When Does CAT5 Start to Radiate?

And why PoE is the silent saboteur of your RF noise floor

Introduction

Ethernet is not supposed to radiate. Twisted pair cabling like CAT5e or CAT6 was designed specifically to not act like an antenna. Under normal conditions, it's a balanced transmission line: two wires carrying equal and opposite currents, with their fields canceling out. That’s the theory. So why does your shack's noise floor jump 20 dB when you power your next-gen SDR with an Ethernet cable?

Let’s talk about the devil in disguise: Power over Ethernet.

Why Twisted Pair Works (Until It Doesn’t)

Unshielded twisted pair (UTP) cables rely on differential signaling. Each Ethernet pair carries signals as equal and opposite voltages. When done properly, this creates a “silent” cable from an RF standpoint: no net current, no net radiation. It's beautifully symmetrical.

But here’s the catch: only the signal pairs are balanced. And PoE breaks that balance wide open.

The PoE Problem: Common-Mode Currents Enter the Game

Power over Ethernet injects DC power onto the Ethernet pairs—typically 48 V. This is done using phantom powering, where the DC is superimposed on the same wires that carry the differential data signal.

However, that DC has to return somewhere.

In a non-isolated PoE injector or cheap power supply (spoiler alert: 95% of them), the return path finds its way into your shack’s grounding system or even worse — your coaxial cable shields or audio ground. This unbalances the cable, turning it from a twisted-pair transmission line into a common-mode radiator. And yes, it radiates spectacularly well at HF and VHF.

The result? Broadband hash noise every 62.5 kHz, 125 kHz, or 250 kHz, depending on the Ethernet PHY clocking — dancing across your waterfall like Christmas lights from hell.

What Makes CAT5 Start to Radiate?

Let’s be blunt: CAT5 starts to radiate when symmetry is broken.

This happens when:

1. The Ethernet signal is no longer purely differential — e.g. due to ground potential differences between devices.
2. PoE is injected without proper common-mode filtering or isolation.
3. A device’s ground is poorly decoupled, creating ground loops.
4. The cable acts as an unintended monopole — especially when long UTP cables are run indoors or near antennas.

Remember: once common-mode current flows, the cable becomes an antenna, especially at odd multiples of λ/2 for the common-mode component.

Symptoms You’ll Notice

• A stable pattern of digital birdies across HF and VHF
• Noise that increases when you plug in a PoE device
• Noise that disappears when you unplug the CAT5 cable
• Broadband hash every 60–250 kHz, depending on the Ethernet clock

Mitigation Strategies

1. Use shielded CAT6 with grounded connectors — and make sure the shield is bonded properly at one end only.
2. Avoid cheap PoE injectors and splitters — especially the $5 eBay types.
3. Use common-mode chokes — snap-on ferrites or toroidal windings on both ends of the Ethernet cable (3–6 turns).
4. Galvanically isolate the Ethernet using fiber — the ultimate fix for RF-sensitive environments.
5. Route cables away from antennas — keep Ethernet cables low and perpendicular to radiating elements.

Real-World Example

In our lab, a 12-meter CAT5e cable from a generic PoE switch to a remote SDR module created an S5 noise floor from 1.8 MHz to 30 MHz. After replacing it with:

• A fiber Ethernet link (MCU → media converter → fiber → media converter),
• Ferrite chokes on both ends,
• And isolating the PoE supply ground,

the noise dropped below S1 across all HF bands.

Conclusion

Ethernet isn't the enemy — PoE done wrong is.

Balanced systems are a beautiful thing… until someone throws a 48 V DC wrench into the equation. So next time someone says “my CAT5 is radiating,” tell them: it’s not the cable — it’s the current on it that’s radiating.

If you're serious about low-noise HF reception or sensitive SDR work, ditch PoE or isolate it properly. Common-mode current isn’t a theory — it’s a killer.

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