Don’t Make This Mistake: Why RG402 is the Worst Choice for 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!”

Here’s the trap — you might think:

“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.

But common mode currents ride on the outside of the shield, and:

The outside of the coax doesn’t participate in skin effect in 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.

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 .... that's the only one that will work with 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 is also valid for all copper pipe types of coax cables (like RG-401, RG-402 and RG-405)

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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.