korea university ubiquitous lab. chapter 2. rf physics ph.d chang-duk jung
TRANSCRIPT
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Korea University Ubiquitous LAB.
Chapter 2. RF physics
Ph.D Chang-Duk Jung
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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
Radio frequency vs. wavelength
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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
Factors affecting RF signal (cont’d)
• In a medium• Distortion
- Change in signal attribute
• Noise- Unwanted signal
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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
Antenna characteristics
• Polarization• Impedance • Voltage standing wave ratio (VSWR)• Resonance frequency • Directivity • Gain• Beamwidth
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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
Isotropic radiator
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Korea University Ubiquitous LAB.
Beamwidth
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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
Radiated power output
• Effective radiated power• Interrogator transmit power• Transmission lines • Antenna gain• Link margin
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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
Transmission lines (cont’d)
coaxial cables
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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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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Korea University Ubiquitous LAB.
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