The Ruthroff transformer (Voltage Balun/UNUN) explained
The Ruthroff transformer is a voltage-mode transmission line transformer (TLT) designed to transform impedance by manipulating voltage ratios. Invented by Clyde L. Ruthroff (US Patent 3037175A), it is simpler and often more efficient than current-mode designs when the goal is impedance matching. But it has a drawback: it does not suppress common-mode currents, which makes it less suitable for balanced systems without additional measures.
Key Characteristics
- Type: Voltage transformer
- Purpose: Efficient impedance transformation
- Construction: Single tapped winding or autotransformer design
- Common Ratios: 1:1, 4:1, 9:1
- Best For: EFHWs, Windoms, random wires where voltage matching dominates
How It Works
- The primary takes coax input power.
- The secondary (tapped winding/autotransformer) steps up or down voltage.
- It balances voltage but not current — so coax shields can carry unwanted RF.
- Common-mode mitigation requires separate ferrite chokes or hybrid designs.
Common Types
1:1 Ruthroff Voltage Balun
- Use: Feed balanced antennas like Windoms, EFHWs
- Construction: Single winding, center-tap
- Limitation: No CMC suppression — coax often radiates
4:1 Ruthroff Voltage Balun
- Use: Match ~200 Ω antennas to 50 Ω coax
- Application: Classic Windoms, OCFs
- Limitation: Needs additional CMC choke for clean operation
9:1 Ruthroff UNUN
- Use: Match 450–900 Ω random wires to 50 Ω
- Advantage: Simple, efficient voltage step-down
- Limitation: Often noisy without a serious common-mode choke
Ruthroff vs. Guanella
| Feature | Ruthroff (Voltage) | Guanella (Current) |
|---|---|---|
| Type | Voltage transformer | Current transformer |
| Impedance Matching | Yes (1:1, 4:1, 9:1) | Yes (1:1, 4:1, 9:1) |
| CMC Suppression | No | Yes |
| Construction | Tapped winding/autotransformer | Separate lines on ferrite |
| Losses | Lower at HF, efficient voltage step | Higher at large ratios |
| Best For | EFHWs, Windoms, random wires | Balanced dipoles, loops, verticals |
When to Use a Ruthroff
- When impedance transformation efficiency is critical
- When voltage balance is sufficient
- If CMC chokes can be added externally
- If Guanella losses are excessive at the target ratio
Final Takeaway
Ruthroff transformers shine as simple, efficient impedance transformers — especially in EFHWs and high-impedance UNUNs. But they do not suppress common-mode currents. Always pair them with serious chokes, or use hybrid designs if your goal is both impedance match and RF cleanliness.
Mini-FAQ
Ruthroff Transformer – Quick Answers
- What’s the main advantage? — High efficiency for impedance transformation.
- What’s the main weakness? — No inherent common-mode current suppression.
- Where is it best used? — EFHWs, Windoms, random wires with an external CMC choke.
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