Debunking Myths in Common-Mode Choke VNA Measurements
Updated August 21, 2025
In recent years, many blogs, videos, and forum posts have claimed to show how to measure common-mode choke performance with a VNA. Most are flawed because they rely on differential-mode S-parameter techniques that simply cannot evaluate common-mode suppression. This article breaks down the common myths and shows the correct way to measure common-mode impedance (Zcm).
Common but Incorrect VNA Methods
1. DG8SAQ VNWA Differential Port Fixture
Claim: Use two VNA ports and a differential fixture to measure S21.
Reality: This only measures differential insertion loss. It doesn’t isolate or excite the common-mode path.
2. VE2AZX Balun Jig
Claim: A balun jig with VNA can measure choke suppression.
Reality: It again measures the differential path. Without true common-mode excitation, results are misleading.
3. S21 Through-Choke with 50-Ohm Load
Claim: Connect VNA across choke and measure S21.
Reality: Unless common-mode is intentionally excited (via center-tapped transformer, for example), it measures differential response, not CM attenuation.
The Right Way: Measure Common-Mode Impedance (Zcm)
The direct method is measuring Zcm. One option is a single-port VNA test:
- Short both choke terminals together and connect to one VNA port.
- Reference to ground through a high-value resistor (≈10 kΩ) to stabilize floating CM reference.
- Measure S11 and calculate: Zcm = 50 × (1 + S11) / (1 – S11).
What This Simulates
This setup applies equal voltage to both wires in phase — simulating common-mode excitation. The far end may be floated (to see peak Zcm) or terminated (to simulate real conditions).
Important: Single-port Zcm testing is useful for trends and comparisons, but not definitive at HF. Above a few MHz, parasitics and fixture effects dominate. Cheap VNAs in particular can give misleadingly “flat” or “good-looking” results.
The Gold Standard: EMC-Compliant Current Injection
The most reliable method is the EMC-standard injection method:
- Inject known CM current via transformer
- Measure current reduction across frequency
- Directly quantify choke suppression
This aligns with CISPR, IEC, ISO, and MIL-STD protocols. These methods inject real CM signals and measure true suppression — not differential artifacts.
The Fallacy of “Exotic” Coax Chokes
Claims of revolutionary performance from semi-rigid coax or tubing chokes often stem from flawed VNA differential tests. Without true CM excitation, results provide false confidence while the choke underperforms in practice.
Why All This Matters
Common-mode currents are a leading cause of:
- RFI and shack noise
- Distorted radiation patterns
- Receiver desense
- RF hot spots on equipment
If your test doesn’t excite the common mode, you’re not testing what matters.
Conclusion: Measure the Mode That Matters
Many hobby VNA test setups measure only differential paths. A choke must be measured in the mode it is meant to suppress — common-mode. Always ask:
Does this setup actually stimulate common-mode current?
If not, the results are irrelevant.
Mini-FAQ
- Why are VNA S21 tests misleading for chokes? — They measure differential insertion loss, not common-mode suppression.
- What is the correct parameter? — Zcm, the common-mode impedance, measured with CM excitation.
- What’s the best lab method? — EMC-standard current injection with calibrated current probes.
- Why do coax-based “exotic” chokes fail? — Their performance claims are based on flawed differential tests.
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