Wave propagation

How a wave travels from transmitter to receiver — a big chunk of the exam.

The ionosphere. The Sun ionises the upper layers of the atmosphere, which can reflect HF waves (< 30 MHz) back to the ground. By chaining several reflections (ground ↔ ionosphere), an HF wave circles the globe: that's "DX" propagation. The layers (named D, E, F) rise and fall with the time of day and solar activity.

• The D layer (daytime) absorbs low frequencies → 80 m carries far at night, poorly by day.
• The F layers reflect the higher frequencies (20, 15, 10 m), mostly by day.

MUF / LUF. At any moment there's a maximum usable frequency (MUF) above which the wave punches through the ionosphere without returning, and a lowest usable frequency (LUF) below which it's absorbed. You operate between the two.

VHF/UHF. Above ~30 MHz the wave usually crosses the ionosphere: propagation is mostly line of sight (range ≈ the horizon, a little beyond). Hence the value of high spots and repeaters. Exceptional modes exist: sporadic E (summer reflections in low VHF), tropo ducting, moonbounce (EME).

Path loss & fading. Signals weaken with distance and fluctuate (QSB, see Alphabet phonétique, code Q et report RST). It all comes down to the received SNR.

Related: Les ondes radio · Antennes