# 'How to Cheat': The Physics of RFID--Part IV

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Editor's Note: Miss Part I? Part I Part II, and Part III

**The Travel Adventures of RF Waves**

When an RF wave travels from the transmitter to the receiver, it can be affected by various factors discussed in the following sections.

*Absorption *

When an RF wave strikes a material object, some of its energy will be absorbed by the object depending on the frequency of the wave and the material of the object. Water and objects containing water, such as liquid products, wood, and food, are especially good at absorbing RF waves. UHF waves, due to their shorter wavelengths, are more susceptible to absorption than LF and HF waves.

*Attenuation*

Attenuation in general means a decrease in the amount of something. In RF physics, it means the decrease in amplitude (strength) of the RF signal (wave). Attenuation is the opposite of amplification. It can occur when the signal is traveling from the source to the antenna through the transmission line or during propagation from the transmitter antenna to the receiver antenna. It can occur due to a number of reasons, such as absorption and dispersion.

*Dielectric Effects*

Dielectric effects refer to a medium's capacity to retain charge. As a result, an electromagnetic wave traveling through a dielectric medium is slowed down. The strength of this effect is measured by a quantity called the dielectric constant whose value is different for different materials. Dielectric effects also detune the signal--that is, shift its frequency to a value that is not in resonance with the frequency for which the antenna is tuned.

*Diffraction*

Diffraction refers to the bending of an EM wave when it strikes the sharp edges or when it passes through narrow gaps. Due to diffraction, the receiver antenna will not receive the wave energy that it would have otherwise.

*Free Space Loss *

If the space through which the RF wave travels is free of all obstructing material and as a result there are no affects such as absorption, reflection, refraction, and scattering, there will still be some loss in signal strength, called free space loss (FSL). This loss occurs simply due to the way a wave travels. An RF wave transmitted from a source travels in all directions in the form of an expanding sphere (called a wavefront), and therefore the power density (power per unit of surface area of this sphere) decreases as a result of this spreading out. If R is the distance from the transmitter antenna, the surface area of the sphere with radius R around the antenna is 4πR2. Therefore, the power density (and hence the signal strength) of a propagating wave at a point in space is inversely proportional to the square of distance of this point from the transmitter antenna. In other words, the free space loss will be directly proportional to the square of this distance. In addition, the loss is inversely proportional to the square of the wavelength of the propagating wave.

The FSL is measured using the following equation:

FSL = (4πRk/λ)^{2}

FSL (dB) = 10 log (4πRk/λ)^{2} 10 log (4πRfk/c)^{2}= 20 log (4πk/c) + 20 log R + 20 log f

=>

FSL (dB) = 20 log R = 20 log λ)< + K

where:

K = 20 log (4πk/c)

and k is a constant that depends on the communication link and the units used for distance and wavelength.

*Interference*

Interference is the interaction between two waves. The signal wave can interact with other waves that it meets on the way to its destination. A resultant wave is produced as a result of interference, and the receiver receives the resultant wave. The interference can be constructive, in which case the resultant wave has a larger amplitude, or destructive, in which case the resultant wave has a smaller amplitude than the original wave.