Coax Length Before the Choke: Why It Matters for EFHW Antennas
EFHW Coax Lengths and Choke Placement: Lessons from DC4KU
Reference: Werner Schnorrenberg – DC4KU, “Endgespeister Dipol mit Gegengewicht und Mantelwellensperre” (2019). Field-verified by RF.Guru designs and measurements.
Many common-mode and noise issues in end-fed antennas can be traced back to one simple oversight: the coaxial feedline is part of the antenna unless you stop it.
DC4KU’s 2019 work shows that improperly managed end-fed systems radiate from the coax — and not just during TX. They also pick up household noise, often adding 3+ S-units of baseline QRM, especially on low bands.
Capacitors Only Where Needed
While many commercial EFHW designs rely on galvanically coupled transformers and fixed capacitor tuning, RF.Guru EFHWs use a fully decoupled winding architecture and add capacitors only where they truly help:
- On 20–10 m flat-top EFHWs (to detune harmonics cleanly)
- On 10 m and 12 m vertical EFHWs (to optimize current distribution)
- Not on full-size 160–80, 80–40, or 40–20 flattop EFHWs
Recommended Coax Lengths Before the Choke
Per DC4KU, using FlexBury 7 mm coax (VF = 0.86). Place the choke at ~0.05 λ from the feedpoint — the coax segment acts as a functional counterpoise.
| Band | Wavelength (λ) | 0.05 λ (electrical) | Physical length (VF=0.86) |
|---|---|---|---|
| 160 m | 187 m | 9.35 m | ≈8 m |
| 80 m | 83 m | 4.15 m | ≈4 m |
| 40 m | 41 m | 2.05 m | ≈2 m |
| 20 m | 20 m | 1.00 m | ≈1 m |
| 10 m | 10 m | 0.50 m | ≈0.5 m |
Can You Roll Up Excess Coax?
Yes — and often you should. As long as you’ve implemented:
- A proper RVS ground rod
- A counterpoise wire (2–4 m or more, depending on band)
…you can loosely roll the coax in coils or bury it without performance loss. Just avoid tight coupling near metal objects.
What Happens If Coax Is Too Short?
DC4KU’s lab data demonstrates how coax length and choke placement affect currents and noise:
| Setup | Common-mode current | RX noise floor (80 m) |
|---|---|---|
| No choke, no counterpoise | ~40 mA | ≈−75 dBm (S8) |
| Counterpoise only | ~20 mA | ≈−75 dBm (S8) |
| Choke at feedpoint + counterpoise | ~5 mA | ≈−93 dBm (S5) |
| Choke 0.05 λ before feedpoint | ~0 mA | ≈−93 dBm (S5) |
Even with a perfect transformer, RF finds its way back — usually via the coax shield, router, PC, or even the neighbor’s lamp.
Best Practice (RF.Guru EFHWs)
- Use enough coax before your choke — don’t skimp.
- Always install a proper common-mode choke.
- Add a 1–4 m counterpoise wire terminated on an RVS rod.
- Roll up extra coax if needed; no need for perfect straight runs.
Don’t Trust SWR Alone
Even with a “perfect” 1.1:1 SWR, your shack may still radiate — especially if:
- Coax is too short
- Choke is placed too close to the transformer
- No counterpoise or RVS rod is used
Our transformers avoid the high self-capacitance issues DC4KU flagged, and only use capacitors where beneficial.
Final Word
Your EFHW isn’t just about the wire and transformer — it’s a system, and the coax is part of it until you stop it.
“Etwas strahlt immer. Die Frage ist nur: was?”
— Adapted from DC4KU
Mini-FAQ
- How much coax before the choke? — About 0.05 λ, adjusted for coax VF.
- Can coax be coiled? — Yes, loosely or underground, if counterpoise and ground are provided.
- Does low SWR mean no common-mode? — No — SWR doesn’t guarantee isolation.
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