Over-the-air television reception can sometimes change without warning. A station that has been reliable for months may suddenly pixelate, freeze or disappear, then return later without any adjustment to the antenna or television.
Solar flares are frequently blamed when this happens, and it would be too absolute to say that activity from the Sun could never affect a television receiving system. Scientific research does show, however, that the best-established radio-blackout effect of a solar flare occurs mainly below 30 MHz, well below the VHF and UHF frequencies used for over-the-air television. For recurring warm-weather reception problems, the more common atmospheric explanation is tropospheric propagation, including a more pronounced form known as tropospheric ducting.
When the Atmosphere Extends a TV Signal
Over-the-air television broadcasts use VHF and UHF radio frequencies. Under ordinary conditions, these signals travel primarily within the radio horizon, with their usable range shaped by transmitter height, receiving-antenna height, terrain and the curvature of the Earth.
The atmosphere is not always uniform, however. Changes in temperature, humidity and air pressure alter its radio refractive index. Under certain stable weather conditions, the refractive index can change rapidly with altitude, causing VHF and UHF signals to bend more strongly toward the Earth than they normally would.
In a sufficiently strong refractive layer, radio energy can become partially confined between atmospheric boundaries or between the atmosphere and the Earth’s surface. This is tropospheric ducting. It can carry a television signal far beyond its normal coverage area, sometimes across several states or over a large body of water.
Location: The lower atmosphere, where weather occurs.
Frequencies affected: VHF, UHF and higher-frequency terrestrial radio services.
Possible result: Distant TV signals travel beyond their normal service areas and interfere with local stations using the same RF channel.
Primary flare effect: X-ray radiation increases ionization in the ionosphere and can weaken HF radio, especially from 3 to 30 MHz.
Separate radio-noise effect: Some solar events also produce broadband solar radio bursts that can extend into VHF, UHF and higher frequencies.
Possible result: Exceptional radio bursts can raise the received noise level or interfere with some radio systems, although this is not considered a common explanation for seasonal terrestrial OTA television problems.
Why a Stronger Distant Signal Can Make Reception Worse
At first, receiving a signal from hundreds of miles away may sound beneficial. For local television reception, it can create a significant problem.
Broadcast stations in different television markets are permitted to reuse the same physical RF channels when they are normally far enough apart to avoid interference. During a ducting event, a distant station using the same RF channel as a local station can unexpectedly reach the same receiving antenna.
A television tuner may then receive two unrelated digital broadcasts on the same frequency. This is known as co-channel interference. Depending on the relative strength and timing of the signals, the picture may pixelate, freeze or disappear completely.
An amplifier generally cannot correct this condition because it does not distinguish between the local signal and the unwanted distant signal. It amplifies both. A more directional antenna or a careful change in antenna aim can sometimes improve the ratio between the desired and interfering signals, but severe ducting often must subside before normal reception returns.
Where and When Ducting Is More Common
Tropospheric ducting can occur throughout the United States, but it is not equally common everywhere. It is favored by stable atmospheric conditions, strong temperature or moisture gradients, high-pressure weather patterns and long, unobstructed propagation paths.
NTIA research based on atmospheric measurements found elevated ducting to be more common across much of the eastern half of the United States, with the California coast also showing elevated occurrence. Geography, local climate and the direction of the signal path all influence whether a particular household experiences interference.
Ducting is generally more noticeable during summer and early fall, when warm surfaces, persistent high pressure and differences between daytime and nighttime temperatures help form stable atmospheric layers. Events commonly develop after sunset, overnight or near sunrise and may weaken as daytime heating mixes the lower atmosphere.
Why Solar Flares Receive the Blame
The belief that solar flares disrupt antenna television comes from a valid scientific fact: activity from the Sun can interfere with radio systems. The important distinction is that solar activity can affect radio reception through more than one mechanism, and those mechanisms do not affect every frequency in the same way.
The best-established flare effect: According to the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center, X-ray and extreme-ultraviolet radiation from a sufficiently strong solar flare increases ionization in the ionosphere’s lower D-region on the sunlit side of Earth. The resulting absorption primarily degrades HF radio signals in the 3-to-30 MHz range. Those frequencies are below the VHF and UHF bands used for terrestrial television.
A separate possible mechanism: Some solar events also produce intense broadband solar radio bursts. Unlike ionospheric HF absorption, this is radio-frequency energy arriving directly from the direction of the Sun. Documented extreme events have produced interference across portions of the VHF, UHF and microwave spectrum, so a direct effect on a radio receiver at television frequencies cannot be ruled out categorically.
That possibility should not be confused with the idea that weakened signals below 30 MHz routinely enter a television tuner and disrupt its picture. A properly functioning TV receiver normally rejects frequencies outside its tuning range. Out-of-band signals can create interference only under special receiver-overload or nonlinear-mixing conditions, and the published research reviewed for this article does not establish flare-altered HF signals below 30 MHz as a common cause of consumer OTA television problems.
Solar flares and related space-weather events can also affect satellites, navigation systems, electrical infrastructure and some satellite, microwave or timing systems used by broadcasters. These effects are scientifically documented. For a household experiencing reception trouble that recurs during calm summer evenings or early mornings, however, tropospheric ducting and the resulting co-channel interference remain the more likely atmospheric explanation.
Why the Problem Can Appear at the Same Time Each Year
Because favorable ducting conditions can return during the same season, a household may experience reception problems at approximately the same time each year. A station may become unreliable during calm summer evenings or early mornings and then return to normal later in the day.
That repeating pattern can make a properly functioning antenna, preamplifier or tuner appear to be failing intermittently. The equipment may not have changed at all. Instead, the atmosphere has temporarily allowed an unwanted distant broadcast to reach the antenna on the same frequency as the desired local station.
Once the inversion or refractive layer breaks down, the distant signal usually fades and local reception returns to normal. Tropospheric ducting is temporary, but its seasonal nature explains why some OTA viewers encounter remarkably similar reception problems year after year.
Scientific sources
The description of solar-flare radio blackouts is based on information published by the NOAA Space Weather Prediction Center . The discussion of broadband solar radio-burst interference is supported by peer-reviewed space-weather research documenting exceptional interference across frequencies extending into the VHF, UHF and microwave ranges.
The discussion of tropospheric refractivity, elevated ducts and their occurrence across the United States is based on research published by the NTIA Institute for Telecommunication Sciences .