The Normal Troposphere
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 density of the troposphere reduces with height. This is hugely important in propagation and in understanding the more unusual propagation mechanisms. Radio waves are bent towards the surface of the Earth and hence propagation distances are typically enhanced. Here’s more.
Most amateur propagation in the troposphere relies on diffraction. There’s seldom a free-space path between the two stations, even from hilltop to hilltop. Some terrain objects intrude into the path. The object with the greatest effect is the Earth’s bulge. Were it not for diffraction the loss between transmitter and receiver would be total and anything other than a free-space path wouldn’t work. So how does diffraction work and how far can radio amateurs get at VHF?
Many of the pages here cover discuss incidence of k-factor fading – slow fading caused by changes in the refractivity of the troposphere. Typically, in UK and western Europe, the refractivity gradient, dN/dh (∆N), is -40 N/km. N is in units of refractivity. This corresponds to an Earth radius factor of 4/3 and an effective radius of 8500km. This page discusses time for which k exceeds the median and hence describes lift conditions such as diffraction loss for low percentages of time.
The notion of height gain is simple enough. Given a long-distance path (of say 300km) where the Earth’s bulge intrudes into and significantly obstructs the line of sight, path loss is estimated using a nomograph. Effectively, there’s only one variable – path distance. If there is a significant antenna height above ground at either or both stations some correction is appropriate , thereby reducing the diffraction loss. This is termed height gain.
Ducts are formed by a change in the atmosphere’s refractive index from the normal -40N/km to -157N/km or below. This is the negative beam-bending case where the obstruction of the Earth’s bulge into the radio path disappears, and the stations are effectively ‘looking at one another’. If a duct is formed, the loss approaches the Free Space Loss. Ducts at VHF and UHF are rare around the British Isles.