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 or try differential antennas, but overlook a simple and highly effective method: a galvanic isolation transformer (1:1 UNUN) placed on the RX line.

Let’s look at why it works, what it breaks (by design), and why it’s strictly RX-only.

Common-Mode Noise: The Hidden Path

Your coax doesn’t just carry the desired differential signal inside the shield. The outside of the braid can behave like a long-wire sensor, picking up conducted and radiated hash from SMPS, Ethernet, LED drivers, PV inverters, and the neighbor’s everything. That common-mode current rides the braid right into the receiver front-end—often louder than the antenna itself.

Why a Choke Isn’t Always Enough

A good common-mode choke adds series impedance on the braid and is essential—but not perfect:

  • Effectiveness can drop at the low end of HF for a given ferrite mix and turn count.
  • Feedline length, routing, and grounding can create alternate return paths around the choke.
  • Stray capacitances can partially bypass the choke.

Result: some noise still gets through.

Enter Galvanic Isolation (1:1 UNUN Transformer)

A 1:1 isolation transformer (unbalanced-to-unbalanced) breaks DC continuity between the coax braid on the antenna side and the receiver ground. That discontinuity prevents braid currents from completing their loop into the RX input and shack ground.

Important: This must be a transformer with separate windings. A 1:1 autotransformer does not provide galvanic isolation.

What isolation does for you:

  • Blocks braid-borne noise currents from coupling into the RX front-end.
  • Kills ground loops between feedline and shack ground reference.
  • Makes the receiver “see” the antenna’s differential signal, not the building’s noise field.

RX-Only—Here’s Why

  • On TX, you need a return path. Breaking DC continuity at the feedline can cause high voltages, unstable SWR, or RF-in-the-shack.
  • Isolation transformers for RX are sized for microvolts/milliwatts, not transmit power.

Best Placement

  • At the shack entry: put the isolation transformer just before the receiver.
  • At the antenna: keep a proper common-mode choke at the feedpoint to reduce noise pickup on the run of coax.

This two-stage approach—choke at antenna + isolation at RX—forms an effective noise barrier.

UNUN vs Choke on RX: What Each Does Best

Aspect 1:1 Isolation UNUN (Transformer) Common-Mode Choke
Primary function Breaks DC continuity (galvanic isolation); blocks braid loops into RX Adds series impedance to braid to suppress common-mode currents
DC continuity No (isolated primary/secondary) Yes (line remains continuous)
Noise loop control Excellent—loop cannot close through RX ground Good—reduced current, but loop may still exist
Frequency coverage Broadband when wound correctly; core choice matters Broadband with right mix/turns; mix-dependent at LF/HF edges
Insertion loss (RX) Low when properly designed Low; increases with very high choking impedance
TX suitability Not suitable (isolation breaks return path; power handling low) Yes—if designed for power and heat (QRO-rated choke)
Best placement At the receiver/shack entry At the antenna feedpoint (and sometimes at the shack)
Main drawback RX-only; needs proper core/winding to avoid LF roll-off Can be bypassed by stray capacitance; layout sensitive
Related reading: Common-Mode and Return Currents on Coax: Your Feedline Is Fighting You

Mini-FAQ

  • Is a 1:1 autotransformer OK here? — No. Autotransformers share a conductor and don’t provide galvanic isolation.
  • Will isolation affect antenna bias or preamp power? — Yes; provide DC power separately (bias-tee before the transformer) or use an isolated feed.
  • Static buildup risk? — Consider a high-value bleed resistor (e.g., 220 kΩ–1 MΩ) or a surge protector to ground on the RX side for ESD/static, while keeping RF isolation intact.
  • Which ferrite? — For HF RX isolation transformers, high-µ mixes (e.g., 73/75/77 family) help at low HF; design and winding style set the LF roll-off.

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Joeri Van Dooren – 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.