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Chameleon Knowledge Base · The Complete Online HF Antenna Handbook

What Is the F1 Layer?

Learn what the F1 Layer is and how it contributes to daytime HF propagation.

Getting Started HF Fundamentals Reviewed 2026-07-14
Short Answer: Learn what the F1 Layer is and how it contributes to daytime HF propagation.

Explanation

Overview The F1 Layer is the lower portion of the F region of the ionosphere. It normally forms during daylight hours and merges with the F2 Layer after sunset. Typical Altitude The F1 Layer generally forms between approximately 90 and 150 miles (150 to 240 km) above the Earth's surface. Characteristics Exists primarily during daylight. Merges with the F2 Layer at night. Supports medium-distance HF propagation. Importance Although less influential than the F2 Layer, the F1 Layer contributes to daytime HF propagation and affects usable operating frequencies. Applied to Chameleon Products Understanding the behavior of the F1 Layer helps operators select appropriate HF bands during daytime portable and fixed-station operation. Related Articles What Is the F2 Layer? What Is MUF? What Is Critical Frequency? 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.