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Chapter 2. RF physics

Ph.D Chang-Duk Jung

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RF propagation

• Radio frequency propagation• Defined as the wireless transmission of

radio waves from one place to another• By using RF propagation, you can transmit

information between a reader and a tag

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Radio frequency vs. wavelength

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Radio frequency vs. wavelength (cont’d)

• Calculate the wavelength and frequency

• ƒ is the frequency• с is the speed of light(approximately 300,000,000 )

• λ is the wavelength

c

f

sm

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Factors affecting RF signal

• In a non-perfect free space• Free space loss • Attenuation• Scattering• Reflection• Refraction• Diffraction• Absorption• Superposition

• Phase• Standing wave

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Factors affecting RF signal (cont’d)

• In a medium• Distortion

- Change in signal attribute

• Noise- Unwanted signal

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Communication methods

• Communication between the reader and the tag occurs through a process called coupling– Inductive coupling– Electromagnetic backscatter coupling– Close coupling

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Antenna field performance

• The importance of understanding antennas– Both tags and readers use their antennas to

communicate with each other– To ensure a successful communication

between readers and tags, it is important to understand the characteristics of an antenna

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Antenna characteristics

• Polarization• Impedance • Voltage standing wave ratio (VSWR)• Resonance frequency • Directivity • Gain• Beamwidth

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Antenna characteristics (cont’d)

• Polarization– Direction of oscillation of the electromagnetic waves

• Impedance – Measure of resistance to an electrical current when a

voltage is moved across it

• Voltage standing wave ratio (VSWR)– The ratio of the maximum RF voltage to the minimum RF

voltage in a standing wave pattern

• Resonance frequency – Related to the electrical length of the antenna

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Antenna characteristics (cont’d)

• Directivity– The ability of an antenna to focus in a particular

direction while transmitting or receiving energy

• Gain– The ratio of the power needed for an antenna to produce

the same field strength in a specific direction

• Beamwidth – It is the angle between two half-power (3 dB) points of

the main lobe in the antenna pattern – This angle is defined as a beamwidth when referenced

to the peak effective radiated power of the main lobe

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Isotropic radiator

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Beamwidth

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Reflective and absorptive materials

• Absorptive materials– liquids, copy paper, and frozen items

• Reflective/refractive materials– metals, foil bags, and anti-static bags

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Radiated power output

• Effective radiated power• Interrogator transmit power• Transmission lines • Antenna gain• Link margin

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Effective radiated power(ERP)

• It is the output of an RFID reader antenna • It is used for RF power accounting • It includes the losses in the transmission

lines, and the gain of the antenna• ERP = RF power (dBm) – cable loss (dB) +

antenna gain (dBi)

continued

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Interrogator transmit power

• The actual amount of power of the RF energy that an interrogator produces at its output

• Calculate the power by using dBm, decibels in units of milliwatts (mW)

• To convert dBm to mW, use the equation – dBm=10×log (power in milliwatts)– For example, a reader transmitting energy at

1000 mW would be 30 dBm

continued

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Transmission lines

• A transmission line is the material medium for transmitting energy– Coaxial cables– Impedance of a transmission line– Cable loss– Return loss

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Transmission lines (cont’d)

coaxial cables

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Antenna gain

• The gain of an antenna is equal to 10×log(power out/power in) and is measured in decibels

• The gain of an antenna is directly related to antenna aperture

• The antenna aperture is tuned to the frequency of the reader antenna

continued

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Link margin

• Link margin is a way of quantifying equipment performance – Transmit power– Tansmit antenna gain– Receive antenna gain– Minimum received signal strength or level

Lmargin = TXpower + TXant gain + RXant gain – RSL

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Unit summary

• Compared radio frequency with wavelength, discussed the factors affecting RF signals, and also discussed the communication methods

• Discussed antenna characteristics and identify reflective and absorptive materials

• Calculated ERP

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Review questions

• Define effective radiated power and what constituents influence it?

• List the phenomena that affect the propagation of an RF signal.

• Given the frequency of an RF wave, which one of the following formulas can you use to calculate its wavelength? A. f = λ/c B. f = d/λ C. f = c/λ D. f = π /λ