Any shortwave radio enthusiast likely knows this common knowledge: the 20-meter band is suitable for communication during the day, while the 40-meter band becomes optimal in the evening and at night, with the 20-meter band’s efficacy diminishing. But why is this the case?
Let’s focus our discussion on skywave propagation, where our radio waves are reflected by the sky to reach distant locations. Ground wave and line-of-sight propagation will not be part of today’s exploration.
In the realm of radio communication, the ionosphere plays a pivotal role, acting like a mystical mirror, reflecting radio waves emitted from the Earth. The Sun, a colossal star in the universe, exerts a profound impact on the state of the ionosphere through its activity cycles. In this article, we will delve into the composition of the ionosphere, the influence of solar cycles, and how these natural phenomena collectively mold the propagation paths of radio waves.
Strata and Characteristics of the Ionosphere
The ionosphere, a segment of Earth’s atmosphere, is composed of several primary layers, each existing at different altitude ranges and exhibiting distinct characteristics during the day and night. Let’s explore each layer, moving from those closest to Earth’s surface to those further away.
- D Layer
- Altitude: Approximately 50 to 90 kilometers (31 to 56 miles).
- Day: Present. Formed by the ionization of atmospheric gas molecules by solar ultraviolet light, it primarily absorbs lower-frequency radio waves, especially in the medium and shortwave bands.
- Night: Virtually non-existent. The reduction of solar radiation causes electrons and ions to recombine, significantly diminishing the D layer’s ionization.
- E Layer
- Altitude: Roughly 90 to 150 kilometers (56 to 93 miles).
- Day: Present. Formed by solar ultraviolet light ionization, it can reflect medium to high-frequency radio waves, allowing long-distance communication under certain circumstances.
- Night: Gradually fades. Electrons and ions recombine as solar radiation decreases.
- F Layer
The F layer divides into two sub-layers during the day: F1 and F2, merging into one layer at night.
3.1 F1 Layer
- Altitude: About 150 to 220 kilometers (93 to 137 miles).
- Day: Present. Generally reflects medium to high-frequency radio waves.
- Night: Merges with the F2 layer.
3.2 F2 Layer
- Altitude: Approximately 220 to 450 kilometers (137 to 280 miles) during the day, potentially higher at night.
- Day: Present. Capable of reflecting high-frequency radio waves and is commonly used for long-distance communication.
- Night: Present. Although ionization weakens, it maintains a degree of ionization, supporting long-distance propagation of high-frequency radio waves.
Understanding the characteristics and variations of these ionospheric layers is crucial for radio communication and predicting radio wave propagation conditions.
Performance of Shortwave Bands Under Various Conditions
4.1 80-Meter Band
- Day: Strongly absorbed by the D layer, resulting in poor communication.
- Night: The D layer disappears, and the F layer reflects effectively, suitable for long-distance communication.
4.2 40-Meter Band
- Day: Moderate D layer absorption, suitable for medium-short distance communication.
- Night: Effective F layer reflection, suitable for long-distance communication.
4.3 20-Meter Band
- Day: Weak D layer absorption, good reflection from E and F1 layers, suitable for long-distance communication.
- Night: Typically insufficient F layer reflection, possibly unsuitable for long-distance communication.
4.4 10-Meter Band
- Day: During periods of robust solar activity, the F layer reflects well, enabling long-distance communication.
- Night: Generally unsuitable for communication as the F layer’s reflective capability significantly weakens.
Impact of Solar Cycles
The solar cycle, averaging an activity shift from minimum to maximum every 11 years, significantly influences the ionization of the ionosphere and the propagation of radio waves.
2.1 Solar Activity and the Ionosphere
Enhanced solar activity typically elevates the ionosphere’s ionization, altering its reflection and absorption properties for radio waves.
2.2 Solar Activity Cycles and Radio Communication
Peak periods of solar activity might bring about higher Maximum Usable Frequencies (MUFs), but may also introduce more communication instability.
Now, do you understand why the 20-meter band is suitable for communication during the day, the 40-meter band opens up in the evening and at night, and why the 20-meter band becomes less effective for communication at night?