Propagation

HF band propagation reasons and effects

High-frequency (HF) radio propagation, spanning from the 160-meter to the 6-meter band, is influenced by multiple factors, including the time of day, seasonal variations, and the 11-year solar cycle. Understanding these influences helps amateur radio operators and communications professionals optimize their transmissions for effective long-distance communication.

HF Propagation as imagined by DALL-E

Propagation by band and time of day

Each HF band has distinct propagation characteristics that vary depending on the time of day:

  • 160m (1.8 – 2.0 MHz) and 80m (3.5 – 4.0 MHz): These bands are predominantly nighttime bands. During the day, they experience strong D-layer absorption, limiting long-distance communication. However, at night, lower absorption allows for enhanced long-distance propagation via skywave reflection, often experiencing strong signals during winter months due to reduced atmospheric noise.
  • 40m (7.0 – 7.3 MHz): This band serves as a transition band between day and night propagation. During daylight, it supports regional communication via ground wave and short skip skywave. At night, it opens up for long-distance (DX) propagation.
  • 20m (14.0 – 14.35 MHz) and 17m (18.068 – 18.168 MHz): These bands offer the best all-around propagation for long-distance communication. They remain open throughout the day, with nighttime openings possible during periods of high solar activity.
  • 15m (21.0 – 21.45 MHz) and 12m (24.89 – 24.99 MHz): These bands are primarily daytime bands, relying on sufficient solar radiation to ionize the F-layer for long-distance contacts. During solar maximum periods, they can stay open well into the evening.
  • 10m (28.0 – 29.7 MHz) and 6m (50.0 – 54.0 MHz): These bands require high solar activity to support long-distance F-layer propagation. Sporadic-E propagation can occur, especially in the summer months, enabling surprising short-to-midrange contacts.

Seasonal effects on HF propagation

The seasonal changes affect HF propagation due to variations in ionospheric ionization levels and atmospheric conditions:

  • Winter: Low atmospheric noise on lower bands (160m and 80m) enhances DX capabilities. Higher bands (10m and 6m) often struggle due to reduced solar heating and ionization.
  • Spring & Fall: These seasons offer the most stable propagation across all bands. The equinoxes allow balanced daylight across the hemispheres, leading to favorable propagation on mid-to-high HF bands.
  • Summer: Increased thunderstorms lead to higher noise levels on lower bands, reducing their effectiveness. However, sporadic-E propagation on 6m and 10m thrives, creating excellent short-range contacts over a few hundred to a few thousand kilometers.

Solar cycle impact on HF propagation

The sun undergoes an approximately 11-year cycle of solar maximum and minimum, dramatically impacting HF band conditions.

  • Solar Maximum: Increased solar activity leads to more sunspots and greater ionization of the ionosphere. Higher bands, such as 10m, 12m, and 15m, remain open longer and support worldwide DX communication. MUF (Maximum Usable Frequency) rises, allowing higher-frequency bands to support long-haul propagation.
  • Solar Minimum: With fewer sunspots, higher bands become unreliable for long-distance communication. Operators must rely on lower bands (160m to 40m) for consistent contacts, with long-haul communication occurring primarily at night when ionospheric absorption decreases.

Other influences on HF propagation

Apart from solar activity, geomagnetic storms play a critical role in HF propagation. Solar flares and coronal mass ejections (CMEs) can cause sudden ionospheric disturbances (SIDs), which lead to blackouts on HF bands, particularly on the sunlit side of the Earth. Additionally, auroral propagation, driven by geomagnetic activity, can enable unique communication paths in polar regions but may also disrupt conventional ionospheric reflections.

Another important factor is the presence of tropospheric effects, though more common in VHF/UHF bands. Temperature inversions and ducting can sometimes enhance HF signals, particularly at higher frequencies near the 6m band. Atmospheric absorption, humidity, and local terrain can also impact signal strength, particularly in lower-frequency bands where ground wave propagation plays a role.

HF propagation is a dynamic and ever-changing phenomenon influenced by time of day, seasonal variations, and the solar cycle. By understanding these elements, radio operators can maximize their chances of establishing effective communications, whether for amateur radio, emergency response, or professional applications.

Vy 73 de Gregor, DO5SSB