It’s difficult for radio amateurs to understand all the various propagation mechanisms without an understanding of the core benchmark of the normal lower atmosphere or troposphere.
The normal, non-ionised atmosphere comprises mainly atoms of nitrogen and oxygen, water vapour, and lesser concentrations of other gases like argon. The density of atoms and molecules typically reduces with height.
The fact that the density of the troposphere reduces with height is hugely important in propagation. It’s shown in the diagram below.
A radio wave propagates in a straight line in a vacuum. This is shown in blue above, with the wave front advancing from Station A. This signal would be lost to space with low energy arriving at Station B. Communications with that station would likely be impossible.
A radio wave propagating in a normal troposphere shown in white would encounter a decreasing particle density with height. The lower parts of the wave front would be slowed down more than the upper parts. As a result, the ray representing propagation would be refracted towards the higher density at the bottom of the troposphere. The ray would be bent towards the surface of the Earth.
As a result of this refraction, communications with Station B may be possible.
To avoid having to draw curved lines between stations in a normal troposphere, it’s more convenient to distort the Earth’s radius and draw a straight line between the stations.
To enable this, an effective Earth radius of 4/3 is used representing the radius exceeded for 50% of time. This effective Earth radius is 8,495km.
This distortion of the Earth’s radius to yield an effective Earth radius is shown below.
In fact, the effective Earth radius varies with time for any path. The degree to which this beam bending occurs depends on the air pressure, the temperature, and the density of water in the troposphere and those parameters vary day by day, hour by hour, and even minute by minute.
This phenomenon of beam bending is hugely important to radio amateurs.
Firstly, as noted above, radio waves don’t travel in straight lines in the troposphere. They are bent towards the surface of the Earth and hence propagation distances (over those determined by straight line ray tracing) are typically enhanced.
And secondly, the effective Earth radius goes well above 4/3 for small percentages of time. This means that any path that ordinarily contains obstructions blocking the signal might be flattened. A path that ordinarily suffers huge diffraction loss (and hence does not support communications) may become a free space path for a short time. Radio amateurs call this a ‘tropo opening’ and say that ‘lift conditions’ exist. More on this second point in another blog.