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. In real-world systems, some reflection occurs due to impedance mismatches, creating a reflected wave traveling backward on the center conductor and 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 inside the coax (center conductor and shield interior) and do not radiate—unless there’s a major defect or shield discontinuity.

A common misconception is that skin effect is caused by VSWR. That is incorrect: skin effect depends only on frequency, not standing-wave ratio. VSWR affects power delivery efficiency, not current penetration depth.

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

At high frequencies, RF currents concentrate near conductor surfaces:

• Outer surface of the center conductor
• Inner surface of the shield

This is a normal phenomenon—not harmful on its own.

However, when antennas are unbalanced (e.g., end-fed or off-center-fed), the return path is incomplete. Then current starts flowing on the outer surface of the coax shield. This current is often mislabeled as “common-mode,” but in reality it’s a stray return current on the outer shield—a skin-effect current launched directly by the transmitter due to feedpoint imbalance.

The physical direction of the coax cable has nothing to do with the electrical direction of current flow. RF current is defined by its instantaneous phase relationship, not by the way the cable is routed. That’s why this concept can feel counterintuitive — the coax may lead away from the antenna, yet electrically, part of the return current can still be flowing “toward” it.

TX ───── coax ─────> ANT

Center conductor:      I_DM  ───────────────>
Shield (inner surface):        <───────────  I_DM   (DM return)
Shield (outer surface): I_stray  ───────────>         (unwanted; “third leg”)
                                         [choke here blocks I_stray]

By reciprocity, the same current paths apply during receive mode, but with arrows reversed. The stray return current can also be driven from both ends—either from the antenna feedpoint or from noise sources in the shack.

ANT ───── coax ─────> RX

Center conductor:      I_DM  ───────────────>   (to RX)
Shield (inner surface):        <───────────  I_DM   (DM return)
Shield (outer surface): I_stray  ───────────>   (from feedpoint toward shack)

These outer-surface stray return currents meet along the feedline and can distort the radiation pattern, shift impedance, and raise the receive noise floor—exactly what happens when the coax becomes a “third leg antenna.”

True Induced Surface Currents: External RF Pickup on Conductors

True stray or induced surface currents refer to RF picked up or coupled onto conductors, not driven by the transmitter. Examples include:

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

These currents:

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

Table: Skin-Effect Return vs. Induced Stray Current

Ferrite Chokes and Coax Selection

To block these outer-surface currents, a ferrite choke or current balun is placed around the coax. These rely on magnetic coupling to present high impedance to the stray current while leaving the differential pair untouched. For this to work, the shield must be magnetically accessible. Coaxes with solid foil or tube shields (like RG402) prevent magnetic penetration and render chokes ineffective.

Foil or double-shielded coax improves internal leakage performance but:

• Does not stop imbalance-driven outer-surface currents
• Does not prevent externally induced noise
• Has too large a bend radius for ferrite winding

Z ≈ 2 π f L μᵣ N²

The Best Coax for a Choke

• Use braided-shield coax (e.g., RG58, RG174, RG316, RG142)
• Avoid solid or foil-shielded types (e.g., RG402, RG214)

Wrapping inappropriate coax on ferrite and expecting choking action is a common trap—it may look effective but leaves imbalance-driven stray currents untouched.

Do’s & Don’ts: Choking Coax Correctly

✅ Do:

• Use coax with braided shielding
• Place chokes near the feedpoint
• Use #31 or #43 ferrites for HF
• Add multiple turns through cores for higher impedance
• Distinguish driven imbalance from induced noise

❌ Don’t:

• Use solid-shield coax like RG402
• Expect foil shields to work with ferrites
• Confuse outer-surface stray current with externally induced pickup
• Assume double-shielded coax eliminates imbalance
• Rely on expensive coax to “fix” RF problems

Summary: Know Your Lanes and Causes

• Differential current = center conductor ↔ inner shield
• Reflected current = still differential mode
• Stray return current = outer shield, imbalance-driven
• Induced current = externally coupled noise
• Skin effect = frequency-dependent surface flow
• Choke design = requires magnetic coupling
• Double-shield coax ≠ fix for imbalance

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. So does the road design.

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