Propagation

5A1: Speed of Light

Radio waves are a form of electromagnetic radiation and travel at the speed of light — approximately 300,000,000 metres per second (3 × 10⁸ m/s) in free space.

• This is the fastest speed possible in the universe
• Light, radio waves, X-rays, etc. all travel at this speed
• Actual speed may vary slightly depending on the medium

5A2: Wavelength, Frequency, and Speed

This means that as frequency increases, wavelength decreases, and vice versa.

5B1: Ground Wave

The ground wave follows the curvature of the Earth's surface. It's useful for medium wave broadcast and LF/MF amateur bands.

• Propagates along the ground, gradually losing strength
• More effective on lower frequencies
• Affected by ground conductivity (sea water is best, dry ground is worst)
• Provides reliable local coverage

5B2: Line of Sight

On VHF and UHF frequencies, radio waves travel primarily in a line of sight — they don't follow the Earth's curve significantly.

• Range depends on antenna height and terrain
• Higher antenna = greater range
• Obstacles (hills, buildings) can block the signal
• This is why VHF/UHF repeaters are placed on high ground or tall towers

5C1: The Ionosphere

The ionosphere is a layer of the Earth's atmosphere that contains charged particles (ions) created by solar radiation. It can reflect radio waves back to Earth, enabling long-distance communication.

The ionosphere has several layers:

5C2: Skip Distance and MUF

• Skip distance — the distance between the transmitter and the point where the reflected signal first returns to Earth
• Maximum Usable Frequency (MUF) — the highest frequency that can be reflected by the ionosphere at a given time
• Above the MUF, signals pass straight through into space
• The MUF varies with time of day, season, and solar activity

5C3: HF Propagation

Sky wave propagation is how HF radio signals travel around the world. The signal bounces between the ionosphere and the Earth's surface.

• Signals can travel thousands of kilometres by multiple "hops"
• HF propagation conditions change throughout the day
• Better propagation generally during high solar activity
• The 20m band (14 MHz) is often the most reliable for long-distance work
• Lower frequencies (80m, 40m) are better for regional contacts, especially at night

5E1: Sporadic E Propagation

Sporadic E is a propagation mode caused by small, intense patches of ionisation in the E layer of the ionosphere (90–150 km altitude). It allows VHF signals to travel much further than normal line of sight.

• Most common on VHF — particularly the 6m (50 MHz) and 2m (144 MHz) bands
• Occurs unpredictably, often in summer (May–August)
• Typically lasts minutes to hours
• Skip distances of 500–2,000 km are possible
• The E layer patches are thought to be caused by wind shear and temperature inversions

5F1: The Solar Cycle

The Sun goes through an approximately 11-year cycle of activity. This has a major effect on HF propagation.

• Solar maximum — more sunspots, more ionisation, higher MUF, better HF propagation
• Solar minimum — fewer sunspots, lower MUF, reduced HF propagation
• Sunspots are cooler, darker areas on the Sun's surface — more sunspots = higher solar activity

5F2: Solar Effects on Propagation

Solar events can significantly affect radio propagation:

• Solar flares — sudden bursts of energy that increase D-layer absorption, causing a fade-out of HF signals (especially on lower frequencies)
• Geomagnetic storms — caused by coronal mass ejections, can disrupt the ionosphere and cause signal fading and distortion
• Increased solar activity raises the MUF, allowing higher frequencies to be used for long-distance communication
• Solar indices (SFI, A-index, K-index) are used to describe current conditions — higher SFI generally means better HF propagation