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Radio propagation is how radio signals travel from one antenna to another. The same transmitter and antenna can reach Australia one hour and nowhere the next, depending on the time of day, the season, the sun and the band. VHF and UHF signals mostly travel line of sight, which is why repeaters sit on hills, while HF signals refract from the ionosphere and can circle the planet. The sun's 11-year cycle governs how high the usable frequencies reach, and effects such as sporadic E carry VHF signals far beyond the horizon. Read more…