Why VNA Coax Shield Tests Don’t Prove Your Choke Works
Updated August 21, 2025
Why Measuring Coax Shield with a VNA Doesn’t Prove Chokes
Many hams think they’ve cracked the code:
“Just connect the VNA to the braid of your coax, clip on ferrites, and measure insertion loss. If S21 drops, the choke is working!”
Unfortunately… this still isn’t a valid way to measure common-mode suppression.
What They're Doing
Typical process:
- Take a piece of coax.
- Strip both ends so the shield is exposed.
- Connect VNA Port 1 and Port 2 directly to the shield; center conductors float.
- Place a ferrite choke somewhere on the shield.
- Sweep S21 and call attenuation “choke performance.”
It looks clever. After all, common-mode current flows on the shield’s outside. So driving the braid should simulate reality, right?
Why That’s Still Wrong
1. You’re Not Exciting True Common-Mode Current
By connecting only to the braid, you’re driving it as a conductor relative to ground, not simulating imbalance-driven common-mode. With no ground plane/counterpoise, the return path is undefined. You’re not recreating real-world shield behavior.
2. Setup Artifacts Dominate
Any S21 loss you see may come from:
- Skin effect losses at HF/VHF,
- Reflections at the shield’s open ends,
- Stray capacitive/inductive leakage, not ferrite suppression.
The VNA just sees “two conductors with loss.” It doesn’t know you’re trying to measure common-mode.
3. Ferrite Behavior Isn’t Linear in This Mode
Ferrites suppress common-mode when current flows relative to an external return. Driving the braid differentially between two ports can push ferrites into misleading regimes, making results meaningless for real choke performance.
How to Measure It Right
If you want real choke data:
- Use a current probe on the coax to measure real RF current reduction.
- Employ a balanced injection method (balun/hybrid) to drive common-mode currents properly.
- Follow K9YC’s jig method with ground reference and measure suppression under real-world excitation.
The Takeaway
Measuring through the braid with a VNA does not equal common-mode testing. You’re driving the shield as a wire, not simulating its role as part of an imbalanced antenna. Loss on the braid ≠ choke effectiveness.
Don’t trust SWR dips or S21 loss unless your test recreates actual RF current paths.
Mini-FAQ
- Why doesn’t VNA braid testing prove choke performance? — Because it doesn’t replicate common-mode excitation paths.
- What’s a reliable way to measure chokes? — Current probe on coax, or proper jig injection methods with ground reference.
- Can S21 loss still be useful? — Only as a rough indicator of added impedance, not real-world suppression.
- Why are ferrite results misleading here? — You’re exciting them in differential mode, not common-mode.
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