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ISM-Band and Short Range Device Antennas

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ISM-Band and Short Range Device Antennas Application Report SWRA046A - March 2005 − Revised August 2005 ISM-Band and Short Range Device Antennas Matthew Loy / Iboun Sylla (Editors) High-Speed and RF ABSTRACT This application report discusses antenna fundamentals and the various types of antennas used for short range devices. Fundamentals are presented along with practical design principals. Contents 1 Antenna Basics . 3 1.1 Fundamental Definitions. 3 1.2 Antenna Characteristics. 4 1.3 Reflection, Matching, and Tuning. 8 2 Antenna Types and Their Features. 11 2.1 Half-Wave Dipole . 11 2.2 Quarter-Wave Monopole. 13 2.3 Transversal Mode Helical Antenna. 15 2.4 Small Loop Antennas. 18 2.5 Rules of Thumb for the Antenna Design. 22 2.6 Antenna on the TRF6901 Reference Design. 23 3 RF Propagation . 24 3.1 Path Loss . 24 3.2 Multipath Propagation Effects. 26 4 Examples and Measurements. 28 4.1 Test Module Schematics. 28 4.2 PCB Monopole Antenna Module. 28 4.3 Transversal Mode Helical Antenna Module. 31 4.4 Loop Antenna Module. 32 5 References . 35 Appendix A Antenna Manufacturers. 36 List of Figures 1 Isotropic Radiator . 5 2 Antenna Equivalent Circuit . 6 3 Reflection at a Discontinuity. 8 4 Standing Waves Due to Reflection. 9 5 Series and Parallel Capacitors and Inductors in the Smith Chart. 10 6 Half-Wave Dipole Antenna. 11 Trademarks are the property of their respective owners. 1 SWRA046A 7 Radiation Pattern of a Half-Wave Dipole Antenna. 12 8 Building Up the Quarter-Wave Monopole. 13 9 Matching of a Short Loaded Stub Antenna. 15 10 Loaded Stub PCB Antenna With Matching Components. 15 11 Inverted-L Antenna and Inverted-F Antenna. 16 12 Helix Antenna on a Ground Plane. 17 13 Small Loop Antenna With Differential Feed. 18 14 Single-Ended Loop Antenna. 18 15 Equivalent Schematics of the Small Loop Antenna in Figure 13. 19 16 Efficiency of Small Loop Antennas for 5% Tolerance. 20 17 Example of a Tapped PCB Loop Antenna. 21 18 PCB Antenna Layout on the TRF6901 Reference Design. 23 19 Fresnel Zone Between Transmitter and Receiver. 24 20 Free Space Path Loss (PL) For Four Short Range Frequency Bands. 25 21 Multipath Propagation . 26 22 ISI Due to Multipath Propagation. 27 23 Test Module Schematics . 28 24 Layout of the PCB Monopole Antenna. 29 25 Vertical Radiation Pattern of the Stub Module (Upright). 29 26 Horizontal Radiation Pattern of the Stub Module (Flat). 30 27 Horizontal Radiation Pattern of the Stub Module Close to the Human Body. 30 28 Layout of the SMT Helical Antenna. 31 29 Horizontal Radiation Pattern of the Helix Module. 31 30 Horizontal Radiation Pattern of the Helix Module Close to the Human Body. 32 31 PCB Loop Antenna . 33 32 Radiation Pattern of the Loop Antenna Module. 34 33 Radiation Pattern of the Loop Antenna Module Close to the Human Body. 35 List of Tables 1 Length of Half-Wave Dipoles and Quarter-Wave Monopoles in Free Space and On the PCB. 14 2 Features of Short Range Antennas. 22 3 Measured Path Loss Coefficients and Standard Deviations /5/. 25 4 Typical Floor Attenuation Factors /5/. 26 5 Typical Values of Partition Losses. 26 2 ISM-Band and Short Range Device Antennas SWRA046A 1 Antenna Basics 1.1 Fundamental Definitions Antennas are the connecting link between RF signals in an electrical circuit such as a PCB and an electromagnetic wave propagating in the transmission media between the transmitter and the receiver of a wireless link. In the transmitter, the antenna transforms the electrical signal into an electromagnetic wave by exciting either an electrical or a magnetic field in its immediate surroundings, the near field. Antennas that excite an electrical field are referred to as electrical antennas; antennas exciting a magnetic field are called magnetic antennas correspondingly. The oscillating electrical or magnetic field generates an electromagnetic wave that propagates with the velocity of light c. The speed of light in free space c0 is 300000 km/s. If the wave travels in a dielectric medium with the relative dielectric constant εr, the speed of light is reduced to: c c + o Ǹå r We can calculate the wavelength from the frequency f of the signal and the speed of light c using the formula: l + c f Using common units, the equation: wavelength in meters + 300 frequency in MHz is often used for the wavelength in free space. If the wave travels in a dielectric medium, for instance in the PCB material, the wavelength has to be divided by the square root of εr. We can distinguish three field regions where the electromagnetic.
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