HomeHFAn Amateur Radio Operator’s Guide To Understanding HF Propagation

There are many websites offering chtharts and graphs related to HF propagation conditions http://www.swpc.noaa.gov/. These display measurements of solar activity and their impact on Earth’s magnetosphere, ionosphere and atmosphere. On NOAA’s web site, there is a tremendous amount of information about the sun. Learning what some of these measurements are and how to interpret them is crucial to understanding how they impact RF propagation

The Solar Cycle: Our Sun goes through 11 year cycle during which its activities or “weather” increase and decrease progressively. During these cycles, the sun’s magnetic poles literally flip north to south and south to north. The solar cycle has a direct influence on RF wave propagation. As I put this article together, we are at the part of the cycle referred to as “solar maximum”. The sun’s magnetic poles have just “swapped” places. This is normally an extremely active point in the solar cycle, but this time, the sun is very quiet. We just never know what to expect from one solar cycle to the next.
The number of spots on our Sun precludes the number of solar flares that may occur, which in return will generate all sorts of measurements that are very useful in understanding and most importantly, predicting radio signals propagation.
A propagation condition report is information relative to the solar weather and it will affect radio signal propagation conditions back here on earth. Most reports use terminological abbreviations such as SFI, SN, N, K, A. What do they mean? Let’s review a few of them and how they can help you visualize radio signal propagation weather:

SFI index: Solar Flux Index: Is a gauge of the level of solar particles and magnetic fields reaching our atmosphere and their impact . In other words, this value tells us about solar winds reaching our planet and their influence on HF propagation conditions. For this measurement, the higher the number, the better HF propagation will be. The index value is an indication of propagation on bands between 10 meter and 20 meter (IE: 10m,12m,15m,17m,20m). The scale is a number between 30 and 300, and can be interpreted as:

  • < 70 = Propagation poor.
  • 80-90 = Propagation somewhat low
  • 90-100 = Propagation average
  • 100-150 = Propagation good
  • >150 = Propagation ideal

 

High SFI values have almost no influence on 30m,40m,80m and 160m bands. SFI value over 150 indicates ideal HF propagation conditions and people with small HF installations can begin exploiting these conditions. At these high SFI values, you might consider stopping what you are doing and take advantage of these conditions while they last because they are far and few between.

SN: Sunspot Numbers: This value is the visible number of spots on the Sun’s surface. Traditionally, the higher the number, the better the ionization of our atmosphere which will help create great HF propagation conditions. The range of SN can be between 0 and up to 250, sometimes more.
We need not go into the types of sun spots or what induces them to flair or launch unimaginably huge hunks of plasma, hundreds of times the size of earth into space. Lets just say that some sun spots interact with other solar prominences and erupt. These eruptions have basically two outcomes. Some just send out a burst of high energy particles others involve overly twisted and extended lines of magnetic flux that eventually snap free of the sun and send the ejecta hurling into space. Some of this ejecta occasionally finds it’s way to earth and slams into the magnetosphere. When this happens all kinds of things happen. The high energy particles can damage satellites, power grids and electronic devises. These can and do cause total radio blackouts.
High SN numbers are indicative of the potential for an increase in solar flaring, but before they erupt into solar flares, the sun spots can create conditions that are conducive to HF propagation. If sun spots produce flaring, the resulting surge of charged particles interacting with the magnetosphere will usually inhibit HF propagation, even create total radio blackouts on all bands. The sun is rotating. This is why radio propagation conditions can be excellent for a few days, then go way down until ether more sun spots form or the existing spots rotate toward us again, which is between 18-25 days later.
When SN numbers over 100, you can expect good propagation conditions, when and if these spots are facing us.
During solar minimums (Low or no sun spots), greatly reduced propagation conditions going on for years at a time, and this until the next solar cycle. When these conditions occur, Amateur Radio operators often tun down to using lower frequencies (IE: 30m,40m,60m,80 and 160m), and watch for events that will to create temporary propagation conditions, such as sun rise and sun sets. I will admit even when conditions were poor, I made my furthest contacts when the Sun was rising or when it was setting.
SN numbers can be interpreted as:

  • <50 = Propagation conditions potentially very bad
  • 50-75 = Propagation conditions attenuated
  • 75-100 = Propagation conditions might be good
  • 100-150 = Propagation conditions should be idea
  • >150 = Propagation conditions possibly exceptional

 

Important: Solar flux (SFI) and Sun spots (SN) numbers need to be high AND sustained to make a major impact on propagation. In other words, a single day high numbers will have very little impact, but on the opposite end of things, high numbers sustained for more than 5-7 days will have a positive impact on propagation on the HF bands. The longer high numbers are sustained, better the propagation will become.

