Wave Crests Heinrich Hertz Speed of Light Radio waves travel through space at a speed of approximately 300,000,000 meters per second. Electricity travels through a wire and light travels through fiber optics at a speed of approximately 200,000,000 meters per second. Electromagnetic Waves
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Radio waves travel through space at a speed of approximately 300,000,000 meters per second.
Electricity travels through a wire and light travels through fiber optics at a speed of approximately 200,000,000 meters per second.
Television & FM Radio
802.11b & 802.11g Wireless LANs
Visible Light & X-Rays
A radio wave has a wavelength of 2 meters. Calculate the frequency in hertz.
f = C
f (hertz) = 300,000,000 (meters/second)
2 meters (wavelength or l)
f = 150,000,000 hertz
A radio wave has a frequency of 10,000,000 hertz. Calculate the wavelength in meters. Since we know the formula for calculating frequency, we can solve it for the wavelength as follows:
l = C
l (meters) = 300,000,000 (meters/second)
l = 30 meters
Fourier's theorem states that any complex wave is the sum of a fundamental sine wave and its multiples (also called its harmonics).
The same idea, but in more detail:
Fourier's theorem states that any complex waveform is the sum of sinusoids (sine waves, the simplest kind of waveform). The complex waveform will be composed of a fundamental frequency of a sine wave, to which are added other sine waves of various amplitudes that are all multiples of the fundamental frequency.
green: y = sin(x) + 0.333333 sin(3x) + 0.2 sin(5x) + 0.142857 sin(7x)
pink + white + red + cyan
yellow: y = sin(x) + 0.333333 sin(3x) + 0.2 sin(5x)
pink + white + red
blue: y = sin(x) + 0.333333 sin(3x)
pink + white
pink: y = sin(x)
Simplex mode, where the transmission always travels in one direction from a transmitter to a receiver.
Half-duplex mode, where communications travel in both directions, but not at the same time.
Full-duplex mode, where the information travels over a radio link in both directions at the same time. Full-duplex mode requires two separate radio carriers operating on different frequencies.
Line of sight waves
The frequency spreads of the three categories have some overlap. As an example, frequencies between 500 kHz and 1.5 MHz (the AM radio band) are classified as MF and exhibit some of the characteristics of both ground and sky waves under certain conditions.
They can travel very long distances.
They are used by the military for communicating between land-based stations or aircraft and submarines.
Ground waves are dependable. They are relatively immune to atmospheric interference or propagation variations.
They require very large antenna structures.
They are expensive.
They are limited in the amount of information (bits per second) they can carry.
Outside of military and government applications, there is no practical commercial
wireless application for ground wave frequencies below 300 kHz.
They support long distance communications with relatively modest transmitter power and antenna requirements.
They can be used for point-to-multipoint voice service or low-speed radio teletype services on a global scale.
They support an economical maritime safety communications service.
Sky wave propagation frequencies extend over a relatively large portion of the spectrum (3 MHz to 30 MHz or approximately 27 MHz of bandwidth).
Sky wave propagation depends upon the ionosphere, which is not a stable medium.
The frequencies for sky waves are not suitable for carrying high-capacity data transmission circuits.
The sky wave frequencies are not suitable for most emerging wireless technologies such as cellular and wireless broadband applications.
They have a limited range, which limits co-channel interference.
They can be limited to a very small transmission angle with parabolic antennas.
The have a huge spectrum (37 MHz to the currently highest useable frequencies), which can be used in multiple ways to carry large amounts of information.
At some frequencies they are sensitive to atmosphere conditions, such as rain.
They require a large amount of equipment to cover a large area, such as multiple cell phone towers.
The square looking cell phone antenna is really an array of a large number of small antennae, all electronically controlled. They achieve a very tightly controlled cell telephone beam, that can be changed and re-directed dynamically..
Multiple transmitters are electronically controlled so that a signal is beamed directionally because multiple copies of the signal are broadcast slightly out of phase. The multiple copies reinforce each other in some directions and interfere with each other in other directions.
D = distance between the antennas (miles)
F = frequency (GHz)
Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs
Figure 3-18: Absorption
Figure 3-20: Scattering
Figure 3-19: Reflection
Total Reflection Loss to scattering
When the angle of incidence is so The difference between a mirrorshallow that refraction can’t occur. and a flat surface is the diffusion.
Figure 3-21: Refraction
The change in direction of a wave due to a change in its speed when a wave passes from one medium to another.
Figure 3-22: Diffraction
The bending, spreading and interference of waves passing by an object or through an aperture
Figure 3-23: VSWR
Co-channel interference is caused by another transmitter using the same frequencies.
Adjacent channel interference is caused by a a transmitter (possibly the same transmitter) using either the frequencies above or below the signal frequencies.
Elecromagnetic Interference (EMI) is a general term covering any degradation of a signal because of other electromagnetic fields mixing with it.