Ham Radio
EFHW Transformer Turns Ratios

EFHW Transformer Turns Ratios Reference

End-Fed Half Wave (EFHW) antennas require an impedance transformer to match the high impedance of the antenna feedpoint (typically 2450–4050 Ω) to the 50 Ω output of your transceiver.

The transformer uses a specific turns ratio to achieve the desired impedance transformation ratio.

Transformer Winding Reference Table

Below is a reference table showing common winding configurations for EFHW transformers, including the resulting impedance ratios and output impedances.

Primary Turns Secondary Turns Turn Ratio Impedance Ratio Output Impedance
2 14 1:7 1:49 2450 Ω
2 16 1:8 1:64 3200 Ω
2 18 1:9 1:81 4050 Ω
3 21 1:7 1:49 2450 Ω
3 24 1:8 1:64 3200 Ω

Understanding the Relationship

Turn Ratio

The turn ratio is simply the ratio of secondary windings to primary windings:

Turn Ratio = Secondary Turns ÷ Primary Turns

Example: 14 turns ÷ 2 turns = 1:7

Impedance Ratio

The impedance ratio is the square of the turn ratio:

Impedance Ratio = (Turn Ratio)²

Example: (1:7)² = 1:49

Output Impedance

The output impedance is calculated by multiplying the input impedance (50 Ω from your transceiver) by the impedance ratio:

Output Impedance = 50 Ω × Impedance Ratio

Example: 50 Ω × 49 = 2450 Ω

Practical Notes

Core Selection

Most EFHW transformers use ferrite toroids such as:

  • FT-240-43 (larger, higher power handling)
  • FT-140-43 (compact, good for QRP and portable)
  • FT-114-43 (small, suitable for low power)

Most Common Configuration

The 2:14 turns (1:7 ratio / 1:49 impedance ratio) is the most widely used EFHW transformer configuration because:

  • Simple to wind (fewer turns)
  • Good match for typical EFHW impedance (~2450 Ω)
  • Proven performance across HF bands
  • Compact and efficient

Choosing the Right Ratio

  • 1:49 (2:14 or 3:21) — Standard EFHW, best all-around choice
  • 1:64 (2:16 or 3:24) — Higher impedance, useful for some multiband setups
  • 1:81 (2:18) — Highest impedance, less common but useful in specific designs

Power Handling

Power handling depends on:

  • Wire gauge used for windings
  • Core size and material
  • Number of turns (affects core saturation)
  • Proper heat management

Always test your transformer at low power first and monitor for excessive heating.

Related Projects

This reference table applies to many antenna projects, including:

  • DG1JAN UniBalun (49:1 UnUn configuration)
  • xOTA antenna (49:1 and 64:1 versions)
  • 221-UnUn (EFHW with WARC band links)
  • Custom EFHW builds

Further Reading

73!