Debunking Common Myths in Common-Mode Choke Measurements with a VNA
In recent years, numerous videos, blogs, and forum posts have emerged claiming to show how to measure common-mode choke effectiveness using a VNA (Vector Network Analyzer). Unfortunately, many of these methods are fundamentally flawed—they don’t actually measure what they claim to. Most rely on differential-mode S-parameter techniques, which are inherently incapable of evaluating common-mode suppression.
In this article, we’ll break down the most commonly misapplied test methods, explain why they fall short, and clarify how to correctly measure common-mode impedance (Zcm) using both lab-proven and EMC-compliant industry standards.
Common but Incorrect VNA Methods
1. DG8SAQ VNWA Differential Port Fixture
Claim: Use two VNA ports and a differential fixture to measure S21 across the choke.
Reality: This setup measures differential-mode insertion loss, not common-mode suppression. It does not excite or isolate common-mode current paths, so any results are irrelevant to choke performance.
2. VE2AZX Balun Jig
Claim: Use a balun-style jig with a VNA to evaluate a choke’s S21.
Reality: Most of these jigs also measure the differential path through the device. Without intentional excitation of common-mode current, the measurement is misleading at best.
3. S21 Through-Choke Measurement with 50-Ohm Load
Claim: Connect the VNA across the choke with a 50-ohm load and measure S21.
Reality: Again, this test evaluates the differential path unless a true CM excitation method (e.g., center-tapped transformer) is used. As such, the results say nothing about CM attenuation.
The Right Way: Measure Common-Mode Impedance (Zcm)
The most direct method to assess a choke’s real-world effectiveness is to measure its common-mode impedance. One approach is a single-port VNA test:
- Connect both terminals of the choke together and to a single VNA port using coax.
- Leave the other end of the choke floating or terminated appropriately.
-
Measure S11 (return loss) and calculate:
Zcm = 50 × (1 + S11) / (1 - S11)
What This Means in Practice:
When we say "connect both terminals together and to a single VNA port," we're setting up the choke to simulate common-mode excitation, where both conductors carry the same voltage in phase relative to ground. Here's how this works:
- Short the choke's terminals together at one end. This forces any signal from the VNA to appear equally on both wires, simulating a common-mode current.
- Connect 10 kΩ from the shorted pair (both terminals tied together) to ground, then the resistor is referenced to the common-mode node. This stabilizes the floating common-mode reference without creating a significant return path, and preserves CM symmetry.
- Connect this shorted pair to the VNA port using a coaxial cable. This applies the VNA signal equally to both conductors, without a differential return path.
- Leave the far end of the choke floating or terminated with a known impedance (e.g., 50 Ω)., depending on your test setup. Floating gives a clearer view of the choke’s peak Zcm. A 50-ohm dummy load simulates real-world conditions better. Avoid grounding it to prevent introducing a differential-mode return path.
This method allows the VNA to see only the common-mode impedance presented by the choke, enabling an approximation of its real-world effectiveness. However, it remains vulnerable to test fixture asymmetry, parasitic elements, and environmental interference.
Important: While this single-port method can be helpful for trend analysis or comparing chokes, it is not a definitive or reliable way to assess performance at HF. At frequencies above a few MHz, parasitic effects, coaxial reflections, and test setup geometry can dominate the measurement. Users of low-cost VNAs should be especially cautious not to draw strong conclusions from such setups.
If your measurement shows little variation across different choke types or coax cables, or results look unusually consistent (e.g., <1 dB return loss across 30 MHz), it’s likely your test fixture is not isolating the common-mode path properly. Misinterpreting these results could lead to false confidence in ineffective designs.
The Gold Standard: EMC-Compliant Current Injection Method
To obtain reliable, repeatable, and regulatory-grade results, we use the EMC-standard injection method. This approach:
- Injects a known common-mode current (usually via a transformer)
- Measures current reduction across frequency
- Directly quantifies how well a choke blocks CM currents
This method is aligned with the standards outlined in CISPR, MIL-STD, IEC, and ISO EMC protocols. These standards have undergone decades of peer review and global regulatory adoption.
What EMC Standards Actually Use
Real-world EMC testing involves:
- Bulk Current Injection (BCI) and Direct RF Injection (DRFI) to stimulate CM paths
- High-impedance current probes to measure CM currents accurately
- Single-ended or triaxial S11 setups with tightly controlled terminations
Differential-mode S21 tests are not part of these procedures, because they fail to simulate the actual noise paths that chokes are intended to suppress.
These standards aren’t arbitrary—they represent the accumulated best practices of the global EMC engineering community, and they’re critical to CE compliance and product safety.
The Fallacy of “Exotic” Coax-Based Chokes
A popular trend among hobbyists involves using semi-rigid coax like RG402 or copper tubing, often presented as revolutionary choke designs. These claims are typically based on flawed measurements using differential-mode methods.
Without proper CM excitation and isolation, such tests provide false confidence. In reality, these chokes may have poor common-mode impedance and be ineffective or even harmful in actual RF systems.
Why All This Matters
Common-mode currents are a major source of:
- RFI (radio frequency interference)
- Pattern distortion
- “RF in the shack” problems
- Receiver noise pickup
If your test setup isn’t exciting the common mode, you're not testing what matters.
Evaluating chokes with differential S-parameters tells you nothing about how they behave when real-world CM currents are present.
Conclusion: Measure the Mode That Matters
Despite their popularity, test methods promoted by otherwise reputable sources like DG8SAQ or VE2AZX frequently fail to isolate and stimulate the common-mode path. Chokes should be evaluated in the mode they are designed to suppress—and that mode is common.
Always ask yourself:
Does my test setup actually excite a common-mode current?
If the answer is no, the measurement is likely irrelevant.
Appendix: Known Misapplied or Misleading Techniques
- DG8SAQ VNWA Differential Test Fixture (e.g., on YouTube or forums)
- VE2AZX balun test methods using differential setups
- MFJ-style balun/choke analyzers lacking CM isolation
- Direct S21 through-measurements without CM excitation
- Any test that does not isolate or stimulate the common-mode path
- Claims of choke superiority based on exotic coax (e.g., RG402) with no proper CM testing
If you're uncertain about your measurement setup, ask yourself this simple question:
“Am I stimulating and measuring the common-mode path?”
If not, the data you're collecting could be dangerously misleading.
Interested in more technical content like this? Subscribe to our notification list — we only send updates when new articles or blogs are published: https://listmonk.rf.guru/subscription/form
Questions or experiences to share? Feel free to contact RF.Guru or join our feedback group!
Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.