John Gendron, NJ4Z
One of the most often ask question of me by new Hams is; “How do I know
what kind of propagation there is going to be today?”
That is a loaded question, because we know as experienced Hams, propagation and
the conditions for propagation can change rapidly, and vary in different areas. This is
especially true for 6 and 10 meters. My go to answer is “It depends, what band,
what time of day and between what two points.” I get a lot of befuddled looks, and
some looks of frustration. What most new Hams do not realize understanding
propagation is part science and part art. The science part is somewhat easy, there is plenty of data to help
sort that out. The art, well that takes experience to learn what time of day to look for different points on the
globe on differing bands and what season is better for what bands. No one knows for sure what propagation
will look like on any given day, but there is data that can be used to make an educated guess, and experience
will give you more accuracy in that guess.
The Sun is the engine that drives electromagnetic wave propagation in the earths Ionosphere. The sun’s
radiation energizes the earth’s ionosphere, creating positive ions and free electrons. When the ionization is
high, the abundance of free electrons allows for the refraction of radio waves.
As we learned in our license study courses the Ionosphere is made up of multiple layers of ionization. The
layers all respond differently to solar radiation and to radio frequency waves. These layers are also not
uniform in density of ionization. Think of it more like cotton candy that a solid surface, in certain areas the
density will be higher and allow for higher maximum usable frequencies (MUF). With higher MUF, there is
better propagation on more bands. Again, this tends to be a local factor and not homogenous throughout
During the climb to the maximum (solar max) of the 11-year solar cycle, solar output and magnetism vary
greatly. The variations in the solar magnetic field as it is switching polarity, creates areas of concentrated
magnetic flux. These areas will appear darker than the surrounding area, because the magnetic flux inhibits
convection and produces lower surface temperature. We call them Sunspots. The bright areas surrounding
sunspots release extra radiation. This extra radiation is both good news and bad news for Hams. The extra
radiation produces more ionization in the Ionosphere and better propagation, but the increase in activity
from sunspots and coronal holes releases more high energy particles from the sun. These high energy
particle streams depending on speed and density can create disturbances in the Magnetosphere. The
Magnetosphere is the magnetic field that surrounds the Earth created by Earth’s spinning molten core. This
magnetic field protects Earth from the intense solar radiation and gamma rays from deep space phenomena.
As a dense and/or fast stream of particles interacts with the Earth’s Magnetosphere field it can create
geomagnetic storms (Solar Storms). Solar storms create high currents and increase plasma in the
Magnetosphere. They can also create disturbances in the Ionosphere, creating Ionospheric storms. These
storms greatly degrade HF propagation and can create radio blackouts.
For a better understanding of the mechanisms that drive Radio Wave propagation please visit: www.radioelectronics.com/info/propagation/radio_prop_list.html
For our purposes here we are going to talk about the four data points (the science part) that will help you
most in beginning to understand how solar weather effects radio propagation. These data points are Sunspot
number (SN), Solar Flux Index (SFI), and the two geomagnetic activity data points the K index and the A index.
These 4 data points can be found on many websites on the internet. Most of the Amateur Radio Spotting
clusters will have solar data listed. The site www.QRZ.com provides the solar-terrestrial data on its front
page. www.hamqsl.com/solar.html is a wealth of information and has a Solar-Terrestrial data widget you can
add to your own webpage or QRZ page. The National Oceanic and Atmospheric Agency (NOAA), has a
specific area of their website dedicated to Solar / Space Weather and how it effects radio propagation,
https://www.swpc.noaa.gov. NOAA also broadcasts Solar Weather Data at 18 minutes after the hour on
WWV (5MHz, 10MHz, 15Mhz and 20MHz). Additionally, the sites www.speaceweathernews.com and
www.spacewaetherwoman.com are incredible sources to better understand solar weather.
Having the solar-terrestrial data, we can now use that data to make informed predictions of propagation. The sunspot (SN) number is the number of sunspots currently on the solar surface. The higher values in sunspots should lead to higher radiation output from the sun and in turn higher levels of ionization of the Ionosphere. These higher-levels ionization leads to more free electrons, creating favorable conditions for radio propagation. With an increasing number of sunspots, the Solar Flux Index (SFI) is the radio flux or noise the sun emits at 2800MHz (10.7cm) increases. The Solar flux is closely related to the amount of ionization and electron concentration in the upper Ionosphere (F2) layer. Thus, it is a good indication of conditions for long distance radio communications. The solar flux index can range from 50 to 300. Low values of SFI will translate to low MUF and poor conditions for overall propagation. As the value of the SFI increases the MUF will rise, as will conditions for propagation. Although, high SFI values can translate into noisy bands and D-layer absorption. It is important to note that the ionization takes time, usually a few days of high SFI values will translate to good propagation.
The two geomagnetic indices the K(p) and A(p) index are related. The K index is measured at observatories all
over the globe. Each of the observatories has a calibrated magnetometer. A measurement taken at 3-
hour intervals and the maximum deviations from geomagnetic quiet are recorded. A mathematic formula
is applied to the deviation number and the K index for that location is determined. The K index can range from
0 to 9. As you increase in value the more severe the disturbance to the Magnetosphere and Ionosphere.
The Kp (K planetary) is the average of all the 3-hour K indices taken globally. The A index is derived from the K
index. Table 1 shows the conversion of the local K index to the A index. The Index can vary from 0 to 400,
and as with the K index the greater the value the greater the disturbance. This conversion allows for a daily
average of the condition of the Magnetosphere and Ionosphere. The A index is the average of the converted
8 K index values over a 24-hour period. Being that the A index is for a 24 hours period it will be more indicative of overall condition predictions vs the 3-hour Kp index. Keep in mind that a geomagnetic storm,
disturbance in the Magnetosphere and a ionospheric storm, disturbance in the Ionosphere are very different.
Ionospheric storms will be more adverse on radio communications than a geomagnetic storm.
Using these figures to interpret overall propagation is quite simple the higher sunspot numbers will normally
lead to higher solar flux. The increase in solar flux over a few days period will produce greater ionization of
the upper levels of the Ionosphere (E, and F layers) producing favorable conditions for the refraction of radio
waves. This will allow for a higher maximum usable frequency. SFI numbers above 150 will vastly improve
propagation and numbers above 200 will provide peak propagation. Lower numbers in the K and A indices
will provide a more stable communication paths for RF propagation. As the K and A indices values increase
the greater the instability will be and suppress the MUF, a solar or ionospheric storm at a Kp 7+ or Ap 81+
would cause an HF communications blackout.
There several websites and software programs that are available to provide HF propagation from point to
point, at differing times of day and on differing bands. These sites and programs are very useful for DX
chasers who want to understand when and on what bands to work DX stations and the possibility of success
for those communications. One of the best tools for HF propagation prediction is VOACAP (Voice of America
Coverage Analysis Program) is free for download, easy to use and very accurate. https://www.voacap.com.
Give it a try.