Galvanic Isolation with a 1:1 UNUN on RX Mitigates Common-Mode Noise
In receive-only systems, common-mode noise pickup is often what sets your weak-signal “noise floor”—especially in urban environments. Many hams add common-mode chokes or experiment with more balanced antennas, but overlook a simple extra barrier on the RX line: a true galvanic isolation transformer (a 1:1 unbalanced-to-unbalanced transformer) placed in series just before the receiver.
Important clarification: this is a transformer, not a choke. A choke increases common-mode impedance, but it does not decouple because it keeps the coax shield electrically continuous. A true isolation transformer uses two separate windings so there is no shared ground reference (no direct conductive bond) between the antenna-side coax and the receiver/shack side.
Let’s look at why it works, what it breaks (by design), and why it should be used only on RX-only paths.
Common-Mode Noise: The Hidden Path
Your coax doesn’t just carry the desired differential signal between center conductor and shield. The outside of the braid can behave like a long-wire pickup, collecting conducted and radiated hash from SMPS, Ethernet, LED drivers, PV inverters, and other local sources. That common-mode current can ride right into the shack and get referenced into the receiver input via the receiver’s ground/chassis.
Why a Choke Isn’t Always Enough
A good common-mode choke adds series impedance to current on the outside of the coax shield and is often essential—but it’s not always the final word:
- Effectiveness can drop at the low end of HF for a given ferrite mix and turn count.
- Feedline length, routing, and bonding/grounding can create alternate return paths that reduce real-world choke effectiveness.
- Stray capacitances can partially bypass a choke at some frequencies.
Result: some braid-borne noise can still make it into the receiver environment.
Enter Galvanic Isolation (1:1 UNUN Transformer)
A 1:1 RF isolation transformer (unbalanced-to-unbalanced) transfers the desired differential RF signal magnetically while keeping the two sides electrically separate. In other words, it intentionally breaks the conductive connection between the antenna-side coax shield and the receiver/shack ground reference.
Important: This must be a transformer with separate windings. A 1:1 autotransformer shares a conductor and does not provide galvanic isolation.
Also important (easy to miss): To actually get “no shared ground reference,” the connector shells must not be bonded together through a common metal panel/enclosure. If both coax connectors are grounded to the same metal box, you’ve re-created a shared reference and defeated the point of galvanic isolation.
What isolation does for you (in practice):
- Greatly reduces conductive braid-noise injection into the receiver by removing the direct shield/ground loop path.
- Breaks ground-loop paths between the incoming feedline shield and the receiver/shack reference.
- Helps the receiver “reference” the antenna signal rather than the building’s local noise environment.
Note: no real transformer is “perfectly isolated” at RF—there is always some stray capacitance between windings. But a well-executed isolator can still provide a very meaningful reduction in common-mode coupling in noisy installations.
Transformer vs Choke (and Why the Transformer Decouples)
A choke works by making common-mode current hard to flow (high impedance), but the shield remains electrically continuous end-to-end. That means there is still a shared ground reference between antenna-side coax and receiver-side coax. The isolation transformer is different: it removes the shared reference entirely (conductively), so the common-mode loop can’t simply “carry on” into the RX input the same way.
RX-Only—Here’s Why
- This approach is intended for a receive-only feedline or a receive-only port (e.g., after proper T/R switching and protection). It is designed to deal with microvolts/milliwatts, not transmit power.
- A small-signal isolation transformer can overheat, saturate, arc, or fail if transmit power is applied (especially with mismatch or strong nearby RF fields).
- For transmit-capable lines, use a TX-rated solution (typically a properly designed common-mode choke/current balun at the feedpoint, plus correct station bonding and protection). Do not assume a receive isolator is safe to use on TX.
Best Placement
- At the shack entry / receiver input: place the isolation transformer as close as possible to the receiver so the “receiver side” coax is short and stays inside the quietest part of the shack layout.
- At the antenna: keep a proper common-mode choke at the feedpoint to reduce noise pickup and unwanted current on the outside of the coax over the full run.
This two-stage approach—choke at antenna + galvanic isolation at the receiver—forms a very effective noise barrier in many real-world RX-only installations.
UNUN vs Choke on RX: What Each Does Best
| Aspect | 1:1 Isolation UNUN (Transformer) | Common-Mode Choke |
|---|---|---|
| Primary function | Breaks conductive shield/ground continuity; removes the shared ground reference into the RX input | Adds series impedance to suppress common-mode current on the outside of the feedline |
| DC / conductive continuity | No (isolated primary/secondary windings) | Yes (feedline remains electrically continuous) |
| Decoupling (shared reference) | Excellent—no direct conductive shared ground reference between sides | No—shield is still the same conductor end-to-end |
| Noise loop control | Excellent for breaking conductive loop paths into the receiver/shack reference | Good—reduces current, but loop/path can still exist |
| Frequency coverage | Broadband when wound correctly; core and winding geometry set LF roll-off and HF behavior | Broadband with the right mix/turns; mix-dependent at LF/HF edges |
| Insertion loss (RX) | Low when properly designed; can rise if undersized (LF roll-off) or poorly laid out | Low; can increase if pushed to extreme choking impedance with lossy materials |
| TX suitability | Only if specifically designed and rated for TX. A receive isolator is RX-only. | Yes—if designed for power and heat (QRO-rated choke) |
| Best placement | At the receiver/shack entry (in the RX-only path) | At the antenna feedpoint (and sometimes at the shack) |
| Main drawback | RX-only; finite isolation due to stray capacitance; must avoid accidentally bonding the two sides | Can be bypassed by stray capacitance; layout and routing sensitive |
Mini-FAQ
- Is this just a choke (1:1 choke balun)? — No. A choke is not a decoupler; it keeps the shield continuous. This article is about a true two-winding transformer that creates no shared ground reference between antenna-side coax and receiver-side coax.
- 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. A galvanic isolation transformer breaks DC continuity. If you must inject DC, do it on the antenna side of the transformer (or use a separate power feed), and make sure the injector arrangement does not reintroduce a shared ground reference.
- Static buildup risk? — Consider proper surge/ESD protection and a bleed path on the antenna side (or at the entry point) so static has somewhere to go without bonding the two sides of the transformer together.
- Which ferrite? — For HF RX isolation transformers, high-µ mixes (e.g., 73/75/77 family) can help at the low end of HF; core size and winding style set the LF roll-off and usable bandwidth.
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