HF Propagation

The ionosphere is a layer of Earth's atmosphere located between 60 and 1000 km altitude, where solar radiation ionizes atoms creating free electrons. These electrons enable the reflection of HF radio waves back to Earth.

The ionosphere is divided into several layers:

• D Layer (60-90 km): present only during daytime, absorbs lower frequencies
• E Layer (90-150 km): medium-distance reflection, occasionally Sporadic-E
• F1 Layer (150-250 km): present during day, merges with F2 at night
• F2 Layer (250-500 km): the most important for DX propagation

When an HF radio wave reaches the ionosphere, it can be:

• Reflected: returns to Earth enabling long-distance contacts
• Absorbed: energy is dissipated (typical for lower frequencies during daytime)
• Transmitted: passes through the ionosphere and is lost in space

The critical frequency (foF2) is the maximum frequency reflected at vertical incidence. For distance contacts, the MUF (Maximum Usable Frequency) can be 3-4 times higher.

Propagation conditions vary based on:

• Solar cycle: every ~11 years, more sunspots = better propagation on higher bands
• Time of day: D layer disappears at night, opening lower bands
• Season: higher bands work better in winter in the illuminated hemisphere
• Geomagnetic activity: solar storms can degrade or block propagation

The skip zone (dead zone) is the area between the end of ground wave coverage and the beginning of sky wave reception. This distance varies with:

• Frequency used (higher frequencies = longer skip)
• Ionospheric layer height
• Antenna radiation angle

For DX contacts, antennas with low radiation angles are preferred.