SFI Range Condition
0 – 69 Poor
70 – 99 Fair
100 – 300 Good

Solar Flux Index

What good is it and why do I care?

HF Propagation Impact

The Solar Flux Index (SFI) is a critical measurement that directly affects HF band propagation from 0-30MHz. The SFI measures solar radio emissions at 2800MHz, which correlates strongly with ultraviolet radiation that ionizes the Earth’s upper atmosphere, creating the ionospheric layers we depend on for HF communications.

Why SFI Matters for HF Operations: The ionosphere’s electron density determines how well it reflects HF signals back to Earth. Higher SFI values indicate increased solar activity, producing denser ionization and better propagation conditions across most HF bands.

Operating Conditions by SFI Range:

SFI 0-69 (Poor Conditions):

  • Expect challenging propagation with weak signals and limited DX opportunities
  • Higher HF bands (20m, 17m, 15m, 12m, 10m) may be completely closed during daylight
  • Rely primarily on 80m and 40m for reliable communications
  • NVIS propagation on 80m/40m becomes your primary tool for regional contacts
  • Contest operations severely hampered, especially on higher bands

SFI 70-99 (Fair Conditions):

  • Moderate propagation with intermittent openings on mid-bands
  • 20m typically reliable during daylight hours with moderate DX potential
  • 17m and 15m may show brief openings, particularly around solar noon
  • 40m remains solid for regional and some DX work
  • Digital modes become more effective on marginal bands

SFI 100-300 (Good Conditions):

  • Excellent propagation across all HF bands
  • 20m, 17m, 15m, 12m open consistently with strong DX signals
  • 10m may open for sporadic E and F2 propagation during solar maximum periods
  • Lower noise floors and stronger signal strengths
  • Ideal conditions for contest operations, DXpeditions, and weak-signal work
  • Enhanced propagation paths support both short and long-distance communications

Monitor SFI daily through space weather services to optimize your operating schedule and band selection for maximum effectiveness.

Technical SFI Measurement and Significance

The Solar Flux Index represents the measurement of solar radio emissions at 2800MHz (10.7cm wavelength), expressed in solar flux units (SFU) where 1 SFU = 10⁻²² watts per square meter per hertz. This measurement serves as a proxy for solar extreme ultraviolet (EUV) and X-ray emissions that drive ionospheric ionization processes.

Measurement Methodology: SFI measurements are conducted using calibrated radio telescopes, with primary data collection occurring at the Dominion Radio Astrophysical Observatory (DRAO) in Penticton, British Columbia, Canada. The observatory employs a 1.8-meter parabolic antenna system specifically designed for solar flux monitoring. Measurements are taken at local solar noon to ensure consistent solar elevation angles and minimize atmospheric absorption effects.

Scientific Significance: The 2800MHz frequency was selected because it originates from the solar chromosphere and lower corona, regions directly influenced by magnetic field activity and thermal processes that also drive EUV production. The correlation coefficient between SFI and EUV flux measurements typically exceeds 0.85, making SFI an excellent proxy measurement for ionospheric research.

Ionospheric Response Characteristics:

  • SFI 0-69: Minimal F2 layer ionization, critical frequencies below 4MHz, limited F2 propagation support
  • SFI 70-99: Moderate F2 layer development, critical frequencies 4-8MHz, supporting propagation up to 20MHz
  • SFI 100-300: Enhanced F2 layer ionization, critical frequencies exceeding 8MHz, supporting propagation across the entire HF spectrum

The measurement provides essential input data for ionospheric modeling systems including IRI (International Reference Ionosphere) and empirical propagation prediction models such as VOACAP and PROPLAB-PRO. Temporal variations in SFI correlate with the 11-year solar cycle, 27-day solar rotation period, and short-term solar flare activity, making it indispensable for space weather monitoring and prediction systems.

In a Nutshell for the Radio Ham:

What is SFI? The Solar Flux Index (SFI) is a number that tells you how active the sun is. Think of it as a “sun strength meter” that directly affects how well your HF radio signals travel.

The Simple Rule:

  • Higher SFI = Better HF conditions
  • Lower SFI = Worse HF conditions

What the Numbers Mean:

SFI 0-69 = Poor

  • HF bands are mostly dead
  • Stick to 80m and 40m
  • Forget about 20m, 15m, 10m – they won’t work well

SFI 70-99 = Fair

  • 20m works during the day
  • 40m is reliable
  • 15m might have some activity

SFI 100-300 = Good

  • All HF bands working great
  • 20m, 15m, 10m are alive with DX
  • Perfect time for contests and working distant stations

Bottom Line: Check the SFI before you operate. If it’s below 70, focus on the lower bands (80m/40m). If it’s above 100, fire up 20m, 15m, and 10m for some great DX hunting. The sun controls our hobby – SFI tells you what the sun is doing.