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Chameleon Knowledge Base · The Complete Online HF Antenna Handbook

What Is the Solar Flux Index (SFI)?

Learn what the Solar Flux Index (SFI) is and how it helps predict HF amateur radio propagation.

Propagation & DX Space Weather & Propagation Forecasting Reviewed 2026-07-14
Short Answer: Learn what the Solar Flux Index (SFI) is and how it helps predict HF amateur radio propagation.

Explanation

Overview The Solar Flux Index (SFI) is a measurement of radio noise naturally emitted by the Sun at a wavelength of 10.7 centimeters (2800 MHz). Amateur radio operators use the SFI as one of the primary indicators of expected HF propagation conditions. Higher SFI values generally indicate greater ionization in the ionosphere, which can improve long-distance communication on many HF bands. Typical SFI Values SFI General HF Conditions Below 70 Poor to Fair 70–100 Fair 100–150 Good 150–200 Very Good Above 200 Excellent (depending on geomagnetic conditions) Important Notes Higher SFI generally favors higher HF bands. SFI alone does not determine propagation quality. Geomagnetic conditions remain equally important. Monitoring SFI Space weather websites. Propagation forecast services. Amateur radio propagation tools. Applied to Chameleon Products Operators using broadband Chameleon HF antenna systems can take advantage of favorable SFI conditions by selecting the amateur band that offers the best propagation at a given time. Related Articles What Is the K-Index? What Is the A-Index? What Is the MUF? How Do Solar Flares Affect Amateur Radio? Related Products All Chameleon HF Antenna Syste

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.