Last Updated on April 21, 2026 by John Berry
Practical antennas can be designed for a task. Engineers design each antenna to exploit the most favourable propagation mode for the path and chosen signal frequency. By choosing the frequency and antenna according to path we can bias our technologies to exploit ground wave, space wave or skywave.
Let’s now explore the characteristics of those three principle modes.
Low frequencies, for example, propagate well along the ground. Medium and high frequencies propagate well skyward, and VHF and above propagate most favourably in the troposphere.
The dominant mode depends on the frequency and the physical environment.
Ground wave
Ground wave comprises two components – surface wave and space wave. Surface wave is the primary wave for frequencies from a few kilohertz up to around 3MHz. If the antenna design permits, surface wave supports propagation along the boundary between the ground and the atmosphere. I’ve stylised this below.
Surface wave propagation loss depends on the ground conductivity along the path, and in the vicinity of the stations. Ground conductivity is measured in Siemens per metre (S/m). Since values in the UK and Europe are low, it’s normally in millisiemens per metre (mS/m). The higher the ground conductivity, the lower the path loss between the stations.
Space wave
Space wave propagation occurs where the wave travels through the troposphere (the region from the ground to about 12km up). Ideally, space wave propagation requires that the path between transmitter and receiver is free-space or near free space. But radio amateurs seldom find this the case and typically exploit heavily obstructed paths. I’ve shown the concept below as a lightly obstructed near-free-space path.
In amateur radio, space wave propagation typically exploits propagation by diffraction, with much of the Fresnel zone obstructed. I describe the concepts of the Fresnel zone and propagation by diffraction elsewhere.
Skywave
Sky wave propagation occurs when the antenna biases radiation skywards to be reflected back to Earth by the ionosphere.
Only certain frequencies arriving at certain angles are returned by the ionosphere. These frequencies lie in the 500kHz to 144MHz range, and more particularly in the 1MHz to 30MHz range. I’ve shown this situation in the following image.
The ionosphere comprises several regions – the D, E and F regions. Each has very specific characteristics, reacting to the incident wave differently depending on the state of the Sun, the signal frequency and angle of incidence (approximately 90 degrees minus the launch angle) of the wave. There are some other variables and I cover those elsewhere in this site.
Summary
If use of an isotropic antenna were possible, the operator would have no say in how the wave would propagate. If we were to transmit, the wave would launch, propagate, and be received – or not.
But things are not that simple. If radio amateurs use specific antennas, one or other propagation mode can be deliberately exploited.
We can predict ground wave performance using the GRWAVE computer program. We can predict sky wave using the VOACAP family of programs. And we can predict space wave using the free-space loss equation modified by calculations of diffraction loss.