Galvanic Isolation with a 1:1 UNUN on RX Mitigates Common-Mode Noise

In receive-only systems, common-mode noise pickup is often the limiting factor for weak signal reception — especially in urban or noisy environments. Many hams add chokes, ferrite beads, or try differential antennas, but often overlook a simple and highly effective method: a galvanic disconnect using a 1:1 UNUN.

Let’s explore why it works, what it breaks, and why it's strictly for RX use only.

Common-Mode Noise: The Hidden Path

Your coaxial feedline doesn’t just carry differential RF signals inside the braid — it also acts as a long wire antenna on its outer shield, picking up stray RF and noise (from switch-mode power supplies, Ethernet, LEDs, etc.). This common-mode current flows all the way into your receiver, where it couples directly into the input of your SDR or preamp, bypassing your antenna entirely.

In effect, your coax becomes a noise antenna, often overpowering the real signal.

Why Chokes Aren’t Always Enough

A common-mode choke (typically made from ferrite beads or windings on toroids) can suppress some of this unwanted current. But suppression is never perfect:

  • Ferrite efficiency drops at lower frequencies
  • Coax length and grounding layout affect choke effectiveness
  • Some noise simply bypasses the choke due to stray capacitance

Choking is necessary, but not always sufficient.

Enter the Galvanic Disconnect: The 1:1 UNUN

A 1:1 UNUN — especially one wound as an autotransformer on a ferrite toroid — provides something that a choke alone cannot: galvanic isolation.

What does that mean?

It means the coax braid is no longer electrically bonded to the shield or ground plane of your receiver system.

By interrupting that path, you:

  • Prevent common-mode noise currents from coupling into the antenna port
  • Eliminate loop currents between coax braid and receiver ground
  • Force the RX front-end to see only the differential signal from the antenna element

It’s like pulling the plug on the noise — the coax is now dead-ended for RF current flowing on its outside.

Why This Only Works on Receive

This technique is only valid in receive-only systems because:

  • A galvanic break prevents any DC or RF return path, which is critical for transmitting
  • With no return current path, a TX antenna can't radiate efficiently
  • On transmit, such a configuration could result in floating voltages, high SWR, or RF burns

A 1:1 UNUN used this way isolates — it does not match or balance. It’s a passive, receive-only solution for keeping the noise out.

Where to Place It

The best results come when you place the UNUN:

  • Right before the receiver, at the point where the coax enters your shack
  • Combined with a high-quality choke further out, ideally at the antenna feedpoint, to suppress common-mode at both ends

This two-stage approach (choke at antenna + UNUN at RX) forms a noise barrier.

Final Thoughts

Using a 1:1 UNUN as a galvanic disconnect is one of the most underrated tricks in the RX toolbox. It doesn’t work miracles — but in high-noise environments, especially with active antennas, it can be the difference between silence and intelligible signals.

But remember: never use this approach for TX. This is strictly a receive-only tactic — and when applied properly, it’s one of the most effective ways to shut the door on common-mode noise.

This is not a magic bullet, but when used right, it's close.

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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.