Why RG402 Coax Fails Miserably in Common Mode Chokes
Are you about to use RG402 coax for your next common mode choke?
!!! STOP !!! Put down the semi-rigid pipe!
You’re about to waste your time—and ruin your choke’s performance.
Let’s break down why RG402 is a terrible idea for common mode chokes, even though it looks like a good one on paper. Especially if you think “skin effect” is your magic bullet. It’s not.
At First Glance: RG402 Looks Perfect
- Solid copper outer shield (great conductivity!)
- PTFE dielectric (ultra stable!)
- Low loss up to GHz (true!)
- Beautifully clean impedance (check!)
So what’s the problem? The common mode choke isn’t about what’s on the inside of the coax—it’s all about the outer shield in common mode.
Myth: “RG402 has better skin effect, so it must be better!”
“Skin effect means the RF rides on the surface. RG402 has a smooth, pure copper outer shield. That must be perfect for choking common mode currents!”
Completely Wrong.
The Truth About Skin Effect in Common Mode
Skin effect applies to differential signals—the signal and return current flowing on the center conductor and inner wall of the shield.
Common mode currents ride on the outside of the shield, and:
The outside of the coax doesn’t participate in skin effect the same way. The outer surface is directly affected by your ferrite material.
So what matters most is how well the outer surface couples to the ferrite.
The current you measure on the outside of the coax shield can be viewed in two ways:
- Outer-surface return (sheath current): Part of the RF return forced by antenna/feed asymmetry. It shares phase with the wanted RF and follows the path the geometry allows.
- Strict “common-mode” (EMC sense): Same-direction current defined relative to an external reference plane/field. Different reference → different name.
Physics is the same in both views: skin effect isolates inner vs outer surfaces; the inner surface carries the TEM return, and the outer surface can carry a separate surface current that radiates and couples noise. Mitigation is identical: raise the outer-path impedance (high-Z choke), improve symmetry, and use proper counterpoise/ground.
And here’s where RG402 fails.
Why RG402 Sucks at Being a Choke
1. Solid Shield = Poor Magnetic Coupling
RG402’s hard copper tube doesn’t couple well with ferrite cores. It forms an effective electromagnetic barrier between the current and the ferrite. That means your choke is starved of magnetic flux—so it doesn’t choke much of anything.
Braided shields (like RG316, RG142) are porous and flexible, which allows better flux interaction and absorption by the ferrite.
2. You Can’t Wind It
Try wrapping RG402 around a toroid. You’ll get one turn in—and then a snapped tube or a kink. It’s like trying to tie a knot with a copper pipe.
Chokes need multiple tight turns through ferrite for maximum impedance. RG402 just isn’t built for that.
3. Not Built for External Currents
Common mode chokes work best when the shield lets external RF currents interact with the ferrite.
RG402’s hard shield is designed to keep fields out, not let ferrite in. You’re fighting its design goals.
Use RG142 or RG400 PTFE coax for high power—or use twin PTFE wire for balanced lines.
Bottom Line
RG402 is designed to shield, not to choke.
Using it in a common mode choke is like using concrete for a garden hose. It just doesn’t work.
Save your time, your ferrites, and your coax budget.
Use the right tool for the job. Let RG402 stay in the microwave shack where it belongs.
This information also applies to all copper pipe coax types (RG-401, RG-402 and RG-405).
Mini-FAQ
- Why is RG402 bad for chokes? — Its solid copper shield blocks ferrite coupling, starving the choke of flux.
- Does skin effect help? — No, skin effect applies to differential signals, not outer-surface currents that radiate.
- What coax should I use? — RG142, RG400, or twin PTFE wires are better for building effective high-power chokes.
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