Stop Calling Your Choke “40 dB” — The Hidden Math They Never Tell You
Updated 30 November 2025
The misunderstanding behind “40 dB rejection”
Most ham-radio choke charts show a dramatic “40 dB rejection” peak. But this number is almost never a property of the choke itself. It is a property of the system you plug the choke into.
A choke has a measurable quantity: common-mode impedance ZCM(f). A choke does not have an inherent dB figure. The moment you turn ZCM into dB, you have already chosen:
- a CM source impedance ZS
- a CM load impedance ZL
- a model for how the current loop behaves
Change ZS or ZL by a factor of ten and your “rejection” may jump by 15–20 dB — even though the choke hasn’t changed at all.
Why S21 and Y21 are not the same thing
Jeff’s well-known Y21 method paper often gets summarized as “S21 and Y21 give the same answer.” That is not what he wrote.
What he actually showed is:
- For his test fixture, shunt capacitance was only ~2–3 pF.
- Between 1–30 MHz, this small parasitic allowed S21 and Y21 to look similar.
- Increase the fixture capacitance to 100 pF, and the S21 result collapses while Y21 stays correct.
In other words:
Y21 is mathematically correct because it de-embeds fixture shunts. S21 is an approximation that only works when parasitics are tiny.
Whether you later convert ZCM to a dB estimate has nothing to do with S21 or Y21. That’s a modeling decision, not a measurement feature.
The only correct model for real common-mode behaviour
Real common-mode is not a 50 Ω → choke → 50 Ω two-port. It is a single CM loop:
CM source → RCM (feedline + antenna + chassis path) → return
The choke contributes a series impedance ZCM inside that loop. The current reduction is:
Attenuation = 20·log₁₀(1 + ZCM / RCM)
There is no “built-in 6 dB.” That myth only exists in the artificial 50 Ω two-port picture — which is not how CM current behaves on a coax.
Why we still show a dB numbers on our tables
Many buyers expect a “dB number,” because competitors publish them. So we include a dB trace derived only from ZCM.
But it is not a universal value. It assumes no real-world ZS, ZL, or RCM.
The impedance curve is the real device property. The dB numbers are just a calculated convenience for readers who think in dB.
Why dB figures without context are misleading
Two chokes may both be labeled “40 dB,” but in different installations:
- One may give 6 dB of real CM reduction
- The other may give 25 dB
Same choke. Different ZS, ZL, and RCM.
This is why publishing only the dB number is misleading, and why the proper way to publish data is simply: ZCM magnitude + phase vs. frequency.
Takeaway
A choke is defined by impedance, not by dB. System-level dB depends entirely on the CM environment, not on the choke alone.
If you remember one line from this article, let it be:
Never trust a single dB number — always look at common-mode impedance.
Mini-FAQ
- Does a choke have a fixed dB value? — No. Only ZCM is intrinsic. dB depends on the system.
- Is Y21 better than S21? — Yes, because Y21 properly removes fixture shunt capacitances.
- Why do manufacturers publish dB? — Market expectation. The impedance curve is the real data.
- Does the 6 dB myth apply to Y21? — No. That error comes from misusing the two-port model.
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