↑ HF Propagation Overview

By Doron Tal, 4X4XM

High-Frequency (HF) propagation refers to the way radio waves at frequencies between 3 and 30 MHz travel through the Earth's atmosphere. Understanding HF propagation is important for amateur radio operators, shortwave listeners, and professionals who rely on long-distance communication. This summary provides a concise overview of HF propagation over the horizon:

  1. Ground Wave Propagation:

    At lower HF frequencies, typically below 2 MHz, radio waves can follow the Earth's curvature and propagate along the ground. This is called ground wave propagation.

    Ground wave propagation is suitable for relatively short distances, and the signal strength diminishes with increasing distance.

  2. Skywave Propagation:

    At higher HF frequencies, typically above 2 MHz, radio waves interact with the ionosphere, a layer of charged particles in the Earth's upper atmosphere, that reflects HF signals back to Earth, allowing for very long-distance communication far beyond the line-of-sight. This is known as skywave propagation. The F-layer of the ionosphere (approximately 150 to 300 km above Earth) plays a crucial role in reflecting HF signals.

    2.1 Skip Zone:

    The skip zone is an area between the point where the ground wave (directly transmitted signal) becomes weak, and the skywave (reflected off the ionosphere) arrives. Within this zone, communication may be challenging.

    2.2 NVIS - Near Vertical Incidence Skywave) can help mitigate skip zone issues. It is a technique where HF signals are transmitted at high angles, allowing them to be reflected back to Earth over short to medium distances. This method is useful for regional communication and is less affected by skip zones.

  3. Ionospheric Layers:

    The ionosphere is divided into several layers (D, E, F1, F2). Each layer has different characteristics, affecting radio wave propagation differently.

    The F2 layer is the most important for long-distance HF communication as it reflects signals back to Earth. The lower layers (D and E) play a complex role, affecting propagation at various HF bands.

  4. Solar Activity:

    Solar activity, particularly sunspots, affects the ionosphere's electron density, influencing HF propagation conditions.

    During periods of high solar activity, the ionosphere becomes more reflective, enhancing long-distance communication. Conversely, during solar minimum, the ionosphere may be less reflective, making communication more challenging.

  5. Day-Night Variation:

    The ionosphere undergoes changes throughout the day due to solar radiation. During the day, the D and E layers are more pronounced, while the F2 layer is more dominant at night.

    This day-night variation can impact the distance and reliability of HF communication.

  6. MUF (Maximum Usable Frequency) and LUF (Lowest Usable Frequency):

    MUF is the highest frequency that can be used for Skywave Propagation between two points at a specific time. It depends on factors like solar activity, time of day, and the distance between transmitting and receiving stations.

    The LUF is the lowest frequency that can be used for reliable skywave propagation between two points at a specific time.

  7. Geomagnetic Disturbances:

    Geomagnetic storms, often caused by solar flares, can disrupt HF propagation by affecting the ionosphere. During such events, communication conditions may vary.

  8. Frequency Selection:

    Different HF frequencies behave differently under various propagation conditions.

    Lower frequencies (e.g., 3-7 MHz) are better for long-distance communication during periods of low solar activity and at night.

    Higher frequencies (e.g., 14-30 MHz) may be suitable for shorter distances and daytime communication.

  9. Antenna Considerations:

    The type and orientation of your antenna can significantly impact HF propagation. Experiment with different antenna types, such as dipole antennas or vertical antennas, to find the one that works best for your specific situation.

    The efficiency of your antenna and its grounding also affect HF propagation.

    Good grounding and the right antenna can improve the effectiveness of your HF communication.

  10. Real-Time Propagation Tools:

    Various online tools and software can predict HF propagation conditions.


Summary: HF propagation is a complex dynamic phenomenon influenced by various factors like solar activity, geomagnetic data, frequency, time of day, and antenna characteristics. Experimenting with different bands, antenna configurations, and staying informed about solar activity can enhance your understanding and help in optimizing communication strategies for effective HF communication. Link to the main page.


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The Understanding HF Propagation Project provides radio amateurs with a detailed overview and tutorials on several aspects of HF propagation.


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