EIRP vs SWR: Why a 4:1 EFOC Beats a Perfect-Match EFHW

Updated November 2025 — clarified EIRP metrics and expanded 40–10 / 80–10 comparison.

This article was sparked by a question many operators ask:

“My EFHW has a perfect match. Your EFOC shows 3:1 SWR. Doesn’t that make it inefficient for DX?”

Short answer: no. And the numbers prove it.

Short Answer

• 2–3:1 SWR on 20–30 m of HF coax adds only ~0.2–0.5 dB of extra loss.
• The real losses are inside a 49:1 EFHW transformer (typically 1.5–2 dB at 100 W).
• A 4:1 EFOC runs near-resonant with transformer losses below 0.2 dB.
• EIRP measurements show 1–2 dB more radiated power from EFOC17/29 on every HF band compared to broadband EFHWs.

If your goal is DX, EIRP wins — not SWR.

What Actually Counts: EIRP, Not SWR

EIRP = power at the feedpoint × antenna gain. For two wire antennas of similar length/height, gain is similar — so efficiency dominates.

You can have: • 2:1 SWR and 96% efficiency, or • 1:1 SWR and under 10% efficiency if the transformer is eating your RF.

SWR tells you nothing about radiation vs. heat. Efficiency does.

The “33% Reflection” Myth

A 2:1 SWR does not mean 33% of the RF is lost.

With a tuner at the shack, power simply bounces a few times and is mostly radiated. The only penalty is extra coax loss due to multiple passes:

• 1.5:1 → +0.1 dB
• 2.0:1 → +0.2 dB
• 3.0:1 → +0.5 dB
• 5.0:1 → +1.1 dB

In contrast, EFHW transformers routinely burn 1.5–2 dB. That is the real loss.

EFHW vs EFOC Electrically

Broadband EFHW (40–10 / 80–10)

• Needs a 49:1–64:1 transformer for 2.5–5 kΩ feedpoints.
• Often uses a series “magic” capacitor to flatten SWR.

The result:

• High internal voltages → 1.5–2 dB loss in ferrite and copper.
• Internal SWR on upper bands can exceed 5–6:1.
• SWR meter smiles, ferrite cries.

EFOC17 / EFOC29 (4:1 Off-Center-Fed)

• Feeds the wire off-center at a few hundred ohms.
• Uses a 4:1 UNUN → far lower voltage, far lower loss.
• Near-resonant on principal bands (40/20/15/10 for EFOC17, and 80–10 for EFOC29).

Measured:

• Transformer loss < 0.2 dB.
• SWR < 2.5–3:1 in sane installations.
• Efficiency 85–97% depending on band.

The EFHW “Magic Capacitor” on 10 m

That capacitor makes SWR look perfect — while internal transformer SWR skyrockets.

Circulating current increases → ferrite heating increases → efficiency collapses.

This is why RF.Guru does not use compensation capacitors for broadband EFHW use.

EIRP Comparison: EFHW vs EFOC

On 40/20/15/10 m — the classic DX bands — EFOC17 radiates 20–35% more signal than a typical broadband EFHW.
On 80/40/20/10 m — EFOC29’s domain — the difference can exceed 50%.

What This Means for Real Installations

• 20–30 m of coax with 2–3:1 SWR costs only a few tenths of a dB.
• The EFOC’s transformer stays cool and efficient even at high power.
• EIRP from EFOC17/29 is consistently higher than from broadband EFHWs on every band they cover.

Conclusion: a modest SWR does not reduce your DX — transformer efficiency does.

Power Handling & Reliability

Because a 4:1 EFOC transformer runs at lower voltage and impedance:

• It comfortably handles legal limit.
• It stays cool on FT8, RTTY, and contest duty cycles.
• It maintains stable impedance without drifting or saturating.

Takeaways

• SWR is not the correct performance metric — EIRP is.
• 49:1 EFHWs hide transformer loss behind nice SWR.
• EFOC17/29 typically beat broadband EFHWs by 1–2 dB (20–50% more radiated power).
• If you want DX, the EFOC is the correct choice.

Put simply:

The EFOC is not inefficient because of 3:1 SWR — it outperforms EFHWs in real-world coax installations where EIRP matters.

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