Thanks to the ionospheric propagation of radio waves, ham radio operators can rely on HF ionospheric radio signal propagation to communicate with fellow hams located way beyond the horizon.
The ionized layers of the ionosphere make HF radio wave propagation possible much beyond line-of-sight distances. These layers can be viewed as our "Seven League Boots" which, by leaps and rebounds, give our ham radio HF signals the ability to travel great distances!
We'll explain why the "F" layer is the most useful ionized layer for DX in a moment. Best of all, sunspot cycles improve RF propagation because they revitalize our ionosphere. The good news is that the 25th solar cycle has begun! Radio operators around the world are looking forward to the increased activity.
This 11 year sunspot cycle graph illustrates the highs and lows of ionospheric propagation of radio waves.
Ionospheric Propagation of Radio Waves in Action
The simplified drawing above shows how the radio wave "C" is refracted by the ionized layer "F", back to the Earth's surface, bouncing off the Earth's surface at a great distance from its origin, and going back up from the "F" as "C1". the layer is broken again and reflected from the earth as “C2” and so on.
The radio signals “A” and “B”, which arrive at the ionized “F” layer at too steep an angle, simply pass through it and are lost in space.
The RF signals gradually lose energy after each refraction through the "F" layer and after each rebound from the Earth's surface...until they are no longer detectable. But by then it will have traveled thousands of kilometers and been heard by countless radio operators and shortwave listeners!
This is the magic of RF radio signal propagation in the ionosphere.
Sunspot Cycle 25
Scott W. McIntosh, Deputy Director of the National Center for Atmospheric Research in Boulder, et al., conclude, in a recent research paper published on October 13, 2020, that the "sunspot cycle 25could be among the strongest sunspot cycles ever observed".
Ionization of the upper reaches of Earth's atmosphere occurs when ultraviolet radiation from the sun collides with hydrogen and helium molecules that are few and far between up there. These collisions detach electrons from the gaseous molecules.
As a result, positive hydrogen and helium ions are generated, and negatively charged free electrons are liberated from their nucleus. These regroup into ionized layers above the earth.
However, ionized layers only form when the sun is "active", which it is for about 9-10 years, every eleven years or so. It's commonly called the 11-year sunspot cycle.
We can see the progression of the last few sunspot cycles in the graph shown earlier. You can obtain more information on the 11-year cycle of sunspots here.
A pictorial diagram of the ionosphere layers responsible for ionospheric propagation of radio waves.
The Ionized Layers and Their Respective Role in HF Radio Wave Propagation
Ionized Layer 'D'
During the day, the ionized layer 'Mostly hinders ionospheric propagation of radio waves. It is the ionized layer closest to the earth's surface. It is located between 60 km and 100 km (37-62 miles) above the earth. In the daytime, it forms under the sun's intense UV radiation and constitutes a barrier preventing radio signals in the 40-meter, 80-meter, and 160-meter bands from getting far and from being heard in the intense atmospheric noise. Meanwhile, signals 10 MHz and above can get through to reach the ionized layers above and make their way beyond the horizon.
Ionized Layer 'E'
The 'E' layer lies between 90 km and 150 km (56-93 miles) above the earth but its most useful portion is located between 95 km and 120 km (59-75 miles) of altitude.
During daytime hours, in theory, layer 'E' could refract 5-20 MHz signals and help them along their way.
However, in reality, the 'D' layer (below) absorbs much of the energy of signals at these frequencies. Only signals in the 7-14 MHz range - transmitted near vertically - will be able to punch through the 'D' layer with enough remaining energy to reach the 'E' layer and be refracted along to reach as far as 1200 km (750 miles) at times.
The periods just before dawn and right after dusk are best to make use of the 'E' layer. At night, the 'E' layer disappears almost completely, while remaining somewhat useful to the propagation of signals in the 160-meter band.
The "Sporadic E" Layer
Sometimes, dense ionized clouds will form suddenly in the 'E' layer and disappear just as suddenly, minutes, rarely hours later. Sporadic 'E' propagation (Es) is useful at frequencies above 28 MHz, in the VHF range, rarely below. We cover their usefulness in extending the reach of VHF signals beyond the horizon on another page of this website. Both 'E' and 'Es' propagation contribute to 50 MHz activity.
Ionized Layer 'F1'
During daytime hours, in summer, this layer will often be useful to the propagation of HF radio signals of the 30-meter and 20-meter bands. Its role in the propagation of HF signals is rather negligible.
Ionized Layer 'F2'
The 'F2' layer forms during daytime hours between 200 km and 400 km (125-250 miles) above the earth. It is higher in altitude in the summer than it is in the winter.
The 'F2' layer plays a major role in the ionospheric propagation of radio waves of the HF spectrum.
It is usually around all year round. At night, layers 'F1' and 'F2' merge into one 'F' layer, a little lower than the daytime 'F2' was located.
The 'F2' ionized layer is present during the major part of a solar cycle.
However, it will sometimes disappear completely for days on end during a deep solar cycle minimum! The 'F2' layer will reach its highest density at the peak of a solar sunspot cycle.
It will then refract toward earth radio signals ranging from 7 MHz to 30 MHz and enable them to reach distances as far as 4000 km from their origin, rebound off the earth to rise again to the 'F2' layer... and repeatedly do so… sometimes to travel right around the earth and come back from behind their point of origin!
During the better nine years or so of a solar cycle, QRP operators (5 watts of radiated power or less), using simple dipoles, can make DX contacts as far and as often as the QRO operators (using up to 200 to 300 times more power) using a multi-element directional antenna!
During such wonderful periods, every radio operator has an equal chance under the sun to make DX contacts.
Ionospheric Propagation of Radio Waves is a Complex Topic
The information I have presented to you in this article is a very brief summary of what could be said about HF ionospheric radio signal propagation. We have only scratched the surface!
Countless scientific publications have covered many aspects of the subject since the discovery of the ionosphere's existence and, later, its role in the propagation of HF radio signals.
Research is ongoing, involving scientists and ham radio operators alike.
For more on our sun's behavior, visit the Solar and Heliospheric Observatory (SOHO).