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Chameleon Knowledge Base · The Complete Online HF Antenna Handbook

What Is Lowest Usable Frequency (LUF)?

Learn what Lowest Usable Frequency (LUF) is and how it affects reliable HF communications.

Getting Started HF Fundamentals Reviewed 2026-07-14
Short Answer: Learn what Lowest Usable Frequency (LUF) is and how it affects reliable HF communications.

Explanation

Overview Lowest Usable Frequency (LUF) is the lowest radio frequency capable of providing reliable communication over a specific path at a particular time. Below the LUF, excessive ionospheric absorption causes the received signal to become too weak for dependable communication. Factors Affecting LUF D-layer absorption. Solar radiation. Time of day. Season. Transmitter power. Receiver sensitivity. Daily Changes LUF is generally highest during daylight hours because of increased D-layer absorption and lowest during nighttime when the D Layer largely disappears. Relationship to MUF Reliable HF communication exists only between the LUF and the MUF. Applied to Chameleon Products Knowing the LUF helps operators determine when lower HF bands such as 160, 80, and 40 meters are most likely to provide dependable communications. Related Articles What Is MUF? What Is the D Layer? What Is Gray Line Propagation? What Is Skywave Propagation? Related Products CHA LEFS 8010 CHA MPAS 2.0

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.