1. Korea University Ubiquitous LAB. Chapter 2. RF physics Ph.D Chang-Duk Jung

2. 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

3. Korea University Ubiquitous LAB. Radio frequency vs. wavelength

4. 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

5. 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

6. Korea University Ubiquitous LAB. Factors affecting RF signal (cont’d) In a medium Distortion - Change in signal attribute Noise - Unwanted signal

7. 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

8. 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

9. Korea University Ubiquitous LAB. Antenna characteristics Polarization Impedance Voltage standing wave ratio (VSWR) Resonance frequency Directivity Gain Beamwidth

10. 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

11. 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

12. Korea University Ubiquitous LAB. Isotropic radiator

13. Korea University Ubiquitous LAB. Beamwidth

14. 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

15. Korea University Ubiquitous LAB. Radiated power output Effective radiated power Interrogator transmit power Transmission lines Antenna gain Link margin

16. 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

17. 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

18. 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

19. Korea University Ubiquitous LAB. Transmission lines (cont’d) coaxial cables

20. 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

21. 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 L margin = TX power + TX ant gain + RX ant gain – RSL

22. 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

23. 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