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Figure 2-6 shows radio wave reflection and multipath created due to reflection
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Metals reflect radio waves while aqueous liquids absorb radio waves
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Diffraction
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Radio signals may also undergo diffraction, which occurs when signals encounter an obstacle and tend to travel around them Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges) The secondary waves resulting from the obstructing surface are present throughout the space, even behind the obstacle, giving rise to a bending of waves around the obstacle, even when a LOS path does not exist between transmitter and receiver At high frequencies, diffraction, like reflection, depends on the geometry of the object, as well as the amplitude, phase, and polarization of the incident wave at the point of diffraction
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Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of such objects per unit volume is large Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel When a radio wave impinges on a rough surface, the reflected energy is diffused
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Radio wave reflection
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CHAPTER 2 The RF in RFID
in all directions due to scattering Figure 2-7 shows radio wave diffraction and scattering
Refraction
Refraction is a change in direction and speed of travel when radio waves move from a medium of one refractive index to another with a different refractive index This is similar to the effect you see when you place a pencil at an angle in a glass of water The pencil seems to bend This phenomenon does not have much importance in RFID application
Fading
Another term frequently used in the RF arena is fading Fading is the variation of signal strength with time It happens due to time dependent random changes in multipath A fade is a constantly changing, three-dimensional phenomenon Fade zones tend to be small, multiple areas of space within an IZ that cause periodic attenuation of a received signal In other words, the received signal strength will fluctuate downward, causing a momentary, but periodic, degradation in radio signal quality One may visualize signal strength with the IZ as a Swiss cheese, where holes in the cheese represent fade zones The fading effect grows worse at greater distances from the interrogator antenna It is impossible to estimate signal strength accurately at various points in an IZ due to the random nature of fades
dB and dBm
Signal levels in radio technology vary by a large magnitude, requiring use of very large and very small numbers To overcome the inconvenience of handling large
FIGURE 27
Indoor radio wave propagation effects
MIKE MEYERS COMPTIA RFID+ CERTIFICATION PASSPORT
numbers, engineers use decibel (dB) to describe signal levels A decibel is one tenth of a Bel A Bel is a ratio of two power levels and is named in honor of Alexander Graham Bell; that s why the B is capitalized Bel and decibel are defined as follows:
P1 Bel = log10 ; P2 P1 dB = 10 log10 P2
P1 and P2 are power levels with the same units dB is a logarithmic measure, and it produces easy to handle numbers for large-scale variations in signals It is useful because system gains and losses can be calculated by adding and subtracting numbers Remember high school algebra: log (a b) = log a + log b and log (a/b) = log a log b Like percentage, a ratio of two quantities, dB has no units Let s consider antenna gain and cable losses, both of which are defined later in this chapter Gain, a ratio of output to input where output is greater than input, is always greater than 1 and thus has a positive dB value Loss, again a ratio of output to input but where output is less than input, has a value less than 1 and thus has a negative dB value The log of numbers greater than 1 is positive and for numbers less than 1 it is negative To make this concept clearer, here are some examples of dB usage:
Gain has positive value while loss has negative value Every time you double (or halve) the power level, you add (or subtract) 3 dB to (or from) the power level A 10 dB gain/loss corresponds to a ten-fold increase/decrease in signal level A 20 dB gain/loss corresponds to a hundred-fold increase/decrease in signal level If a cable has 2 dB loss (a typical value for the most antenna cables), it will lose 37 percent of its signal by the time it gets to the other end If a cable has 20 dB loss, it will lose 99 percent of its signal by the time it gets to the other end
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