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. This makes them prone to common-mode currents (CMC)—unwanted RF that rides along the coax shield. These currents cause:
- SWR instability
- RF feedback into audio gear or the operator
- And worst of all: noise pickup from the environment
That’s why every end-fed system should have two chokes:
- A 30 dB high-CMR choke at the antenna
- A 45 dB high-CMR choke at the shack entrance
This combination is not overkill—it’s best practice, especially for high-power stations or noisy urban environments.
Why Two Chokes? (And Why the 45 dB at the Shack Makes All the Difference)
A 30 dB choke offers around 1,000× reduction in common-mode current.
A 45 dB choke offers around 32,000× reduction.
Let that sink in:
A 45 dB choke suppresses 32× more current than a 30 dB choke.
This makes a huge difference in noise suppression, especially in RF-polluted areas, where the coax shield acts like a receiving antenna for every nearby switching supply, LED dimmer, solar inverter, and powerline.
Why Common Mode Rejection (CMR) Matters for SWR and Noise
- End-fed antennas are high impedance, unbalanced systems.
- Common-mode currents on the coax braid distort the system impedance, destabilizing SWR.
- Chokes block those currents, stabilizing SWR and reducing radiated or picked-up interference.
- On the low bands (160–30 m), where noise coupling is strongest, good chokes make the difference between hearing DX or just hash.
Impact on Different End-Fed Antennas
| Antenna Type | Behavior Without Choke | Benefit of Dual Chokes |
|---|---|---|
| 9:1 End-Fed Random Wire | Coax shield acts as unpredictable counterpoise | Forces current into wire, reduces stray radiation |
| 4:1 End-Fed Near-Resonant | Coax radiates part of signal | Restores symmetry, improves match and pattern |
| 49:1 EFHW (and higher) | Lower CMC by design, but still picks up noise | Greatly reduced receive noise on low bands |
RF.Guru High-CMR Choke Options
| Model | Choking Level | Power | Impedance | Use Case |
|---|---|---|---|---|
| 2025-F-010-30 | +30 dB (1,000× suppression) | 9 kW | 5–10 kΩ | Antenna-side choke, mid-power shack entrance |
| 2025-F-010-45 | +45 dB (32,000× suppression) | 9 kW | 10–20 kΩ | Shack entrance, high-noise areas, QRO stations |
Placement Guidelines
- Antenna-side (30 dB): Place at the end of the coax where it meets the transformer (Unun). Prevents the coax from becoming part of the radiator or counterpoise.
- Shack entrance (45 dB): Essential for protecting sensitive gear and reducing received noise. Think of it as an RF firewall—especially with long coax runs.
For runs longer than ½ wavelength on the lowest band, consider adding a third choke midway along the feedline.
Conclusion
Don't settle for just a single choke. Whether you're trying to stop SWR weirdness, eliminate RF feedback, or just hear more and work more, a dual-choke setup is one of the most effective upgrades you can make to your end-fed system.
- 30 dB choke at the antenna = controlled SWR + proper current flow
- 45 dB choke at the shack = quieter RX + safer shack
A single +30 dB choke solves part of the problem.
Adding a +45 dB choke completes the system—and gives you 32× more suppression right where it counts.
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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.