Optimizing End-Fed Antennas with High-CMR Chokes

End-fed antennas, particularly those using 9:1, 4:1, and 49:1 ununs (and higher ratios), often rely on the coax shield as a counterpoise, which can introduce common mode currents (CMC). These unwanted currents can cause SWR instability, RF feedback, and noise pickup. Using a high-CMR choke can significantly enhance the antenna's performance by isolating the coax shield from acting as part of the antenna system.

Why Common Mode Rejection (CMR) Matters for SWR and Noise Reduction

  • End-fed antennas have a high impedance feed point, making them prone to unwanted RF flowing on the coax shield.
  • These common mode currents alter the impedance seen by the tuner/transceiver, leading to higher and unstable SWR.
  • A high-CMR choke suppresses these currents, ensuring that the antenna operates in a more controlled manner, often resulting in a lower and more predictable SWR.
  • In addition to SWR improvements, a high-CMR choke also provides significant noise reduction, especially on the low bands (160–30m), where common-mode pickup from the environment is higher.

Impact on Different End-Fed Antennas

  • 9:1 End-Fed Random Wire: Typically high-impedance, requiring a counterpoise for proper operation. Without a choke, the coax shield acts unpredictably as part of the counterpoise.
  • 4:1 End-Fed Near-Resonant: These antennas are closer to resonance, making the choke essential to ensure that the coax does not radiate and affect impedance matching.
  • 49:1 EFHW (and higher ratios) (End-Fed Half-Wave): These antennas are often used on resonant harmonic frequencies, leading to less common-mode current compared to 9:1 and 4:1 designs. However, a high-CMR choke still significantly reduces noise pickup, particularly on the low bands (160–30m), where environmental noise is prevalent.

Choosing the Right High-CMR Choke

RF.Guru offers two wideband high-power line isolators:

1. +30dB Wideband Choke (2025-F-010-30)

  • Choking level: 30dB
  • Power handling: 9kW
  • Frequency range: 1.5 – 30 MHz
  • Impedance suppression: 5–10 kΩ
  • Best for: Medium power setups or cases where moderate CMR reduction is sufficient.

2. +45dB Wideband Choke (2025-F-010-45)

  • Choking level: 45dB
  • Power handling: 9kW
  • Frequency range: 1.5 – 30 MHz
  • Impedance suppression: 10–20 kΩ
  • Best for: High-power (>1kW) applications and where maximum common-mode suppression is required.

Which One to Choose?

For high-power setups (>1kW), the +45dB model is strongly recommended due to its superior suppression (10–20 kΩ). The +30dB model is still a good option for lower-power applications where excessive common mode current is not a major issue.

Placement of the Choke

To maximize effectiveness:

  • Primary location: After the recommended length in the manual of the EFOC, EFLW (off-center long wire), or EFHW, ensuring proper isolation of the counterpoise formed by the coax.
  • Optional second location: Near the transceiver, which is beneficial for both common-mode current suppression and noise reduction. This is especially useful if the feedline is longer than half a wavelength, as longer coax runs tend to accumulate more unwanted RF currents.

For more detailed guidance on common-mode RF current and noise elimination for transmit antennas, refer to our in-depth guide here: Optimal Common-Mode RF Current and Noise Elimination for TX Antennas

Conclusion

A high-CMR choke is a key addition to any end-fed antenna system, particularly for high-power applications. The +45dB model is the best choice for >1kW setups, while the +30dB model works well for moderate-power installations. Proper placement of the choke ensures a stable SWR, reduced noise pickup, and minimized RF feedback.

Would you like to fine-tune your high-power antenna system? Consider adding a high-CMR choke to optimize performance!

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.