Comms Table of Contents
- Introduction to Communication Systems
- The Electromagnetic Spectrum
- Intro to Signals
- Intro to Systems
Gain and loss refer to a change in the amplitude of a signals amplitude. Gain, is an increase in the amplitude of the signal, and loss is a decrease.
Gain can occur as an active process, i.e. one where energy is added to the system, such as using an amplifier to increase the amplitude. Or it can occur as a passive process (one where no energy is added) such as focusing energy of a signal in a beam, or reflecting signals so that they add “in-phase”.
Loss can occur due to many things – resistance in a wire, reflections of signals, filters which may purposely be in the system or not, general attenuation of the signal as it propagates away from the antenna.
It is important to know what factors are affecting the gain and loss of a specific system, to be able to quantify them, and if necessary, be able to compensate for them. This is because there is a minimum level that a receiver can sense, and if a signal falls below the level the receiver can sense, then the signal will be lost.
Reflections occur when an electromagnetic (EM) wave strikes a large surface (large means with respect to the wavelength), and bounces off (generally with some absorptive loss). The most obvious example of reflection is light (which is also an EM wave) reflecting off of a mirror. The main problem that reflection can cause is that as the main signal is reflected, multiple copies of the signal can be created, and then received at the receiver. This multiple signal creation/reception is called multipath.
In WLAN systems, multipath can result in holes in the network. In GPS systems, multipath can cause incorrect location calculation.
Another form of reflection can occurs in wired (copper or optical) connections. When a signal travelling down the wire encounters a change in the medium it is travelling in (this change can occur at a connector, a break in the wire, and even a bend in a PCB) some fraction of the signal will reflect back the way it traveled.
Refraction is a change of direction of a wave as it passes from one medium to another. The reason for the change of direction is because EM waves have different velocities in different mediums, so when it moves into a new medium, it will change velocity. But why does this cause a change in direction? The best way to think about this is by using an analogy:
Imagine a line of foot soldiers marching together in a straight line. In the soldiers’ path in front of them is a line drawn on the ground at an angle to the path of soldiers. The speed of the soldiers on the beginning side of the line is regular marching pace, but the speed on the other side of the line must be very slow – they must take baby steps. When the first soldier reaches the line, he immediately starts taking baby steps, but the others are still marching. One by one the soldiers slow down as they reach the line, but the ones that reached the line later have traveled farther forward. The result is that the line of soldiers swings around a certain amount. If you imagine that the wavefront of the EM signal is the line of marching soldiers, and that the line on the ground is a boundary between different media, then you can see how an EM wave will bend when it crosses the boundary.
There is one exception to the bend, and that occurs if the wave hits a boundary between media at right angles. If the wave hits the boundary straight on, then no bending occurs; if you go back to the foot soldier analogy, all of the soldiers would begin taking baby steps at exactly the same time, so the line of soldiers will not bend.
Refraction is very apparent with the EM wave known as light. If you look at an object such as a straight straw that is halfway immersed in water, the straw will look as though it is bent or broken as in the following picture:
Diffraction is the bending of an electromagnetic wave around an object that stands in its way. When an object partially obstructs an electromagnetic wave, the wave will slow down where it runs into the object, and the entire wave will bend around the object. The result is a “diffraction pattern” in the wave where the original wave and the new bent wave interfere with each other.
The following picture shows the diffraction of light around the silk in a spiders web. As the white light from behind the web hits the strands of silk (which are only a couple of times thicker than the wavelength of the light), some of the light is reflected, some is absorbed, and some is diffracted. The different beams of light created from the interaction with the web interfere with each other and create the rainbow of colours you see in the picture.
Scattering is the result of an EM wave striking a rough surface or a large number of small objects. Waves will be reflected in all different directions resulting in destruction of the original signal
Absorption occurs when an EM wave strikes a surface, and all or a large part of the signal is absorbed by the material. Since the energy of the signal is absorbed by the material, the signal is lost.