General High Power EFHW UNUN (49:1 / 68:1 / 70:1) user installation guide & safety instructions

This UNUN is designed for End-Fed Half-Wave (EFHW) antennas and enables efficient impedance transformation from high-impedance antenna feedpoints (typically 2450 to 3500 ohms) to standard 50-ohm coaxial systems. It is suitable for multiband EFHW antennas in the following configurations:

• 49:1: for 20 to 40 meters
• 70:1: for 80 to 40 meters
• 68:1: for 160 to 80 meters

Due to the very high voltages present at the feedpoint (up to 10 kV peak during high-power transmission), strict safety precautions must be followed.

This product is intended for outdoor use in a fire-safe and explosion-safe environment only.

Mounting & Installation

• Mount the UNUN as high as safely possible, using UV-resistant rope or hardware.
• Keep the UNUN out of reach of humans and animals.
• Suspend wire antennas using UV-resistant, weatherproof rope (≥ 5 mm diameter) and decent insulators.
• The wire radiator should be kept away from metallic structures and power lines. Maintain safe clearance in all directions.
• When mounted horizontally or as an inverted-L, ensure the transformer enclosure is oriented with vent holes downward to allow for drainage.
• A solid ground connection at the antenna feedpoint is essential. We offer an optional 75 cm stainless steel ground stake with integrated PL bulkhead connector. This improves transmission stability and significantly reduces received noise.

Grounding & Common Mode

• A common misconception is that EFHW antennas are "no ground required" — this is false.
• The UNUN must be installed with a common-mode choke (1:1 current balun). For optimal performance, the choke should be placed:
  • Either just before the coax enters the shack, or
  • Approximately 0.05 λ (lambda) from the feedpoint or ground stake, depending on coax length.
  • Using multiple common-mode chokes along the coax feed-line can further reduce RF noise and mitigate common-mode currents.

Connections

• Antenna terminal: M6 stainless steel (do not over-tighten; high voltage is present!).
• Coaxial feed: SO-239.
• Protect all exposed connectors with high-quality self-amalgamating tape or silicone sealant or rubber seal!

Antenna Wire Lengths (Typical for Resonance)

• 20–40 m bands: ~21 m radiator (49:1)
• 80–40 m bands: ~41 m radiator (70:1)
• 160–80 m bands: ~81 m radiator (68:1)

Tuning & SWR

• These transformers are not magic boxes; a tuner is can be required to ensure acceptable SWR depending on your installation (height, surroundings).
• Asymmetrical antenna tuners offer better range than built-in tuners.

Power Ratings & Safety Margins

• Rated for continuous use up to:
  • 4 kW PEP (160–40 m) when SWR ≤ 2:1
  • 3 kW PEP (40–20 m) when SWR ≤ 2:1
  • 1.5 kW PEP when SWR between 2:1 and 3:1
  • 750 W PEP when SWR > 3:1 (not recommended for extended periods)
• WARNING: High mismatch leads to excessive heating and corona discharge inside the UNUN.
• Voltages can exceed 10 kV peak at full legal power on low bands.
• Adequate creepage and clearance distances are implemented, and all internal components are HV coated for corona protection.

Safety Instructions

• Read this document fully before installation. Follow amateur radio safety guidelines.
• Never install near or under overhead power lines.
• Do not touch the antenna wire or terminals during transmission.
• Install lightning protection and always disconnect during thunderstorms.
• Never work on the antenna when transmitting equipment is powered.
• Do not open the enclosure, warranty is void upon tampering.
• Inspect the unit regularly for signs of weather damage or corrosion.

Disposal & Symbols

• The product carries the WEEE symbol (crossed-out bin). Dispose of responsibly.
• The High Voltage symbol warns of lethal voltages during transmission.