Autotransformer vs. Ruthroff Transformer in UNUNs and BALUNS
Both autotransformers and Ruthroff transformers are widely used in UNUNs and BALUNs, but they function differently when it comes to impedance transformation and handling common-mode currents.
1. Ruthroff Transformer (Tapped Winding, Isolated Secondary)
- Type: Voltage transformer
- Common Use: Voltage baluns and some UNUNs
- Key Feature: Separate secondary winding provides isolation between input and output
- Advantage: Good for impedance transformation while maintaining correct phase relationships
- Disadvantage: Does not suppress common-mode currents, allowing unwanted RF on coax
-
Examples:
- 4:1 Ruthroff voltage balun for OCF dipoles
- 4:1 voltage UNUN for end-fed antennas
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How It Works:
- Tapped winding transforms voltage based on turns ratio
- Secondary is magnetically coupled, not electrically connected
2. Autotransformer (Tapped Coil, No Isolation)
- Type: Voltage transformer
- Common Use: UNUNs (e.g., 9:1, 49:1, 64:1)
- Key Feature: Single winding with taps — input and output share a common point
- Advantage: Simple and efficient for impedance matching
- Disadvantage: No isolation; common-mode currents can flow unless an external choke is added
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Examples:
- 49:1 or 64:1 UNUN for EFHW antennas
- 9:1 UNUN for random wire antennas
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How It Works:
- A single tapped winding sets the impedance transformation ratio
- Input (feedline) and output (antenna) are electrically connected
Key Differences
| Feature | Ruthroff Transformer | Autotransformer |
|---|---|---|
| Type | Voltage transformer | Voltage transformer |
| Used In | Voltage baluns, some UNUNs | Most UNUNs (9:1, 49:1, etc.) |
| Isolation? | Yes (separate secondary winding) | No (shared connection) |
| Common-Mode Suppression? | No, requires choke | No, requires choke |
| Efficiency | Slightly lower (extra winding) | Higher (direct connection) |
| Example Applications | 4:1 Voltage balun for OCF | 49:1 UNUN for EFHW |
Which One is Better?
- For EFHW and random wire antennas → Autotransformer (9:1, 49:1, etc.) is simple and efficient
- For OCF dipoles and Windoms → 4:1 Ruthroff voltage balun is common but limited in CMC suppression
- For best results: Always add a 1:1 choke after an autotransformer or Ruthroff device
Mini-FAQ
- Which is more efficient? — Autotransformers are usually more efficient since they use fewer windings.
- Which provides isolation? — Ruthroff designs provide electrical isolation, autotransformers do not.
- Do either suppress common-mode currents? — No, both need a separate choke to block RF on the coax.
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