The Hybrid Balun Trap: Why 0.05λ Chokes Work, Hybrids Don’t
Some antenna manufacturers promote “hybrid” baluns — a voltage transformer with a choke wound directly on top of it. While this looks compact, it’s rarely the best practice for end-fed or off-center antennas like our EFOC EFLW and EFHW series. In most cases, a dedicated 0.05λ choke placed further down the feedline performs much better.
Why Hybrids Fall Short
In a hybrid, the choke sits directly at the high-voltage point of the transformer. This means:
- The choke sees extreme voltages and stress it was never designed for.
- It cannot properly suppress common-mode current because the coax shield is still coupled to the transformer’s voltage node.
- Any imbalance in the transformer directly leaks into the feedline, defeating the purpose of a choke.
A hybrid may look elegant on paper, but electrically it mixes two functions that should be separated.
The Case for 0.05λ Chokes
For both EFHW and EFOC antennas, the proven method is to place a current choke about 0.05λ (λ/20) down the feedline. This corresponds to a current maximum point on the coax shield, where a choke is most effective at blocking common-mode current.
The choke placement recommendation is supported by research from German RF engineer Werner Schnorrenberg (DC4KU), published in 2019 as “Endgespeister Dipol mit Gegengewicht und Mantelwellensperre”. He showed that placing the choke at ~0.05λ is consistently more effective than at the transformer itself.
Free-space values. Adjust for coax velocity factor (≈0.66–0.85) for physical length.
| Band | Freq (MHz) | 0.05λ (m) |
|---|---|---|
| 80 m | 3.6 | 4.2 m |
| 60 m | 5.3 | 2.8 m |
| 40 m | 7.1 | 2.1 m |
| 30 m | 10.1 | 1.5 m |
| 20 m | 14.2 | 1.05 m |
| 17 m | 18.1 | 0.83 m |
| 15 m | 21.2 | 0.71 m |
| 12 m | 24.9 | 0.60 m |
| 10 m | 28.5 | 0.53 m |
EFOC vs. EFHW: Counterpoise Matters
The correct choke position depends on whether a counterpoise is used:
- With a counterpoise (short stub or braid): place the choke at ~0.05λ. This isolates the counterpoise as the return path and stops the coax from becoming part of the antenna.
- Without a counterpoise: the coax itself acts as the return. In this case the choke must be placed further down, with distance depending on radiator length and operating bands.
This is why EFHW and OCF antennas without a counterpoise are so feedline-sensitive. The coax is unintentionally part of the antenna system until a choke finally forces it to stop radiating.
Are Hybrids Ever Useful?
In general, hybrids are not recommended for transmit antennas. They combine a choke and transformer at the worst possible location (voltage maximum), exposing the choke to stress and making it ineffective.
One exception may be in low-level receive-only applications (e.g., active whips or small magnetic loops). Here, the convenience of packaging a transformer and choke together can be acceptable, because the system isn’t handling high RF voltages or currents. Even then, performance is usually better with a separate choke positioned away from the transformer.
Practical Takeaways
- Do not rely on “hybrid” transformer+choke units for transmit EFHW/EFOC antennas.
- Chokes are most effective at ~0.05λ down the feedline, as shown by DC4KU and confirmed in practice.
- With a counterpoise: place the choke at ~0.05λ. Without a counterpoise: further down, depending on radiator length.
- Hybrids may be acceptable for RX-only, but not recommended for TX use.
Mini-FAQ
- Is a hybrid transformer+choke a good idea? — Not for TX. It’s at the wrong point and ineffective.
- Where should I place a choke for EFHW/EFOC? — At ~0.05λ down the coax if a counterpoise is present.
- What if my antenna has no counterpoise? — The coax becomes the return, so the choke must be placed further away.
- Does this improve SWR? — Not directly. It improves noise rejection, stability, and tuner behavior.
- Who researched this? — Werner Schnorrenberg — DC4KU, who published detailed studies on choke placement in 2019.
- Are hybrids ever useful? — Maybe in low-power RX-only systems, but rarely in transmit applications.
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