Why the Y21 Method Is the Only Ham Measurement That Actually Works
Most “ham” VNA choke-measurement tricks are only as reliable as their control of the common-mode (CM) path. Some give ballpark trends — very few produce numbers you can trust. The Y21 method is the only ham-accessible approach that consistently lands within reasonable error bounds of the standardised EMC measurement — the same one used in compliance labs. Everything else is prone to artifacts, fixture effects, and calibration drift.
The reliability reality check
Every home-brew or “YouTube” choke-measurement setup suffers from one thing: uncontrolled common-mode return paths. If your fixture or analyzer cables radiate, you’re not measuring the choke — you’re measuring the room.
To separate methods by credibility, here’s a quick reliability scorecard ranked from weakest to strongest:
One-port S₁₁ (pair strapped)
- What it does: Treats the choke as a series element and infers impedance from the reflection.
- Why it fails: A few picofarads of fixture capacitance dominate the result and create fake resonances.
- Verdict: Fine for quick sanity checks, useless for absolute Zcm.
Two-port S₂₁ (series insertion, G3TXQ-style)
- What it does: Feeds the choke between Port 1 and Port 2 and computes Zcm = Z₀ × (2 / S₂₁ − 2).
- Why it’s better: Less sensitive to stray capacitance; gives a decent relative ranking of mixes and turn counts.
- Verdict: Medium reliability if you calibrate at the fixture plane and keep symmetry.
Two-port Y₂₁ (π-model extraction)
- What it does: Measures all S-parameters, converts to Y-parameters, and isolates the true series impedance Zcm = −1 / Y₂₁.
- Why it’s superior: It mathematically separates series and shunt paths, removing the influence of fixture capacitance.
- Verdict: High reliability. The only method that matches EMC-lab results within reasonable error margins.
RF current-probe verification
- What it does: Measures actual CM current reduction in-situ.
- Why it matters: Answers the real question — does it work on the station?
- Verdict: Great for go/no-go checks, not for absolute ohms.
EMC-style fixtures (CDN or triaxial/TEM)
- What it does: Enforces a defined 150 Ω CM path per IEC 61000-4-6.
- Verdict: Gold standard, but far too costly and complex for most amateurs.
Why Y21 stands alone
When you extract Y₂₁ from a full two-port S-parameter set, you effectively de-embed your test jig’s stray capacitance. That’s what EMC engineers pay thousands for in compliance setups — but with a decent NanoVNA and software conversion, you can achieve the same principle.
Even hobby-grade VNAs can handle this if:
- You perform SOLT calibration right at the fixture plane.
- Keep both conductors perfectly symmetric and well-choked on the analyzer leads.
- Apply Y-parameter conversion to extract (−1 / Y₂₁) instead of reading S₂₁ magnitude directly.
The result: data that behaves predictably, scales correctly when adding chokes in series, and doesn’t collapse at higher impedances.
Where the others go wrong
Most “ham” methods fail because they treat the choke as a perfect lumped element and ignore how the environment couples into the measurement. Just 1 pF of stray capacitance has XC ≈ 11.4 kΩ @ 14 MHz — enough to completely falsify an S₁₁ plot of a “good” choke. That’s why S₁₁-only methods show resonant dips that don’t exist in reality. Y₂₁ explicitly removes that term.
How close is Y21 to the real EMC measurement?
Very close. In practice, Y₂₁ measurements with a symmetrical fixture and proper de-embedding track within ±10–15 % of triaxial or CDN results up to 50 MHz. That’s extraordinary for amateur gear and the reason professionals recognise Y₂₁ as a credible approximation of the standardised common-mode measurement.
Quick self-check: Is your result real or artifact?
If your extracted Zcm survives these abuse tests, it’s probably valid:
- Swap conductors A/B — no major change → fixture is symmetric.
- Move the DUT ±10 cm — big change → you’re measuring the room.
- Clamp ferrites on both analyzer leads — no big difference → good isolation.
- Add 1 pF across the fixture and see if your model predicts the shift.
- Stack two identical chokes — impedance should roughly double.
Fail any of those, and you’re not measuring common-mode impedance — you’re measuring your fixture.
Bottom line
- Most ham VNA setups are prone to artifacts and false confidence.
- The Y₂₁ method is the only approach that stands up to technical scrutiny without lab-grade gear.
- For publication-grade choke data, Y₂₁ with fixture-plane calibration or de-embedding is the minimum credible standard.
- To match EMC-compliance accuracy, you’d need a triaxial or CDN system — but that’s overkill for 99 % of ham work.
Mini-FAQ
- Why is Y₂₁ better than S₂₁? It separates the choke’s real series impedance from parasitic capacitances in the fixture, giving stable values even at high Z.
- Can a NanoVNA really do this? Yes. It can capture all four S-parameters; software like NanoVNA-Saver or a Python script can convert to Y-parameters easily.
- Do I need de-embedding? For repeatable results, yes. Measure the open and short of your fixture, then mathematically remove them.
- How accurate is it? Expect ±10–15 % magnitude and a few degrees phase error — remarkably close to EMC-lab figures.
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