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Chameleon Knowledge Base · The Complete Online HF Antenna Handbook

What Is the Lowest Usable Frequency (LUF)?

Learn what the Lowest Usable Frequency (LUF) is and why lower HF bands sometimes become unusable during daylight hours.

Getting Started HF Fundamentals Reviewed 2026-07-14
Short Answer: Learn what the Lowest Usable Frequency (LUF) is and why lower HF bands sometimes become unusable during daylight hours.

Explanation

Overview The Lowest Usable Frequency (LUF) is the lowest radio frequency capable of providing reliable communication between two stations under current ionospheric conditions. Signals below the LUF are usually absorbed by the ionosphere before reaching the receiving station. Why the LUF Exists The primary cause of the LUF is absorption by the D Layer. During daylight hours, lower HF frequencies experience increased absorption, making long-distance communication more difficult. Daily Changes Daytime: LUF rises. Nighttime: LUF falls. This explains why bands such as 80 and 160 meters often perform much better after sunset. MUF and LUF Together Successful HF communication occurs within the range between the LUF and the MUF. This usable frequency window changes continuously throughout the day. Applied to Chameleon Products Understanding the LUF helps operators using Chameleon multiband antennas choose frequencies that are most likely to provide reliable communication under current propagation conditions. Related Articles What Is MUF? What Is the D Layer? What Is Skywave Propagation? How Do I Choose the Best HF Band? Related Products All Chameleon HF Antennas

The exact result depends on the complete station: frequency, geometry, feed line, matching network, return-current path, environment, operating power, and the reference plane of any measurement. A low SWR establishes an impedance relationship at that point; it does not by itself prove efficiency, radiation pattern, compatibility, or safety.

What to Verify

  • Use the newest official product guide or primary service documentation.
  • Confirm the exact model, revision, components, configuration, and operating conditions.
  • Begin tests at low power and change one variable at a time.
  • Do not infer compatibility from connector or thread fit.

Learn Next

  • Antenna Selection: A Mission-First Decision Guide
  • Engineering Design Tradeoffs in Portable HF Antennas
  • Antenna Measurement Reference Planes
  • Understanding Common-Mode Current

Source note: Independently synthesized with reference to The ARRL Handbook for Radio Communications, 99th edition (2022), and The ARRL Antenna Book for Radio Communications, 24th edition (2019). Verify changing regulations, services, software, specifications, availability, and safety requirements against current primary sources.