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.
This page is not intended to be definitive, all knowing, giving a master framework. It will be worked on over the coming years and maybe will ultimately give that. But for now, it’s a guide.
A propagation mode describes the mechanisms by which a signal gets from one station to another. It’s not to be confused by modulation mode which describes how information is impressed on the signal.
Propagation is all-at-once. The wave is launched. One propagation mode proves better than another and the path works over that path for that system value. Radio amateurs can bias their antennas to more likely exploit one propagation mode or another, but it’s a bias, not a switch. Signals still simultaneously propagate by favourable and less favourable modes. And signals still arrive at the distant station by several modes, sometimes causing multi-path interference that manifests as fading.
Signals can propagate in the troposphere, or they can go up to the ionosphere and be refracted, turned, back to Earth.
In the troposphere, signals propagate by ground wave and space wave. Paths are frequently obstructed by terrain and land use features like buildings and trees, so generally, propagation is by diffraction.
At LF and HF, the idea of a space wave is difficult since the Fresnel zone is huge. Instead, signals hug the ground, and the path length is lengthened by the undulation. At VHF and UHF, ground wave is attenuated, and space wave prevails.
At HF and lower VHF, propagation can be by sky wave.
Ground, space and sky all at once
So, you might think, and indeed want, skywave (for example), but you’ll get all the rest too to varying extent.
On any day when working stations on 50MHz during a Sporadic E opening, the PSK Reporter site (https://pskreporter.info/) will show reception of your data signals worldwide. On a good day, there will be three (and maybe even four) groupings:
- Those from your station out to about 500km;
- Those in a band at 1,000km distance, plus or minus about 250km;
- Those out to about 2,400km distance, plus or minus about 500km;
- Those at greater distance from about 3,000 to about 5,000km or more.
Data modes have high system values. As a result, reception is possible at these distances and hence, a result of servers like PSK Reporter, we can now see such banding.
Describing the modes
The diagram below illustrates this graphically, five propagation cases. The table below sums up the diagram with associated propagation mechanisms to give a general model. The model covers tropospheric and ionospheric propagation (via both E and F regions).
|Identity on image||Path||Propagation mode||Losses include||Propagation range limit|
|1||Within the horizon.||Tropospheric line of sight and near line-of-sight space wave. Ground wave at LF and lower HF.||Free Space Loss, multiple knife edges, diffracted sub-path and clutter losses. Ground losses.||5-50km|
|2||Beyond the horizon.||Tropospheric by diffraction.||Propagation by diffraction over rounded (Earth bulge) obstruction.||50-700km|
|3||Skywave||Ionospheric, refracted in E Region during aurora, sporadic E, meteor scatter and other E Region events.||Free Space Loss.||1E, nominally 1,200km. (1,000-1,500)km|
|4||Skywave||Ionospheric refraction in E Region during sporadic E events.||Free Space Loss, reflection loss.||2E, nominally 2,400km (1,900 – 2,800km)|
|5||Skywave||Ionospheric refraction in F Region enhanced by solar radiation. Splits to F1 and F2 layers by diurnally. A function of Maximum Usable Frequency.||Free Space Loss, reflection loss.||1F, 2F and occasionally higher orders (1F1, 2F1, 1F2, 2F2 etc.). Distance 100km to 6,000km.|