RF in the Shack: It's Skin Effect, Not Common Mode
When hams talk about "RF in the shack," it’s often shorthand for common-mode current. But not all RF currents on the coax shield are common mode. Many are due to skin effect in unbalanced or mismatched systems—a different problem with a different solution.
What Is Skin Effect?
The skin effect describes how RF currents travel on conductor surfaces. In coax:
- Inner conductor: forward signal
- Inner shield wall: intended return path
- Outer shield wall: should be quiet
With feedpoint imbalance (end-fed, OCFD, verticals), return current spills onto the outer braid. This is differential return current forced outside by imbalance. It’s skin-effect current, not environmental pickup, and still part of the transmit/receive circuit.
What Common Mode Actually Is
Common-mode current is in-phase noise on both conductors. It comes from coupling with the environment:
- Nearby power or signal lines
- Switching supplies
- Your body or house wiring
It can ride on any surface—outer braid, inner braid surface, or inner conductor exterior—because it’s not part of the intended differential circuit.
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 | No (needs symmetry fix) |
| Common-mode current | Environmental coupling | Any exposed surface | No (exists on RX too) | Yes (with correct ferrite choke) |
Ferrite chokes suppress common-mode noise effectively. But they don’t fix differential imbalance. For that, you need proper baluns or feedpoint isolation.
The Diagnostic Trap
Not every outer shield current is common mode.
- Skin-effect return: driven by imbalance; fixed with a choke or current balun at the feedpoint.
- Common-mode noise: audible on RX, independent of TX power; fixed by shielding, grounding, and chokes.
Why It Matters
Confusing them leads to wasted effort. Covering coax with beads won’t stop RF burns from imbalance currents, and adding a balun won’t fix noise pickup if the problem is common mode.
If you’re getting RF burns or hot mic cables, it’s imbalance (skin-effect return), not common-mode pickup.
Summary
- Skin-effect current: imbalance-driven differential return
- Common-mode current: environmental noise coupling
- Ferrite chokes stop common mode, not imbalance
- End-fed and OCF antennas are most prone to imbalance problems
Always ask: Is this imbalance or noise pickup? Because only the right cure will solve your RF problem.
Mini-FAQ
- How do I know if it’s imbalance? — If the issue scales with TX power (RF in shack, hot gear), it’s imbalance currents, not noise pickup.
- Can ferrites stop imbalance? — No. Ferrites choke common-mode noise, but imbalance needs proper feedpoint choking or a balun.
- Is common-mode present on RX? — Yes. Even with no TX, coax can pick up environmental noise as common-mode current.
- Which antennas are most prone? — End-fed half waves, OCF dipoles, and verticals without good ground systems.
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