Aug 18, 2026 – “Impact of the Sun on Radio Propagation Part 1”

Good evening. This is W1UT and tonight’s topic will be about the “Impact of the Sun on Radio Propagation Part 1.”

This can be a rather involved subject and may require a couple of weeks to even cover briefly. Much more on this topic can be found in various books or a study on the internet.

First let me say a little about radio waves which is a part of the electromagnetic spectrum. Radio waves tend to travel in a straight line from one pint to another, but can also be refracted by our ionosphere. Some low radio frequencies such as the AM broadcast band can actually follow the curvature of the earth at night but poorly during the daylight hours. Have you ever tuned across the AM broadcast band at night and found stations from halfway across the country coming in perfectly well? Whereas FM broadcast radio stations around 100 Mhz do not propagate much beyond the horizon and are greatly affected in mountainous areas. How can HF amateur radio stations below 30 Mhz send and receive a signal from many parts of the world? Answers to these questions are related to the sun and the earth’s ionosphere.

As is the case with life on the earth, our sun provides warmth and the energy needed for life to exist. The energy radiated by the sun also impacts radio signals on earth. The sun rotates on its axis and our planet revolves around the sun and since our planet is tilted on its axis propagation is also affected by daylight vs night and seasonal changes. The sun also tends to follow an ~11 year solar cycle during which time the sun is more active and then more quiet. We are currently in what is called solar cycle 25 which means astronomers have been watching the sun for some 275 years documenting these solar cycles not counting any observations made by much older civilizations.

Each cycle has a period of maximum activity, meaning more sun spots and solar storms and a period of minimal activity. During maximum activity the sun generates more energy in the form of solar flares and solar wind all of which can cause radio blackouts along with polar auroras. This energenic activity can also cause low band VHF over the horizon and enhanced shortwave and propagation across multiple HF bands. I personally have made contacts around the world during these times. Several decades ago during a strong solar maximum and when I had a large directional antenna on a tower I could actually copy my own transmitted signal as an echo going completely around the world. Any of you older hams may have had a similar experience. This means there must have have been multiple hops between the ionosphere and the earth where my signal made it all the way around our planet. This works best under these conditions when using morse code. I have also worked Australia from Maryland years ago on 10m using an old 23 channel CB I had converted using AM for 2 hours during solar cycle 21. The antenna was a simple dipole of 300 ohm TV feedline thumb-tacked to my apartment sheet rock wall. Absolutely amazing!

Our sun is what drives all weather on the earth. The sun’s X-ray and UV radiation hitting the upper layers of our planet’s atmosphere causes our atmosphere to ionize and is what makes it possible for long distance HF and Low band VHF propagation, but it can also attenuate radio signals to the point of a blackout. A powerful enough solar storm could wipe out all man-made satellites and destroy much of our land based solid state equipment – even your radio - and send us back to the pre-industrial era. The most massive solar storm ever recorded happened in September of 1859. It is called the Carrington Event. If this happened again in our modern era the impact to society would be immeasurable.

Next week I will provide more information on how frequency dependent radio waves can refract off the ionosphere. The 2m and 70cm bands are rarely impacted by the ionosphere so this discussion will be more directed to those using or planning to use the HF bands and the 6m band. In many ways using the principles of propagation and understanding how seasonal changes, solar weather and even day or night time can impact your success in making contacts from here in Utah to Europe, Japan, South America or just the east coast is possible. In a way its like fishing.