Wireless+Networking+in+the+Developing+World_Part2

Wireless+Networking+in+the+Developing+World_Part2 -...

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Unformatted text preview: amplitude amplitude wavelength ( ¡ ) wavelength ( ¡ ) time: 1 second Figure 2.1: Wavelength, amplitude, and frequency. For this wave, the frequency is 2 cycles per second, or 2 Hz. Electromagnetic forces Electromagnetic forces are the forces between electrical charges and cur- rents. Our most direct access to those is when our hand touches a door handle after walking on synthetic carpet, or brushing up against an electrical fence. A more powerful example of electromagnetic forces is the lightning we see during thunderstorms. The electrical force is the force between electri- cal charges. The magnetic force is the force between electrical currents. Electrons are particles that carry a negative electrical charge. There are other particles too, but electrons are responsible for most of what we need to know about how radio behaves. Let us look at what is happening in a piece of straight wire, in which we push the electrons from one and to the other and back, periodically. At one mo- ment, the top of the wire is negatively charged - all the negative electrons are gathered there. This creates an electric f eld from plus to minus along the wire. The next moment, the electrons have all been driven to the other side, and the electric f eld points the other way. As this happens again and again, the electric f eld vectors (arrows from plus to minus) are leaving the wire, so to speak, and are radiated out into the space around the wire. What we have just described is known as a dipole (because of the two poles, plus and minus), or more commonly a dipole antenna . This is the simplest form of omnidirectional antenna. The motion of the electric f eld is commonly referred to as an electromagnetic wave . Let us come back to the relation: Speed = Frequency * Wavelength Chapter 2: A Practical Introduction to Radio Physics 11 In the case of electromagnetic waves, the speed is c , the speed of light. c = 300,000 km/s = 300,000,000 m/s = 3*10 8 m/s c = f * ¡ Electromagnetic waves differ from mechanical waves in that they require no medium in which to propagate. Electromagnetic waves will even propagate through the vacuum of space. Powers of ten In physics, math, and engineering, we often express numbers by powers of ten. We will meet these terms again, e.g. in Giga-Hertz (GHz), Centi-meters (cm), Micro-seconds ( ¡ s), and so on. Powers of Ten Nano- 10-9 1/1000000000 n Micro- 10-6 1/1000000 μ Milli- 10-3 1/1000 m Centi- 10-2 1/100 c Kilo- 10 3 1 000 k Mega- 10 6 1 000 000 M Giga- 10 9 1 000 000 000 G Knowing the speed of light, we can calculate the wavelength for a given fre- quency. Let us take the example of the frequency of 802.11b wireless net- working, which is f = 2.4 GHz = 2,400,000,000 cycles / second wavelength lambda ( ¡ ) = c / f = 3*10 8 / 2.4*10 9 = 1.25*10-1 m = 12.5 cm Frequency and wavelength determine most of an electromagnetic wave ¢ s be- havior, from antennas that we build to objects that are in the way of the networks we intend to run. They are responsible for many of the differences between dif- 12 Chapter 2: A Practical Introduction to Radio Physics ferent standards we might be choosing. Therefore, an understanding of the basic ferent standards we might be choosing....
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