The A Index: It’s simply an index of geomagnetic activity derived from a scaled average of the previous 24 hours K-index readings. Your should use this as a reference for general conditions on the bands. Lower A index means better conditions for propagation. This scale goes between 0 and 400, but typically never above 100. This value should be interpreted as:

  • Between 1 and 5 = Best conditions on 10,12,15,17,20 meter bands.
  • Between 6 and 9 = Average conditions on 10,12,15,17,20 meter bands.
  • From 10 and above = Very Bad conditions on 10,12,15,17,20 meter bands.

 

The Ap-index value can be interpreted as:

  • Between 1 and 5 = Best conditions expected on 30,40,80,160 meter bands
  • Between 6 and 9 = Average conditions expected on 30,40,80,160 meter bands.
  • From 10 and above = Bad conditions expected on 30,40,80,160 meter bands.

 

The K-Index: (or Boulder K)is a gauge of geomagnetic activity relative to an assumed quiet-day. Falling numbers mean improving conditions and better propagation particularly in northern latitudes and areas where aurora activity can occur. The scale is between 0 and 9. You never want to see value above 8 because this indicates our planet going thru a solar storm of great intensity. This value can be interpreted as:

  • From 0 to 1 = Best conditions for 10,12,15,17,20 meter bands.
  • From 2 to 3 = Good conditions for 10,12,15,17,20 meter bands
  • From 4 to 5 = average conditions for 10,12,15,17,20 meter bands.
  • From 5 to 9 = Very bad conditions for 10,12,15,17,20 meter bands.

 

The Kp-index value can be interpreted as:

 

  • Between 0 and 1 = Best conditions expected on 30,40,80,160 meter bands.
  • Between 2 and 4 = Good conditions expected on 30,40,80,160 meter bands.
  • Between 5 and 9 = Bad conditions expected on 30,40,80,160 meter bands.

 

There are many other measurements which are somewhat technical but worth looking into if you really want to learn more about solar activities.

Here are a few more definitions:

  • X-Ray: NOAA reported value from A0.0 to X9.9. Intensity of hard x-rays hitting the earth’s ionosphere. Impacts primarily the D-layer (HF absorption). The letter indicates the order of magnitude of the X-rays (A, B, C, M and X), where A is the lowest. The number further defines the level of radiation. Updated eight times daily.
  • 304A: NOAA reported value from 0 to unknown. Relative strength of total solar radiation at a wavelength of 304 angstroms (or 30.4 nm), emitted primarily by ionized helium in the sun’s photosphere. Two measurements are available for this parameter, one measured by the Solar Dynamics Observatory, using the EVE instrument, and the other, using data from the SOHO satellite, using its SEM instrument. Responsible for about half of all the ionization of the F layer in the ionosphere. 304A does loosely correlate to SFI. This measurement report is updated hourly.
  • Ptn Flx: NOAA reported value from 0 to unknown. Density of charged protons in the solar wind. The higher the numbers, the more the impact the ionosphere. Primarily impacts the E-Layer of the ionosphere. Updated hourly.
  • Elc Flx: NOAA reported value from 0 to unknown. This is a report is a measurement of the density of charged electrons in the solar wind. The higher the numbers (>1000), the more the impact the ionosphere. The primary impact is to the E-Layer of the ionosphere. This measurement report is updated hourly.
  • N: NOAA reported value from 0 to 5 about auroral impact. When <2.0, high confidence in Aurora measurement. When >2, low confidence. This measurement report is updated hourly.

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