↑ A Quick Guide to HF Propagation Using Solar Indices

This page is part of the project "Understanding HF Propagation."
by Doron Tal, 4X4XM

Understanding the influence of solar indices on HF radio propagation can help radio operators, amateurs, and professionals to find the best frequency to make a reliable conact.

  1. What Are Solar Indices?
  2. The solar indices provide insight into how the sun's radiation affects Earth's ionosphere, which, in turn, influences HF radio wave propagation.

    The most important solar indices are:

    1. SSN - Sunspot Number: Measures sunspots.
    2. SFI - Solar Flux Index: Tracks solar radio emissions at a wavelength of 10.7 cm (2800 MHz).
      Table 1 illustrates the correlation between these indices and HF propagation conditions.
    3. X-ray Flares: Table 2 categorizes solar flares which are potential for fadeouts of HF communication on Earth.
     
    ConditionsBADLowAverageGoodBetterBest
    MUF (MHz) <12<15> 21> 24> 28> 50
    SSN0255075100125150175200250
    SFI (sfu)6783102124148172196219240273
    Table 1: The solar indices: SSN, SFI
    Reference to the conversion between SSN and SFI.

    High SSN and SFI are good for daytime propagation on all HF bands. Best for higher frequency bands (20 meters and up). Provides better long-distance communication opportunities.

     
    Solar  Flare  Class A  B  C MX
    HF Propagation condBestAveragePoorBAD
    Radio Blackout ScaleR0R1R2R3R4R5
    Table 2: Solar flares may cause Radio blackout

    Higher class of X-ray flares indicate potential for fadeouts of HF communication on Earth, in particular below 20 meters band.

     
  3. What Are Geomagnetic Indices?
  4. The A and K indices refer to geomagnetic activity, characterized by disturbances in Earth’s magnetic field caused by solar wind and other solar phenomena. These disturbances can range from minor fluctuations to major geomagnetic storms.

    1. A-Index represents geomagnetic activity, measuring disturbances in Earth's magnetic field caused by solar events like solar flares or coronal mass ejections (CMEs).
    2.  
    3. K-Index is another measure of geomagnetic activity, calculated from data recorded every 3 hours.
    4. These indices track geomagnetic fluctuations influenced by solar activity, but they do not directly measure the solar phenomena themselves.

      HF Propagation conditionsBestAveragePoorBAD
      Geomagnetic activity index (linear)A 0  4  7  15 27 48 80132207400
      Geomagnetic activity index (log-scale)K0123456789
      Table 3: The geomagnetic indices: A, K
      Reference to the conversion between A and K.

      Low A-Index (0-7) and K-Index (0-2) indicate good geomagnetic conditions for HF communication and lower signal absorption and disruption.


     
  5. Tools for Monitoring Propagation Indices:
    Stay updated on solar indices using various online tools and resources:
  6. Space Weather Prediction Center (SWPC) provides real-time solar and geomagnetic data, including SFI, A-Index, and K-Index.

    DX Cluster Websites: Many DX cluster websites display current propagation conditions and include solar indices.

    Propagation Prediction Software: Use dedicated software like VOACAP (Voice of America Coverage Analysis Program) for detailed HF propagation predictions based on solar indices.


     
  7. Adjust Your Operating Strategy:
      Based on the propagation indices, adapt your operating strategy:

    1. Select Appropriate Bands: Choose HF bands that match the current SFI and geomagnetic conditions.
    2. Optimal Times: Plan your HF operations during daylight hours when the SFI is higher.
    3. Monitor for Changes: Keep an eye on the indices, as they can change rapidly due to solar activity.
    4. Experiment: Try different bands and times to maximize your chances of successful HF communication.

  8.  
  9. Continuous Learning:
  10. HF propagation is a dynamic field, and solar indices can vary daily. Stay engaged with the amateur radio community and learn from experienced operators to refine your understanding and skills in using solar indices for HF propagation.

    By grasping the basics of solar indices and their impact on HF propagation, you can enhance your ability to make effective long-distance HF contacts and enjoy the exciting world of amateur radio and shortwave communication.

Read more about Solar Indices & Ham Radio Propagation


The project "Understanding HF Propagation" provides a detailed overview and tutorials on HF propagation.

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