A large and increasing portion of communications technology is becoming wireless. As more and more processes become wireless, the more complex the infrastructure surrounding them becomes. Understanding the basics of wireless communication is becoming important for the average user.
All forms of wireless communication rely on what is knows as “electromagnetic radiation.” Electromagnetic radiation refers to waves of energy in the electromagnetic field, which carry – sometimes referred to as “propagate” – electromagnetic radiant energy across three dimensional space. Though the term “radiation” has negative connotations in common usage, used here it simply implies that a single point source is giving off – or “radiating” – energy. Electromagnetic radiation isn’t necessarily harmful to humans, however certain frequencies and in sufficient quantities it can be.
Observers perceive electromagnetic radiation in a variety of formats; both radio waves and light waves are forms of electromagnetic radiation, they just happen to have different wavelengths and fall on different parts of the spectrum.
In a vacuum, all electromagnetic radiation travels at the same speed – the speed of light. As electromagnetic waves travel through different substances, their speed and/or ability to transmit begins to change based on the properties of the physical matter and the wavelength of the electromagnetic radiation itself. For example, both light and radio waves are able to pass through the earth’s atmosphere, while only radio waves can pass through the walls of a building as light bounces off the solid structure. In any situation when electromagnetic radiation interacts with any form of matter, the radiation will always lose at least some of its strength as electromagnetic waves interact with molecules of the physical matter itself.
Wavelength and Frequency
In electromagnetic radiation, there is a direct relationship between energy, wavelength and frequency. The shorter the wavelength, the shorter the period between the peaks of two waves. Because all electromagnetic radiation travels at the same speed, as the wavelength becomes shorter, the relative frequency of the of the wave increases, as the period between the peaks of two waves becomes shorter. As the frequency increases, more energy is conveyed over the same period of time, meaning shorter wavelengths with higher frequencies appear to be more energetic when received from a relative vantage point.
As there is a direct relationship between wavelength, wave frequency, and wave energy, there is also a direct correlation between wavelength and the size of the required antenna to transmit/receive a signal. Practically this means that the higher the frequency of a signal, the smaller the receiving antenna needs to be, the implications being that radio waves on the lower end of the frequency of transmission will require significantly larger antennas. For humanitarian agencies, there are real world trade-offs between the usefulness of a certain band of transmission, and how large their radio reception equipment can actually be.
Propagation speed is defined as the length of time it takes for one thing to move to another. The speed of radio propagation in a vacuum is the speed of light, and this speed can be impacted by passing through a variety of transparent or semi-transparent mediums.
Additionally, as different wavelengths of electromagnetic radiation move through any transparent medium, there are subtle and very specific ways in which they are altered or interact with that medium which are governed by a variety of factors. When it comes to using radio or microwave signals within the earth’s atmosphere, there are modes of propagation that impact communication.
Line of Sight Propagation – Line of sight propagation means radio signals can only successfully be received and transmitted if there is no large object blocking the path between the two. Line of sight propagation does not mean that both the transmitter and receiver need to be able to physically see each other – such as a satellite in orbit of the earth – nor does it mean that there has to be completely open space between two objects – such as a VHF radio working inside a structure with radio-transparent walls. Line of sight propagation is important because, hills, large structures, and even the curvature of the earth will limit how far a line of sight signal can go. Most VHF/UHF and microwave radio communications devices are limited by this method of propagation.
Groundwave Propagation – Radio waves can be propagated using what is called groundwave or “surface waves”. Groundwave propagation involves radio waves moving along the surface of the earth and bouncing off solid structures such as hills or buildings. VHF and UHF communications might benefit from groundwave propagation a little, but generally only higher frequency signals benefit from groundwave propagation.
Skywave Propagation – HF radio waves in the earth’s atmosphere propagate using skywave or “skip” propagation. Skywave propagation enables signals transmitted along portions of the HF frequency to bounce off the earth’s ionosphere and oscillate within the earth’s atmosphere well beyond the horizon. Skywaves are able to reach around the curvature of the earth’s surface, sometimes to great distances, however distances are impacted by a complex series of environmental factors.
In practice, all spectrum of radio waves interact with their environment in many different ways, meaning multiple forms of propagation may be possible.
- Absorbed – Radio waves are absorbed and neutralised by large stationary objects like buildings.
- Refracted – As radio waves pass through any medium of varying density, their course may be altered.
- Reflection – Radio waves bounce off stationary or solid objects, sending signals in a new direction.
- Diffraction – The tendency for radio waves to bend towards large objects as they pass over/around them objects.
The combined effects of these different effects creates what is known as multi-path propagation. Multi-path propagation practically results in signals being received in seemingly random or inconsistent ways. It is why signal strength can be increased or decreased by moving one or a few meters in one direction or another, and what may create dead-zones for radio communication.