# Core propagation concepts

Last Updated on November 6, 2023 by John Berry

## Fresnel Zone

The Fresnel zone is important in radio propagation because any path obstruction that impinges the First Fresnel zone causes obstruction loss. Most paths between amateur radio stations are heavily obstructed (unless both are on hilltops) but it doesn’t do to tolerate avoidable loss.  Make sure that your foreground is as clear as you can make it at all azimuths round your station. Often the single variable here is antenna height. Here’s an analysis and explanation.

## Reflection, Refraction or Scattering?

There’s huge ambiguity across the radio amateur books, magazines, and sites when it comes to describing how it is that the ionosphere returns radio waves to Earth. The ambiguity is this. Many writers routinely talk of reflection. Others call it scattering. And some refer to the mechanism as refraction. So, which is it? Here’s an argument and conclusion.

## Ground wave, space wave and skywave

Hams routinely identify skywave propagation, and, to a much lesser extent, talk of space wave and ground wave. But how do these modes come about? It’s to do with the launch angle of the antenna in use and the frequency of operation. Generally, ground wave is used for LF and MF propagation, sky wave is used for HF communications and space wave is for VHF, UHF, SHF and above. Here’s a description and summary.

## Propagation modes

Radio hams frequently jump to conclusions about propagation modes. It’s no surprise. Propagation is complex and a little knowledge can lead to assumptions. This page attempts to give a logic to understanding how far signals can go for a given frequency, time of day and station characteristics. It aims to describe propagation modes, and in so doing, give understanding of propagation range limits. Read about propagation modes.

## Fading

There’s the direct path assuming free space transmission. Paths between radio amateur stations are typically attenuated hugely by diffraction loss (discussed in another blog), with huge diffraction contribution from the Earth’s bulge protruding into the path. And then there are all the reflected paths from buildings, atmospheric layers, aircraft and frankly everything and anything in the vicinity of, and mid-way between, both stations. The result is fading.

## Probability distributions

Amateur radio often exploits signal strengths in the tail of various probability distributions. The term ‘DX’ gives the idea of communication for low, or even very low, percentages of time. If we are to exploit DX, we need to understand the various probability effects. This page describes two distributions – the log-normal and the Rayleigh.

## Propagation bibliography

Here’s a list of papers, books and the like that have informed the pages about radio wave propagation.