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

When dealing with RF noise on the mains feed of a ham shack, many operators instinctively reach for ferrite chokes. While this can help in some cases, it is not the complete solution — especially when your mains cable includes a protective earth (PE).

Let’s break down why, and how to do it right.

The Role of Protective Earth (PE)

Most mains cables carry three conductors: phase (L), neutral (N), and protective earth (PE). For safety reasons PE must always be bonded directly and never passed through a choke. This also means you cannot just pass the whole 3-wire cable through a ferrite — because PE upsets the symmetry of the choke. Instead, you should build a small filter box where:

• L and N are wound together through ferrite cores to suppress noise.
• PE is connected straight through the box, untouched.

PE is essential for human safety. Never disconnect or choke it. For RF suppression, treat only L and N together as a balanced pair.

Common Mode vs. Differential Mode Noise

• Common-Mode (CM): Noise that appears in-phase on L and N together, returning via stray paths such as PE, ground, or antenna feedlines. Winding L and N together on ferrite effectively blocks this.
• Differential-Mode (DM): Noise between L and N, like what Ethernet-over-mains devices generate. Ferrites don’t stop DM, since they see this as “normal” opposite current. You need a proper EMI filter with capacitors and inductors designed to shunt DM noise.

When to Use a Schaffner Filter

A dedicated mains filter is the best solution for both CM and DM noise. The Schaffner 60A filter is a proven choice for ham stations because:

• Bigger filters mean physically larger inductors and capacitors.
• This translates to higher attenuation of both CM and DM interference.
• Although most stations don’t draw 60A, the oversizing ensures deeper noise suppression margins.

For typical amateur stations up to 2.5 kW output, this filter alone is usually sufficient — no extra ferrites are needed. Extra ferrite choking only becomes necessary at very high power levels, such as in civil AM broadcast installations, where residual RF can overwhelm even large filters.

Practical Strategy

• Never choke PE — only L and N together in a dedicated filter box.
• Use a high-current mains filter (like the 60A Schaffner) for combined CM and DM suppression.
• Reserve additional ferrite chokes for extreme power levels beyond amateur ranges.

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