Why a Simple Choke on Your Mains Cable Isn’t Enough

Ferrite Chokes on the Mains Feed — What Really Works for RF and Why

Many hams reach for ferrite rings to “clean up” their shack’s AC feed — but which conductors should go through the core, and what about the protective earth (PE)? The answer depends on whether you’re fighting common-mode or differential-mode noise, and whether you’re chasing RF on the mains cable or on the safety-earth itself.

1. PE-Only Choke — The Ground-Loop Cure

Schurter and Schaffner both offer earth-line chokes built into their inlet filters (e.g., Schurter KFA, Schaffner FN9233E) for one purpose: stopping RF circulating on the protective earth between devices, or between shack ground and building earth. This type of choke adds RF impedance in series with PE while leaving the L/N filter untouched.

• Use it when: You have hum or RF currents flowing on the green/yellow wire, or between grounded gear interconnected by shielded cables.
• Effect: Breaks the RF loop path by adding impedance to the PE conductor.
• Limitation: It does not block common-mode RF traveling along L+N together on the mains cord itself.

Schurter calls this a “ground-line choke” and explicitly shows it as a loop-breaker for high-frequency earth currents, not as a mains cable choke.

2. All-Conductors-Through-One-Core — The Cable Choke

Würth Elektronik and GM3SEK both recommend routing L, N, and PE together through a single ferrite toroid or clamp-on sleeve. This creates a high impedance only for the common-mode RF flowing along the cable, while canceling out the normal 50/60 Hz differential current.

• How it works: At mains frequency, the current in L and N cancel; PE adds little. For common-mode RF, all three conductors carry in-phase current, so the choke resists it strongly.
• Goal: Stop the mains cord from acting as an antenna or RF return rail.
• Placement: On the cord leaving the metal equipment enclosure — as close to the box as practical.

Including PE in the bundle doesn’t “break symmetry.” The ferrite sees the vector sum of currents, not individual winding balance — so the 3-wire choke behaves exactly as intended for CM suppression.

3. Choosing the Right Ferrite Mix for Ham Bands

Not all ferrites behave the same. Their impedance vs. frequency curve determines how effectively they resist common-mode current. For HF use, you typically need two material types to cover the full range cleanly:

• Mix 31 (Fair-Rite #31, Wurth #74270097): High permeability, high loss below about 10 MHz — excellent for 160 m through 40 m. It gives deep attenuation where CM current tends to be strongest from long cables and power supplies.
• Mix 43 (Fair-Rite #43, Wurth #74270079): Lower permeability and higher Q, so its peak impedance sits between 15 – 50 MHz — ideal for 30 m through 10 m and VHF-edge noise.

Stacking or combining both mixes (e.g., a few turns on #31 plus one or two #43 cores) gives broadband coverage — thousands of ohms from 1 MHz to 50 MHz. This mirrors the EMC strategy in commercial gear, where wideband suppression requires multiple materials tuned to different frequency zones.

For best results, put mix 31 cores on the power inlet or cord choke, and mix 43 near the equipment case or feedline entry where higher-frequency RF leaks in or out.

4. Common-Mode vs. Differential-Mode Noise

• Common-Mode (CM): Noise that appears equally on L, N, and possibly PE with respect to ground or nearby conductors. It’s what makes cables radiate and couples RF between boxes. Solved by ferrite chokes or the CM section of an EMI filter.
• Differential-Mode (DM): Noise between L and N. Examples include switch-mode PSU hash and powerline networking devices. Solved by internal EMI filters with capacitors (L-N) and series inductors in each line.

A ferrite choke only addresses CM noise. For DM suppression, you need a proper EMI filter — ideally one certified to EN/UL standards — that includes both CM and DM sections.

5. Ground Loops vs. Common-Mode — Why You Might Need Both Chokes

Ground loops and CM current are related but distinct:

• Ground loops are circulating currents between interconnected grounds (e.g., between two devices both earthed to different outlets). These loops can inject audio hum or RF feedback.
• Common-mode current flows along the entire mains cable relative to its surroundings and can make the cord radiate RF or import noise into the shack.

That’s why professional EMC practice — and top HF stations — often combine the two approaches:

1. Inside the inlet: Use a PE-only choke or certified filter module with an internal ground choke to stop RF ground-loop current.
2. Outside on the cord: Add a large #31 or #75 toroid choke with all three conductors through it to raise CM impedance on the cable itself. Supplement with #43 cores if upper-HF noise is present.

The combination works because they act at different points in the chain: one isolates the ground, the other quiets the cord across the HF spectrum.

6. Differential-Mode Filtering — The Third Leg of the Stool

Even a perfect common-mode choke won’t suppress differential switching noise from your PSU or inverter. For that, a proper L/N filter with capacitors (X and Y types) is required. Schaffner’s high-current filters, like the 60 A series, include DM filtering elements rated for continuous RF attenuation up to several MHz — making them ideal for high-power amateur stations.

7. Putting It All Together

For the cleanest result:

• Install a certified EMI filter at the equipment inlet (Schaffner FN9233E, Schurter KFA, etc.) that handles both CM and DM noise and optionally includes a PE choke.
• Add a 3-wire cable choke (L + N + PE) on the power cord to suppress external CM currents.
• Use mix 31 ferrites for low-HF and mix 43 for higher-HF suppression.
• Keep PE connections intact — never interrupt or bypass them for safety reasons.
• For tough RFI cases, consider a separate PE choke at the inlet to block RF ground-loop current while preserving safety bonding.

8. Decision Guide

• RF current on PE / hum loops? — Add a PE-only choke at the inlet (per Schurter guidance).
• Mains cord radiates or brings in RF? — Add a 3-wire cable choke on L + N + PE using #31/#43 cores (per Würth and GM3SEK).
• Switching PSU noise between L and N? — Use a proper EMI filter (Schaffner or Schurter).
• For best results? — Combine all three: DM filter + 3-wire cable choke + optional PE-only choke at inlet.

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