Last Updated on March 31, 2026 by John Berry
To understand radio wave propagation, you must look at the matter occupying the space or atmosphere above us. You must understand what’s up there that supports radio wave propagation. The atmosphere is not a void. It is a fluid made of neutral gases, water vapour, and charged particles. These physical components determine how your signal propagates from transmitting station to receiving station in the frequency range from 1.8 MHz to 1300 MHz.
There are two zones or media.
The Neutral Medium: A Lens Made of Air
The lower atmosphere consists mainly of Nitrogen (N2) and Oxygen (O2). We term these gases and a small amount of others as ‘air’. These gases have mass and a measurable density. Air density decreases as you move higher into the atmosphere. This change in density creates a negative gradient or rate of change from ground level to around 60km up.
Radio waves travel slightly slower in dense air than in a vacuum. Because the density reduces with height, the top of a radio wavefront travels faster than the bottom. This changing refractive index with height causes the wave to bend back toward the Earth. We call this bending process refraction.
We use a model called the Standard Atmosphere when discussing this beam bending. This beam bending is very significant and important for signals in the VHF and UHF frequency ranges.
The Plasma Medium: The Ionosphere
Above about 60 km, the atmosphere changes from a neutral gas to a plasma. High-energy photons from the sun strike gas atoms. This energy incident on the gas atoms knocks electrons loose. This creates a “soup” of free electrons and positive ions.
An ionosphere of electrons, positively charged ions and neutral atoms
The cloud comprising negative electrons and positive ions becomes stable or quasi-stable under two conditions – first, that neutral atoms/molecules are interspersed (as shown above) thereby weakening the repelling and re-combining forces, and second, that the Earth’s magnetic field, and the various tides and winds of the Thermosphere cause particles to group. To a lesser extent, interplanetary geomagnetic forces, such as geomagnetic interaction between the Earth and the Sun, also have an effect.
For the radio amateur, the density of free electrons is the most important parameter.
The diagram shows free electrons (black), liberated by the energy from the Sun. These charged particles, by their presence in the alternating electric field of the incident wave (red), oscillate (black-grey). This oscillation occurs at the signal frequency and creates a secondary alternating field which radiates a wave isotropically (blue). It’s Faraday’s law at the atomic level. In a dense plasma, there are many electrons affected and hence many re-radiations, and the mechanism is efficient at propagating the wave onward. This is the concept of reflection.
It is sometimes thought that this turning is simply continuous refraction within the plasma. I discuss elsewhere on this site the discontinuities that allow those dense layers to reflect.
The Metallic Trace: A Research Gap
Regions comprising metallic vapour also exist. Meteors burning up in the atmosphere leave behind ions of Magnesium (Mg+) and Iron (Fe+).
Scientists believe these ions help form clouds in the E region at around 100km up. These clouds allow VHF signals to travel thousands of kilometres. However, we do not fully understand how these ions compress into thin, dense clouds. This is a known research gap that requires further academic study.
Support for radio wave propagation
The atmosphere acts as a complex, dynamic medium. Between about 1.8 MHz and the lower end of SHF at about 1300 MHz, your signal interacts with different atomic particles. In the lower layers, it is gas density and humidity that matter. In the upper regions, it is the density of free electrons.
Understanding these particles and their behaviour is the first step to mastering propagation.