How to Measure SWR and Resonance on an EFHW Antenna
End-Fed Half-Wave (EFHW) antennas are popular multibanders, but SWR measurements can be misleading if you don’t isolate feedline and common-mode effects. Here’s how to do it correctly.
What Are You Measuring?
An EFHW feedpoint impedance is very high (2–5 kΩ), transformed down to ~50 Ω by a 49–70:1 UNUN. When you connect an analyser:
- You’re seeing antenna + transformer + coax + common-mode
- Without suppression, results are distorted
How to Measure SWR Properly
Recommended setup:
- EFHW wire → Matching Transformer (e.g. 49:1)
- 0.05 λ coax jumper
- 1:1 current choke
- Analyser or VNA
Why 0.05 λ of Coax?
- Minimises but keeps predictable transformation
- Ensures repeatable, comparable results
- Provides a stable point for the choke
Examples: 14 MHz → 1.07 m; 1.9 MHz → 7.9 m. Yes, even on 160 m it’s worth doing right.
Why Place the Choke After 0.05 λ?
The choke stops common-mode currents from:
- Radiating and skewing SWR
- Shifting resonance dips
- Changing when you touch coax or analyser
📌 Placing the choke here keeps the antenna system intact but isolates the analyser from stray coupling.
❌ What to Avoid
| Mistake | What Happens |
|---|---|
| Long coax, no choke | Coax radiates, SWR distorted |
| Measuring at radio end | Impedance transformed, meaningless |
| No choke | SWR shifts when coax/analyser is moved |
| Using tuner | Hides antenna behaviour completely |
What Should You See?
- Sharp SWR dip at fundamental (e.g. 7.1 MHz)
- Usable harmonic dips (20 m, 15 m, 10 m)
- Predictable band behaviour
❗ If dips shift: common-mode problem. ❗ If dips look broad/multiple: check environment, grounding, or transformer.
Final Word
- Use 0.05 λ coax
- Choke immediately after
- Measure from that point forward
A clean SWR dip isn’t luck — it comes from measuring the whole EFHW system correctly.
Mini-FAQ
- Why 0.05 λ coax? — It keeps transformation minimal but predictable, giving repeatable results.
- Why choke placement matters? — To block common-mode and stop coax from radiating into the measurement.
- Why not measure at the rig? — Coax transforms impedance, hiding the true antenna behaviour.
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