Coax Length Before the Choke: Why It Matters for EFHW Antennas
Reference:
- Werner Schnorrenberg – DC4KU, “Endgespeister Dipol mit Gegengewicht und Mantelwellensperre” (06.03.2019)
- Field-verified by RF.Guru designs and measurement data
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.
The 2019 work by Werner Schnorrenberg (DC4KU) shows that improperly managed end-fed systems radiate from the coax — and not just during TX. They also receive household noise, resulting in 3+ S-units more baseline QRM, especially on low bands.
While most commercial designs attempt to suppress this by using a galvanically coupled transformer and fixed capacitor tuning, our EFHW designs use a fully decoupled winding architecture, and capacitors only where beneficial:
- Only on the 20–10 m flat-top EFHW (to detune harmonics cleanly)
- Only on the 10 m and 12 m vertical EFHW (to optimize current distribution)
- Not used on our full-size 160-80 80-40 top-band EFHWs and Flattop 40-20 EFHW
Still, a good transformer alone is not enough if the coax is too short before your common-mode choke.
Recommended Coax Lengths Before the Choke (per DC4KU, using M&P FlexBury 7mm, VF = 0.86)
| 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 |
These values are derived from DC4KU's recommendation to place the choke at approximately 0.05 λ from the antenna feedpoint. The short coax segment between the choke and the transformer acts as a functional counterpoise. Measurements use FlexBury 7 mm coaxial cable with a velocity factor of 0.86.
Can You Roll Up Excess Coax?
Yes — and in many cases, you should.
As long as you’ve implemented:
- A proper RVS ground rod, and
- A counterpoise wire (2–4 m or longer, depending on band),
…you can loosely roll up the coax in coils, or even bury it, without risk of performance degradation.
Just avoid tight magnetic coupling near other metal. Coax doesn’t need to be perfectly straight or flat on the ground.
What Happens If Your Coax Is Too Short?
Using DC4KU’s lab measurements as a benchmark:
| Setup | Common-Mode Current (I₄) | 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 0.05λ before feedpoint (coax = rad) | ~0 mA | ~−93 dBm (S5) |
Even with a perfect transformer, RF will find a way back — and that’s usually through your coax shield, router, PC, or the neighbor’s lamp.
Best Practice (as implemented in RF.Guru EFHWs)
- Use enough coax before your choke. Don’t skimp — more is better.
- Always install a proper common-mode choke. Even the best winding design benefits from isolation.
- Add a 1–4 m counterpoise wire at the transformer terminate it on our RVS rod (high impedance path) + current sink
- Roll up extra coax if you don’t have space. No need for a perfect straight line.
Don't Trust Low SWR Alone
Even when your SWR reads 1.1:1, your shack may still be radiating — especially if:
- Your coax is too short,
- The choke is too close to the transformer,
- You don’t use a counterpoise or RVS rod (or bleed resistor and a copper rod)
Our transformers do not require capacitive compensation (except where explicitly designed), and are internally designed to avoid the high self-capacitance problems DC4KU highlighted in legacy designs.
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 tell it otherwise.
"Etwas strahlt immer. Die Frage ist nur: was?"
— Adapted from DC4KU
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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.