Currents on the Coaxial Cable: A Multi-Lane Highway of RF Behavior

Imagine a coaxial cable as a wide multi-lane highway. Each lane carries a different kind of traffic—each with its own direction, behavior, and consequence for your signal integrity. Understanding these "lanes" is crucial for diagnosing and solving many RF issues in ham radio and RF engineering.

The Differential Mode: The Legitimate Highway Flow

The differential-mode current is the signal you want to carry. It flows in one direction on the center conductor and returns on the inner surface of the shield. This is the designed traffic lane, controlled and confined, like well-behaved cars in opposite directions on separate lanes of a highway.

This current stays inside the coax, benefiting from the shield's symmetry to avoid radiation and interference.

When there is no VSWR, the forward wave travels cleanly from the transmitter to the load, and very little energy reflects back. But in real-world systems, some degree of reflection often occurs due to impedance mismatches. This creates a reflected wave, which travels backward on the center conductor and the inner shield surface, still confined to the differential path.

So now you have two differential-mode currents:

  • Forward current from transmitter → antenna
  • Reflected current from mismatch → transmitter

Both are confined to the inside of the coax (center conductor and shield interior) and do not produce radiation—unless there's a major defect or a break in shield continuity.

A common misconception is that skin effect is caused by VSWR. That is incorrect.

Skin effect is purely a function of frequency, not standing wave ratios. Whether the current is forward or reflected, it still hugs the conductor surfaces more tightly as frequency increases. VSWR affects power delivery efficiency, but not how deeply current penetrates into the metal.

Skin Effect: Surface-Limited Behavior, Not a Problem by Itself

The skin effect describes how, at high frequencies, RF currents tend to concentrate on the outermost layer of conductors. For differential signals, this means current flows:

  • Near the outer surface of the center conductor, and
  • Near the inner surface of the shield

This is a normal physical effect, not inherently harmful.

However, when antennas are unbalanced (e.g., end-fed or off-center-fed), there's no proper path for the return current. Then, current starts flowing on the outer surface of the coax shield. This current is often labeled as “common-mode” but this is incorrect.

This is actually a skin-effect return current, launched directly by the transmitter, traveling via the outer braid because the antenna system lacks a proper return path. It’s part of the conducted RF, not a noise current and part of the differential mode ! It’s not coupled or picked up—it’s deliberately driven as part of the feedpoint imbalance.

This distinction is critical. Mislabeling it as common-mode leads to confusion in diagnosis and ferrite placement.

True Common-Mode Currents: External RF Pickup on Conductors

True common-mode current refers to RF current that is picked up or coupled onto conductors—not driven by the transmitter. Examples:

  • Pickup from strong nearby transmitters or lightning
  • Long cables acting as receiving antennas
  • Audio or USB lines affected by RF fields

These currents:

  • Are induced from external fields
  • Flow in equal phase on all conductors with respect to ground
  • Are truly unwanted noise
  • Can exist on coaxial shields, control wires, Ethernet cables, etc.

They can cause:

  • Audio hum
  • USB disconnects
  • Microphone noise
  • Even receiver overload

Table: Skin-Effect Return vs. True Common-Mode

Characteristic Skin-Effect Return Current True Common-Mode Current
Source Driven by transmitter (from imbalance) Induced by external RF fields
Flow path Outer shield (conducted) Outer shield or any conductor (induced)
Radiation Yes, radiates if not choked Yes, often radiates/interferes
Cause Asymmetrical antenna system Nearby RF sources or coupling
Fixed by Choke near feedpoint Ferrites, filtering, shielding

Ferrite Chokes and Coax Selection

To block surface currents effectively, a ferrite choke or current balun is placed around the coax. These devices depend on magnetic coupling to introduce high impedance to unwanted currents.

For this to work properly, the outer shield of the coax must be accessible to the ferrite’s magnetic field. Coax types that use tightly bonded solid foil or solid tube shields (like RG402) restrict this magnetic interaction.

There’s a persistent myth that double-shielded coax (e.g., RG-214 or RG-223), or solid pipe-style coax like RG402, helps mitigate these unwanted currents. That’s not the case.

These types of coax improve isolation and reduce leakage for internal signal integrity, but:

  • They do not stop skin-effect return currents caused by antenna imbalance
  • They do not prevent common-mode noise pickup from the environment
  • Their minimum bend radius is typically far too large for practical use in winding around ferrite cores—making them mechanically unsuitable for use in effective chokes

Some mistakenly assume that wrapping RG402 or foil/braid coax around ferrite will work. They think that because surface currents follow the skin effect, a ferrite choke will suppress them. This is only partly true:

  • Surface (imbalance) currents may be reduced
  • But induced common-mode noise will not be affected—because the ferrite has no magnetic path into the solid or foil-covered shield

Ferrites act on the magnetic fields that surround the conductor. In coax with a solid outer conductor or foil barrier, the magnetic field cannot penetrate, and therefore the ferrite cannot couple to the shield currents effectively. This makes the choke ineffective.

The Best Coax for a Choke: The One with Good Magnetic Coupling

For optimal ferrite choke performance:

  • Use coax with braided shielding that allows good magnetic coupling (e.g., RG58, RG174, RG316, RG142)
  • Avoid coax with:
    • Solid outer conductors (e.g., RG402)
    • Dual shielding that includes a solid foil layer under the braid

Wrapping inappropriate coax around a ferrite and expecting choking action is a common mistake. It leads to frustration and misunderstood results. Worse, it creates the illusion that the system is choked when it's not.

Do’s & Don’ts: Choking Coax the Right Way

✅ Do:

  • Use coax with braided shielding for ferrite chokes
  • Place chokes as close to the feedpoint as possible
  • Use high permeability ferrites (e.g., #31, #43) for HF choking
  • Apply multiple turns through ferrite cores for greater impedance
  • Understand the difference between driven imbalance currents and induced noise

❌ Don’t:

  • Don’t use solid-shield (pipe-style) coax like RG402 in chokes
  • Don’t expect foil-based shielding to couple with ferrites
  • Don’t confuse skin-effect return currents with true common-mode noise
  • Don’t assume a double-shielded coax eliminates the need for choking
  • Don’t rely on high-end coax to “fix” imbalance—it won’t

Summary: Know Your Lanes and Causes

  • Differential current = center conductor ↔ inner shield
  • Reflected current (from VSWR) = still differential mode
  • Skin-effect surface current = outer shield, due to imbalance (driven, not induced)
  • True common-mode current = induced/picked up on conductors
  • Skin effect = higher frequency → current hugs surfaces
  • Choke design = requires magnetic coupling on outer shield
  • Double shield or pipe coax = does not solve imbalance or pickup issues

Treat your coax like a highway. Know where the traffic should flow—and where it shouldn’t. Unbalanced antennas and poor choking allow current to detour onto the shoulder, where it causes trouble. But don’t mislabel every surface current as common mode.
Precision matters. And so does the road design.

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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.