Propagation of VHF Radio Signals

[jimh]

An often mentioned assumption by boaters or others not familiar with radio propagation is that at VHF frequencies communication between stations is limited to "line of sight" (LOS) distances. This is not particularly accurate.

Radio waves are electro-magnetic waves and when they propagate in free space they travel in straight lines, but when they propagate in the Earth's atmosphere they are traveling through a denser medium than a total vacuum, and as a result they are slightly refracted or bent.

The general behavior of radio waves in the atmosphere is they tend to bend in a manner that creates a radio horizon that would be similar to the geographic horizon if the Earth radius were 4/3-larger.

For more details see the REFERENCE article:

Radio Horizon
https://continuouswave.com/whaler/reference/radioHorizon.html

In addition to the typical bending of the signal, other atmospheric conditions can further enhance the propagation distance. Temperature layering in the troposphere can create or enhance bending or ducting of signals over much longer distances than expected under normal conditions.

The actual range of communication between two particular stations is always a product of their combined radio horizons. If one station has a particularly very high antenna, that station will have a much extended radio horizon, enabling longer range communication. If both stations in a communication circuit have extended radio horizons, the resulting range of communication can be much greater than for two other stations with lower antenna and much shorter radio horizons.

The term "skip" refers to strong reflection towards Earth of radio waves that would otherwise continue in a straight line and go into space. Skip mechanisms can occur at various layers of the Earth atmosphere. For very long distance communication, radio signals are routinely reflected off the ionosphere F-layer of the atmosphere. The higher layer is referred to as the F2 layer and can produces "skip" distances of thousands of miles. The degree to which the ionosphere reflects radio waves is a function of the frequency of the radio wave and the intensity of solar radiation which causes the ionization of the molecules in the ionosphere. The intensity of solar radiation varies in 11-year cycles, and at solar maxima the F2 layer can be sufficiently ionized to reflect signals in the High-Frequency range as high as 30-MHz, and in some instances (rare) reflect signals at frequencies above 30-MHz and into the low VHF range.

For a thorough discussion of the Ionosphere, see the WIkipedia article for more information:

https://en.wikipedia.org/wiki/Ionosphere

For more about long range signal propagation at VHF and UHF frequencies, see

TV and FM DX
https://en.wikipedia.org/wiki/TV_and_FM_DX

[jimh]

For a more technical look at radio propagation at 156-MHz (the VHF Marine Band), see article:

VHF Radio Propagation Over Water
https://continuouswave.com/whaler/refer ... Water.html