Last Updated on December 11, 2025 by John Berry
Fresnel Zone
The Fresnel Zone is important in radio propagation. Any object like a hill or forest (or Earth bulge) that protrudes into the First Fresnel zone potentially causes signal loss. Paths between amateur radio stations often suffer excess loss over free space when the First Fresnel Zone is obstructed. At HF, such excess loss is normal, and above about 3GHz, the zone radius is small so the loss reduces. As a topic for discussion, the fresnel Zone is most relevant at VHF and UHF. Here’s an analysis and explanation.
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 relevant for LF and MF propagation, sky wave is exploited 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
Any signal propagating in free space and perhaps suffering excess loss due to obstruction will typically be at a steady level over a short time. But over a longer time of several hours, the characteristics of the atmosphere change. These changes cause the received signal to fade. Likewise if the signal is reflected from objects along the path there may be interaction between direct and reflected waves and rapid fading may occur. Here’s a discussion on fading.
Probability distributions
Amateur radio often exploits steady and faded signals. Signal levels are often in the tail of various probability distributions. The term ‘DX’ gives the idea of communication for low or even very low percentages of time, far from the 99% that we expect from our mobile phones and broadcast radio and TV. If we are to exploit DX, we need to understand the various probability effects and those probability distributions. This page describes two distributions – the log-normal and the Rayleigh.
Communications path geometry
Path geometry is the description of the spatial relationship between the transmitting antennas and the plethora of other solid and gaseous objects between them. Understanding communications path geometry is fundamental to understanding radiowave propagation. Path geometry embraces ray tracing to describe the path from transmitter to receiver via all the obstacles, clouds of plasma, and reflective layers that might support the communication. This page defines path geometry and gives example geometries.
Polarisation distortion
When excited by radio frequency energy, an antenna creates both an electric field and a magnetic field. An antenna’s polarisation is defined by the plane of its electric field giving vertical, horizontal, circular, and mixed polarisation. I describe here polarisation discrimination (loss due to deliberate cross polarisation operation) and polarisation distortion (when atmospheric anomalies twist the polarisation) as the wave propagates. I describe how polarisation distortion reduces the received signal.
Paths are reciprocal
Are paths reciprocal? Answer: yes, it’s a reasonable assumption to make. But we must be more specific for it’s obvious that in some cases they are most certainly not. Here’s an analysis supporting the assumption. And does whether they are reciprocal matter anyway? Understanding the reason why matters if we want to be accurate about the various reasons for received signals encountered.
Path loss
The loss in signal as a wave propagates in an environment between two antennas is termed the path loss. The path can be through, for example, the troposphere alone, via the ionosphere, reflected by the moon, or scattered by rain. Typically, a path loss value can be aggregated by summing all discrete losses from each element.
The effects of vegetation, buildings and rain
I’ve described here the nature and magnitude of the effects of vegetation buildings and rain and therefore how radio hams might think about them. I’ve written it as some recommendations, rather than describing any specific method of treatment. Broadly the effects are modest for radio hams.
Our Sun and its importance
Our Sun is important to all radio amateurs. And I’ve described on this site many propagation phenomena that are caused by the Sun. It’s important therefore that I also include a good high level description of our Sun and how it does what it does. For this I’ve copied and cited a page from Chris Lintott’s book.
The magnetosphere
Here’s a description and a model to show the causal links between the Sun, the magnetosphere, and useful ionospheric propagation. There’s the normal interaction between Sun and ionosphere. There are three major paths (through the model) that disturb the ionosphere from time to time(red, blue, and purple). And then there are three minor paths that provide some anomalous propagation that’s quite fun (brown, orange, and cyan).
Propagation bibliography
Here’s a list of papers, books and the like that have informed the pages about radio wave propagation.