RF in the Shack: It's Skin Effect, Not Common Mode
When hams talk about "RF in the shack," it's often used as shorthand for common-mode current. But here's the catch: not all RF currents flowing on the outer braid of your coax are common mode. Many are caused by skin effect—especially in unbalanced or mismatched systems. And that’s a completely different kind of beast.
What Is Skin Effect?
The skin effect describes how high-frequency AC currents tend to travel along the surface of conductors. In a coaxial cable:
- The inner conductor carries the forward signal.
- The inner surface of the shield serves as the return path.
- The outer surface of the shield should ideally carry no signal at all.
But when there's a feedpoint imbalance—like with end-fed or off center antennas—current begins to flow on the outer (braided) shield. This is not because the coax has become an antenna. It's because the system is forcing the outer shield to act as part of the differential return path. This is skin-effect-driven current. It's not pickup. It's not common mode. It’s still differential mode current!
What Common Mode Actually Is
Common-mode currents are in-phase signals that ride on both conductors in the same direction. They're typically picked up from environmental coupling:
- Nearby power or signal cables
- Switching power supplies
- Even your own body acting as an antenna
These currents can ride on multiple surfaces:
- The outer surface of the braid (most common)
- The inner surface of the braid (often overlooked)
- The outer surface of the inner conductor, depending on geometry and frequency
So, common-mode current can exist on any conductive surface that is not part of the intended signal path.
Skin Effect ≠ Common Mode
| Type | Origin | Surface | Affected by TX power? | Blocked by Ferrite Choke? |
|---|---|---|---|---|
| Skin-effect current | Feedpoint imbalance (differential) | Outer braid (forced return) | Yes | Not always (needs symmetry fix) |
| Common-mode current | Environmental pickup (external noise) | All exposed surfaces | No (happens on RX too) | Yes (with correct choke) |
Ferrite chokes are designed to block common-mode currents by presenting high impedance. However:
- Common-mode noise is usually low-level and easier to suppress.
- Skin-effect currents can be high-level (especially in QRO setups) and need much stronger rejection.
The Diagnostic Trap
Many operators misdiagnose any outer shield current as common-mode.
How to tell them apart:
- Skin-effect return current: Happens with unbalanced antennas. Fix it with a proper feedpoint choke or isolating balun.
- Common-mode noise: Audible on RX. Independent of TX power. Fixed by better shielding and grounding.
They're different in cause, behavior, and solution.
Why It Matters
Confusing the two leads to ineffective solutions. You might cover your coax with ferrite beads and see no improvement—because the problem wasn’t noise pickup. It was imbalance.
If you're getting RF burns or hot mic cables, you're dealing with skin-effect imbalance currents—not common-mode pickup.
Fix the system, not the symptom.
Summary
- Skin-effect current = caused by feedpoint imbalance (forced differential return)
- Common-mode current = in-phase noise from environmental coupling
- Ferrites block common-mode, but are blind to differential imbalance
- End-fed and off center antennas often cause RF-in-the-shack problems due to poor balance
Always ask: Is this system imbalance, or is it noise pickup?
Because your RF problems deserve the right kind of choke.