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