Why a Low EFHW Can “Look Good” on SWR

End-Fed Half-Wave (EFHW) antennas have a reputation for “working even when they shouldn’t.” A classic example is an EFHW installed very low (matchbox and both wire ends around 1 m above ground), yet the SWR looks perfectly usable—sometimes even “resonant.”

That situation is real, but it’s easy to misread. Low SWR only tells you the radio sees something near 50 Ω at the end of the coax. It does not guarantee the radiator is efficient, nor that the wire is behaving like a clean half-wave in a clean environment.

(In other words: SWR is a matching meter, not a performance meter.)

What an EFHW Is Supposed to Do When the Install Is “Clean”

A true EFHW is roughly a half-wavelength of wire on the lowest band. It is fed at the end, which is a high-voltage / low-current point. In a practical HF installation, the impedance at that point is typically in the few-kΩ range—often discussed around ~2–5 kΩ depending on height, ground, and surroundings.

That’s why the common matchbox uses a 49:1 impedance transformer (about 7:1 turns ratio). When the antenna’s end impedance is in the expected ballpark, the transformer can present something close enough to 50 Ω that many rigs can run without a tuner.

(Those numbers are not universal “constants.” They move a lot with height and coupling. That’s the whole point of this article.)

Why Height Matters So Much for End-Fed Wires

When the end of a half-wave wire sits close to ground, two things get you at the same time:

• Capacitive coupling to ground increases at the high-voltage sections (wire end(s), and often the matchbox area).
• Loss becomes easier to “convert” into a nice match (soil loss, transformer loss, and sometimes coax loss).

This is why an end-fed wire that is “textbook” at a decent height can become a completely different electrical system when it’s close to earth, wet vegetation, gutters, fences, or even people moving nearby.

The Matchbox Capacitor: Helpful… and Sometimes Misleading

Many EFHW matchboxes include a shunt capacitor (often on the radio side / primary side of the transformer), commonly in the rough 100–220 pF neighborhood. The intended purpose is usually practical: compensate transformer leakage inductance and stray capacitances, often improving the higher-band SWR behavior.

But electrically, that capacitor plus the transformer’s inductance/parasitics can form something that behaves like a small matching network (think “L-network flavor”), not just a pure impedance transformer. That means the matchbox can sometimes create a neat SWR dip even when the radiator is being heavily detuned or loaded by its environment.

The capacitor isn’t “evil” by default. The problem is when a low, strongly-coupled installation allows the network + losses to produce a match that looks honest—while radiation efficiency quietly collapses.

How a Low EFHW Can “Look Great” While Radiating Less

Here are the common mechanisms that make a poor install look “fine” on SWR:

• Loss-match effect: when ground/nearby-losses/transformer-losses add resistive components, the radio can see something closer to 50 Ω even though less power is radiated.
• Shifted impedance peak: end-loading by capacitance to ground reduces the high impedance you were expecting, changes current distribution, and moves resonance.
• Common-mode current turns the system into “wire + coax + shack”: if the return currents are not controlled (choke placement, counterpoise strategy, bonding), the coax shield and nearby wiring can become part of the antenna. That can stabilize SWR while making pattern and efficiency unpredictable.

This is why a very low EFHW can still make contacts: something is radiating. The trap is assuming that “it tunes” means “it’s working as designed.”

A Practical Height Target for 40–10 m

For 40–10 m end-fed wires (EFHW and end-fed off-center designs), a practical goal is to keep the high-voltage areas out of the “ground-coupled clutter zone.” A simple rule of thumb that works well in real installs is:

(Safety note: EFHW wire ends can reach very high RF voltages even at modest power. Keep ends out of reach and away from conductive objects.)

How to Tell “Good Match” From “Good Radiation”

If you want to diagnose whether you’re seeing a real antenna match or a “loss + coax participation” match, use tests that care about current and field strength—not just SWR:

• Choke A/B test: add or move a proper common-mode choke and watch what changes. If SWR and behavior swing wildly, the coax was likely part of the radiator.
• Raise the system and compare reports: move from ~1 m to a few meters higher and compare WSPR/FT8 reports (or a fixed remote SDR). Compare signal, not SWR.
• Feel the losses: unexplained matchbox heating at power levels that should be easy is a strong hint you’re burning watts in the transformer/network.
• Look beyond SWR: if your analyzer shows a suspiciously “nice” resistive component over a wide range in a messy install, be skeptical—loss often looks like resistance.

Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.

Questions or experiences to share? Contact RF.Guru for antenna setup help and troubleshooting.