacitor ESR, and connection resistance.

2) Higher transformer loss and stress

The altered phase and magnitude of voltage and current can push the transformer into a lossier operating region, increasing core heating and insertion loss before power ever reaches the radiator.

3) Detuned radiator behavior

Trimming the wire to satisfy the LC network rather than true half-wave resonance can degrade current distribution and increase coupling into unintended conductors.

4) Masked common-mode problems

Changing impedance relationships can increase shield current at certain frequencies. Because the SWR looks good, these problems often go unnoticed.

Why a tuner at the shack is often more efficient

A tuner corrects mismatch without forcing an artificial resonance at the antenna feedpoint.

• The wire can be cut for true resonance.
• The tuner usually handles modest mismatch with low loss on HF.
• The system remains predictable as installation conditions change.

Very long coax runs or extreme mismatch may justify a remote tuner — but that is fundamentally different from a fixed shunt capacitor.

Practical takeaways

• Do not use a shunt capacitor as a tuning knob.
• Cut the radiator for real resonance, not the prettiest SWR dip.
• Use proper choking and a tuner to handle residual mismatch.

Bottom line

A shunt capacitor can be a valid compensation element in a transformer design, but it is also a classic trap. It can shift and hide real resonances, increase internal losses, and make system behavior harder to diagnose.

In many real EFHW installations, a clean transformer design without SWR cosmetics, combined with correct antenna length and a tuner for final matching, delivers higher efficiency and more predictable results than relying on a fixed shunt capacitor.

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