Last Updated on February 10, 2022 by John Berry
An isotropic antenna – a point source – launches a wave in all directions in three dimensions. But practical antennas don’t behave like isotropic antennas. They bias their radiation. Generally, that’s because engineers designed the antenna to bias in a desired direction. And this bias is a lot about finding the most favourable propagation conditions for the frequency in use.
We can explore those conditions. If the radiation from the antenna could launch in all directions, we’d find that for a given frequency, some directions propagate more favourably than others. 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 bias of the antenna and the favourable propagation give rise to three models of propagation: ground wave, space wave and sky wave.
Ground wave
Frequencies from a few kilohertz up to around 3MHz will, if the antenna bias permits, support propagation along the ground. This is shown stylised below.
Ground 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) and since normal 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.
Operators can vary the frequency and the antenna type for a given path if hoping for ground wave propagation. The higher the frequency, the greater the loss. Frequencies above about 3MHz have high ground wave loss that renders ground wave unusable for all but local communications. For ground wave propagation, the antenna must bias its energy toward the horizon.
Space wave
Space wave propagation occurs where the wave travels through the troposphere (the region from the ground to about 12km up). Space wave propagation requires that the path between transmitter and receiver is free-space or would be free space if it were not for light or moderate terrain or built or natural environment obstructions. The concept is shown below as a lightly obstructed free-space path. I describe the concept of the Fresnel zone – the dotted lozenge around the path – in a separate page.
Space wave propagation requires that the ground has little or no effect on the antenna radiation pattern at both stations. This constrains space wave to above about 50MHz for practical antenna heights, after which the ground effect peters out. Below that, the ground tends to force the antenna response skywards.
Sky wave
Sky wave propagation occurs when the antenna biases radiation skywards to be returned by the ionosphere. I discuss the detail of this elsewhere on this site.
The key point is that only certain frequencies arriving at certain angles are returned by the ionosphere. These frequencies lie in the 500kHz to 50MHz range, and more particularly in the 1MHz to 30MHz range. This situation is shown in the following image.
As I note elsewhere, the ionosphere comprises several regions – the D, E and F region. Each has very specific characteristics, reacting to the incident wave differently depending on the state of the sun, the frequency and angle of incidence (approximately 90 degrees minus the launch angle). There are some other variables and I cover those elsewhere in this section of the site.
Summary
If use of an isotropic antenna were possible, the operator would have no say in how the wave would propagate. The operator would transmit, and the wave would launch.
By selecting an antenna that biases the radiation in a specific direction, and by selecting a frequency that ground wave, space wave or skywave supports, communications are possible over a desired path. Here’s a summary.
| Propagation mode | Frequency supported | Antenna response |
| Ground wave | 67kHz to 1MHz | Toward the horizon |
| Sky wave | 500kHz to 50MHz | Towards the horizon and skywards |
| Space wave | 30MHz to 100GHz | Biased towards the azimuth and elevation required |
There are exceptions to this neat categorisation, but it’s a basis for understanding.
Ground wave propagation is predicted using the GRWAVE computer program. Sky wave is predicted using the VOACAP family of programs. And space wave prediction is based on the free-space loss equation modified by environmental variables. I discuss each separately on other pages.