Beyond their love of radio, amateur radio operators and shortwave radio broadcasters have one thing in common: They rely on the ionosphere to refract or bend their signals back to Earth, so that they can travel beyond line-of-sight distances.

In turn, the ionosphere’s ability to refract radio signals depends on its level of ionization or charge. The more ionized the ionosphere is, the more likely it is to bend signals back to the ground rather than let them pass through.

Here’s where the sun comes in. The number of sunspots on the solar surface rises and falls over an 11-year period, during what is known as a solar cycle. The more sunspots, the more solar radiation comes to Earth. 

This radiation ionizes the ionosphere, improving the propagation of signals for transmitters and receivers alike. 

Right now is a good time for amateur operators and shortwave broadcasters (and listeners), because the world is just coming off the peak, or solar maximum, of Solar Cycle 25.

“Solar Cycle 25 refers to our current solar cycle, which began in December of 2019 and peaked in October 2024,” said Rob Steenburgh, the science and operations officer (SOO) at NOAA’s Space Weather Prediction Center (SWPC) in Boulder, Colo.

SWPC is the official source in the United States for space weather forecasts, watches, warnings and alerts. “The solar cycle refers to the naturally occurring maximum and minimum in the number of sunspots,” he told Radio World. 

A complete cycle lasts, on average, 11 years. The numbering began with Cycle 1 in 1755.

“The number of sunspots is an indicator of the amount of solar activity and emissions in the X-ray and Extreme Ultraviolet (EUV) wavelengths,” Steenburgh said. 

“Energy at these wavelengths drives the formation of the ionosphere, which is the critical atmospheric component for radio communication in the high frequency (HF; 3–30 MHz) bands. In addition to sunspots, radio users often follow the 10.7 cm wavelength solar flux, F10.7, which is also a proxy for solar activity.”

Sunspots are good

Gilles Letourneau is a respected radio enthusiast and the producer/host of the OfficialSWLchannel on YouTube. He is a big fan of more sunspots. 

In layperson’s terms, “That increased ionization changes the structure of the ionosphere making it thicker and denser and capable of bouncing shortwave signals of higher frequencies with more efficiency,” Letourneau said. “Without that extra energy, higher frequency signals simply go through and are lost into space.”

That said, not every band benefits equally from increased ionospheric ionization. 

“The largest influence of solar maximum is seen at bands above 20 meters (14 MHz), although you can still see some influence on 20m,” said Steenburgh. “Radio waves are refracted/reflected when their frequency is at/below the plasma frequency of the ionosphere. The plasma frequency is directly related to the electron density, and the electron density increases with the increased ionization occurring during solar maximum.”

Letourneau agreed: “The higher the sun’s output of energy in a solar cycle the higher the frequencies are capable of bouncing off the ionosphere. Thus higher frequencies are the first to show improvement in propagation, mostly above 20 MHz like the 15, 12 and 10 meter band and in extreme cases even the 6 meter amateur band (50 MHz).” He added that the increased energy “will improve the 19, 16 and 13 meter international broadcast bands also, making those more reliable.”

Shortwave boon

SW broadcasters are keenly aware of the solar cycles and use this to plan their transmission frequencies accordingly.

“Generally speaking, we need to use higher frequencies during the high points of the 11-year solar cycle, and lower frequencies during the low points, in order to provide the best reception quality for listeners,” said Jeff White, general manager of WRMI shortwave in Okeechobee, Fla., and chairman of the High Frequency Coordination Conference (HFCC). 

WRMI is the largest privately-owned shortwave station in the United States. The recent peak of Solar Cycle 25 is still delivering good results for WRMI. 

“We only use two very high frequencies (15770 and 17790 kHz), and we are finding that these frequencies are providing extremely good reception not only in the intended target areas, but all over the world,” White told us recently. 

“For example, 15770 is beamed primarily to Europe (with some coverage of North Africa and the Middle East), and we are getting reports of excellent reception of 15770 from listeners in places like Australia and South America.”

Other SW broadcasters have been enjoying similarly positive results. 

“Radio New Zealand was received in North America on their 17675 kHz frequency much more reliably last summer than years before,” said Gilles Letourneau. 

“BBC World Service even used the very seldom used 11-meter band on 25790 kHz that was giving an amazing signal due to higher solar activity. Radio France coming into Montreal was much more reliable on high frequencies like 17850 and 21690.” 

Letourneau also reported making amateur radio contacts on 10- and 12-meter bands with Europe and Asia “when it is usually dead in the low part of the solar cycle.”

“During solar maximum, listeners may be able to receive distant stations more reliably, while broadcasters and radio amateurs may find their signals travel farther. It’s a great time to be a DXer,” Steenburgh said. 

However, he cautioned that solar maximum is a double-edged sword. 

“In addition to fantastic propagation conditions, solar maximum sees large X-ray flares that result in periodic short-wave-fade events, and occasionally, radio bursts and noise storms. Solar particle events that can accompany flares lead to polar-cap absorption conditions. 

“Finally, flares can be associated with coronal mass ejections, which bring geomagnetic storms, noisy conditions and maximum usable frequency depressions.”

Useful resources

For those looking to take advantage of Solar Cycle 25 to improve their RF transmissions and reception, There are several websites that address radio propagation, Steenburgh said. 

“SWPC’s website, spaceweather.gov, provides a great deal of information of interest to radio operators and listeners, including the F10.7 solar flux, geomagnetic indices and output from our D-Region absorption.”

Letourneau recommended a similarly named, but distinct, resource for solar max enthusiasts. 

“Spaceweather.com is the first website anyone should know about to learn about and know what is happening with the sun and the solar cycle,” he said.

Which frequencies should we tune into, to exploit the solar maximum? 

Jeff White offered practical advice for the current environment. “Broadcasters can take advantage of the peak of the solar cycle by using higher frequencies (let’s say above 10 MHz), and listeners can look for these transmissions and enjoy better-than-usual reception on these frequencies, including reception well into the local nighttime,” he said. He recommended the HFCC website and the World Radio TV Handbook for station schedules, as well as The Spectrum Monitor for shortwave news.

Limited-time offer

The term “solar cycle” says it all: The peak of sunspot activity does not last, so the time to check out the amateur and SW bands for best performance is now.

“We appear to be moving past the peak of Solar Cycle 25, so the F10.7 numbers are expected to see a gradual decline over the next several years,” said Steenburgh, citing SWPC’s solar cycle progression plot. 

“It’s important to note that even though we’re on the ‘downslope’ of the solar cycle, impactful events can and do still occur. The most impactful storms of the last cycle (24) occurred in September 2017, more than three years after solar maximum in April 2014.”

Letourneau noted that the 11-year solar cycle is not set in stone, because a massive ball of fusing hydrogen is not committed to a firm schedule. In fact, “A solar cycle can last anywhere from 8 to 18 years in length,” he told Radio World. Still, 11 years has become the accepted length based on experience, so this is the standard that astronomers and broadcasters go by.

Now is the time to enjoy the plethora of sunspots and their effect on the ionosphere while you can.

“I expect they will diminish gradually over the next five-and-a-half years,” said White. “At that point, the lower frequencies will provide better reception and larger coverage areas.”

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