Why dB Attenuation Specs on Ham Chokes Are a Mess

(…and why common-mode impedance is what actually matters)

If you’ve browsed ham-radio choke specs lately, you’ve seen it everywhere: “30 dB!”, “40 dB!”, “60 dB!”. Those numbers look impressive, but most of the time they tell you almost nothing about how the choke will behave in your real station. Positive dB values often come from unloaded or unrealistic test conditions that don’t represent your antenna, your tuner, or your feedline.

What a common-mode choke actually does

On a coax feedline:

• Differential mode is the desired signal: equal and opposite currents.
• Common mode is the unwanted RF that travels on the outside of the shield.

A choke simply adds a large impedance in series with that unwanted common-mode path. The bigger the impedance, the more it reduces common-mode current. That’s the real engineering quantity—not the dB “attenuation”.

Where those impressive dB numbers come from

Most published attenuation values come from:

• Unloaded “near-open” tests that exaggerate the attenuation.
• Assuming 50 Ω source and 50 Ω load, because the VNA prefers 50 Ω—even though your antenna system does not.

Both methods produce big dB numbers that look clean on a spec sheet but do not represent real antennas.

The 50 Ω fantasy world vs. real ham antennas

Your real-world feedpoint impedance is almost never 50 Ω except at one frequency, at one height, with one environment. Multiband antennas, ladder line, tuners, verticals with long radial systems—these all create wildly varying impedances in the common-mode path:

• Tens of ohms
• Hundreds of ohms
• Kilohms
• Strongly reactive values

A choke measured as “40 dB” in a 50 Ω lab setup might perform radically differently—sometimes better, sometimes worse—in your actual antenna.

Why attenuation in dB becomes meaningless in complex systems

Common-mode reduction depends on the ratio between the choke impedance and the rest of the CM path. The system matters as much as the choke.

In setups like:

• Tuner → ladder line
• Off-center-fed antennas
• Non-resonant wires
• Feedlines at resonant or antiresonant lengths

…the classic “40 dB” chart might have no relationship to reality.

Common-mode impedance: the spec that actually means something

Engineers use common-mode impedance (in ohms) because it describes the choke itself. It is portable, universal, and comparable across vendors.

Rules of thumb:

• 1 kΩ+ — useful
• 3 kΩ+ — strong suppression
• 5 kΩ+ — excellent, especially if the impedance is mostly resistive

Resistive impedance gives broadband suppression. Highly reactive impedance produces peaks and nulls that depend on feedline length and surroundings.

So why do vendors still publish dB numbers?

Because hams expect them. The “compare the dB” habit is deeply ingrained. Commercially, you must provide them—even if they aren’t the best engineering metric.

We publish dB values too, but always with:

• A clear statement of the test conditions.
• Impedance plots in ohms, which are the numbers that actually matter.

How to read choke specs without getting fooled

• Look for CM impedance vs. frequency first. This is the real performance metric.
• Check if it’s mostly resistive. Resistive = predictable suppression.
• Treat dB as context-dependent. Only valid if the measurement setup is clearly defined.
• Remember your system isn’t 50 Ω. Especially with tuners and ladder line.
• Compare based on impedance. 4 kΩ ≈ twice the isolation of 2 kΩ in most stations.

Wrapping it up

dB attenuation specs are often misleading because they depend entirely on the test configuration. Common-mode impedance in ohms is the honest, comparable, universal specification that lets you understand how a choke will behave in your real installation.

Next time you see a choke boasting “60 dB of attenuation!”, ask:
“Under what conditions? And what’s the impedance in ohms on my bands?”

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