Also known as: radio propagation, propagation
Radio propagation describes how radio waves travel from transmitter to receiver, including line-of-sight travel, reflection, diffraction, and atmospheric effects.1 Which of these dominates depends mostly on frequency: the same terrain that is transparent to one band is a wall to another, so the propagation picture shifts completely as you tune from HF up to microwave.
How it works
A radio wave spreading from a transmitter weakens with distance even in empty space, as its power spreads over an ever-larger sphere; this baseline is free-space path loss, and everything else adds to it. On top of that, several distinct propagation modes carry signals in the real world:
- Line-of-sight — the direct ray, dominant at VHF, UHF, and above. It is bounded by the radio horizon, so antenna height and a clear path matter more than raw transmitter power.
- Ground wave — at low frequencies the wave follows the Earth’s conducting surface and can reach well beyond the horizon; this is how AM broadcast and marine MF work by day.
- Sky wave / ionospheric — at HF the upper atmosphere refracts waves back to earth, allowing “skip” over thousands of kilometres.
- Reflection and multipath — waves bounce off terrain and buildings, arriving by several paths that interfere and cause fading.
- Diffraction — waves bend slightly around edges and over hills (knife-edge diffraction), filling in some shadow behind obstacles.
- Tropospheric effects — ducting and scatter in the lower atmosphere occasionally stretch VHF/UHF ranges far past the normal horizon.
In practice
Frequency sets the recipe. Below a few MHz, ground wave and sky wave rule and signals can travel far. In the HF range (roughly 3–30 MHz), the ionosphere opens and closes paths with the time of day, season, and solar cycle. From VHF upward the sky goes transparent and coverage collapses to line-of-sight plus reflections, which is why land-mobile systems lean on tall repeater sites rather than raw power. Total signal at the receiver is the transmit power plus antenna gains minus the accumulated path loss — the link budget that decides whether a decode is even possible.
Relevance to SDR
Understanding propagation explains why antenna height and a clear path often matter more than the radio, and why a distant hilltop system can beat a closer obstructed one. It also tells the operator which antenna and band strategy fits a target: a line-of-sight UHF trunked system rewards a high, clear vertical, while chasing HF utility stations means working with a fickle ionosphere. GopherTrunk decodes whatever survives the path; it cannot conjure a signal the propagation channel never delivered, so reading the channel correctly is the first step in a successful monitoring setup.
Sources
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Radio propagation — Wikipedia, on line-of-sight, ground wave, sky wave, reflection, diffraction, and atmospheric propagation. ↩