AN4190 Application note Antenna selection guidelines By Placido De Vita Introduction The antenna is a very important component of communication systems. By definition, an antenna is a device used to transform an RF signal, travelling on a conductor, into an electromagnetic wave in free space (transmit mode), and to transform an RF electromagnetic wave into an electrical signal (receive mode). The choice of antenna is very important for a transmitting - receiving communication system. The antenna must be able to radiate or receive efficiently so the power supplied is not wasted. This application note describes the most important parameters to consider when deciding what kind of antenna to use in a short range device application. In the first section of this application note the antenna theory is covered. The main antenna parameters such as radiation pattern, gain, impedance matching, bandwidth, size and others are discussed. In the second part of this document different antenna types are presented. November 2012 Doc ID 023812 Rev 1 1/29 www.st.com Contents AN4190 Contents 1 Antenna theory . 4 1.1 Antenna and radiation pattern . 4 1.1.1 Isotropic, directional and omnidirectional patterns . 5 1.1.2 Principal patterns . 5 1.1.3 Field regions . 6 1.1.4 Radiation pattern lobes . 7 1.2 Radiation density and intensity . 8 1.3 Directivity . 8 1.4 Antenna gain . 8 1.5 Antenna efficiency . 9 1.6 Antenna bandwidth . 9 1.7 Antenna polarization . 10 1.8 Input impedance . 12 1.9 Effective isotropic radiated power . 13 2 Antennas for low power applications . 14 2.1 Linear antenna . 14 2.1.1 Dipole antenna . 14 2.1.2 Monopole antenna . 15 2.1.3 Loop antenna . 17 2.1.4 Spiral antenna . 18 2.1.5 Helical antenna . 20 2.2 Microstrip patch antenna . 21 2.3 Ceramic antenna . 23 2.4 Slot antenna . 24 3 Antenna advantages and disadvantages . 25 4 Reference . 27 5 Revision history . 28 2/29 Doc ID 023812 Rev 1 AN4190 List of figures List of figures Figure 1. Coordinate system for antenna analysis. 4 Figure 2. x - y plane omnidirectional antenna pattern . 5 Figure 3. E and H plane radiation patterns . 6 Figure 4. Field regions . 6 Figure 5. Near and far field regions . 7 Figure 6. Vertical, horizontal,3 and elliptic polarization . 11 Figure 7. Circular polarization . 11 Figure 8. Half-wave dipole antenna voltage and current distribution . 14 Figure 9. Half-wave dipole antenna model and radiation patterns. 15 Figure 10. Quarter-wave monopole antenna design . 16 Figure 11. Inverted-L antenna . 17 Figure 12. Inverted-F antenna . 17 Figure 13. Spiral antenna. 19 Figure 14. Spiral antenna 3D radiation pattern . 19 Figure 15. Helical antenna implementation . 21 Figure 16. Helical antenna axial mode 3D radiation pattern . 21 Figure 17. Rectangular microstrip antenna . 22 Figure 18. 5.8 GHz patch antenna implementation and characteristics . 23 Figure 19. Ceramic antenna . 24 Figure 20. Slot antenna implementation and 3D radiation pattern. 24 Doc ID 023812 Rev 1 3/29 Antenna theory AN4190 1 Antenna theory Antenna performance parameters and the language used to describe antennas can be confusing and sometimes even misleading. While much can be said in general about what constitutes a good antenna, most designs reflect some sort of compromise or trade-off between the various desirable attributes because antenna design involves conflicting goals. Therefore, it is crucial that antenna specifications are reviewed in light of the intended application. A more complete and accurate understanding of the terminology associated with antennas allows the most appropriate antenna for a given task to be specified. A great deal of effort has been made over the years to standardize antenna terminology. The “de facto” standard is the IEEE Standard Definitions of Terms for Antenna, so in this document the main antenna parameters as defined in this standard are given. The purpose of this document is to give a brief and easy description of antenna parameters. For a complete and rigorous description of antenna behavior, the reader is requested to refer to the book “Antenna theory: analysis and design” [1]. 1.1 Antenna and radiation pattern An antenna radiation pattern is defined in the IEEE standard as “the spatial distribution of a quantity which characterizes the electromagnetic field generated by an antenna”. In other words, an antenna radiation pattern or antenna pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates, as shown in Figure 1. Radiation properties include power flux density, radiation intensity, field strength, and directivity phase or polarization. Figure 1. Coordinate system for antenna analysis AM14796v1 In most cases, the radiation pattern is determined in the far field region and is represented as a function of the directional coordinates. That is, in spherical coordinates the distribution of the quantity over Θ and Φ for fixed radius (see Figure 1). The interpretation of an antenna's radiation pattern can become problematic because of the three-dimensional nature of the information. The complexity of a three-dimensional plot of a 4/29 Doc ID 023812 Rev 1 AN4190 Antenna theory radiation pattern can sometimes obfuscate details. In practice, a three-dimensional plot of a radiation pattern is of limited value in presenting quantitative information. Therefore, two- dimensional “cuts” of the radiation pattern are often presented. In particular, cuts in the so- called E and H planes are often presented. 1.1.1 Isotropic, directional and omnidirectional patterns An isotropic radiator is defined as a “hypothetical” lossless antenna having equal radiation in all directions. Although it is ideal and not physically realizable, it is taken as a reference for expressing the directive properties of actual antennas. A directional antenna is one having the property of radiating or receiving electromagnetic waves more effectively in some directions than in others. An omnidirectional antenna is defined as one having an essentially non-directional pattern in a given plane and a directional pattern in any orthogonal plane. An omnidirectional pattern is a special type of directional pattern. An example of an antenna with an omnidirectional radiation pattern in the x - y plane is shown in Figure 2. Figure 2. x - y plane omnidirectional antenna pattern AM14797v1 1.1.2 Principal patterns For a linearly polarized antenna, performance is often described in terms of its principal E and H plane patterns. The E plane is defined as the plane containing the electric field vector and the direction of maximum radiation. The H plane is the plane containing the magnetic field vector and the direction of a maximum radiation. The principal E and H planes are orthogonal planes. It is often sufficient to examine only E and H plane cuts of the three-dimensional radiation pattern. An example of E and H plane radiation patterns is shown in Figure 3. Doc ID 023812 Rev 1 5/29 Antenna theory AN4190 Figure 3. E and H plane radiation patterns AM16434V1 1.1.3 Field regions Various parts of a radiation pattern are referred to as lobes, which may be sub-classified into major or main, minor, side and back lobes. A radiation lobe is a portion of the radiation pattern bounded by regions of relatively weak radiation intensity. Figure 4 demonstrates a symmetrical three-dimensional polar pattern with a number of radiation lobes. A major lobe (also called main beam) is defined as the radiation lobe containing the direction of maximum radiation. A minor lobe is any lobe except the major lobe. Minor lobes usually represent radiation in undesired directions and they should be minimized. The beamwidth is the angle, expressed in degrees, between the half power (-3 dB) points of the main lobe, when referenced to the peak effective radiated power of the main lobe. Figure 4. Field regions AM16435V1 6/29 Doc ID 023812 Rev 1 AN4190 Antenna theory 1.1.4 Radiation pattern lobes The space surrounding an antenna is usually sub-divided into three regions: ● Reactive near field ● Radiating near field, also called Fresnel region ● Far field, also called Fraunhofer region. In the reactive near field region of an antenna, the non-radiating field components dominate. The term reactive near field arises from the fact that for a non-resonant antenna such as an electrically small dipole, reactive power circulates between the reactive near field and the source, an external matching network, or both. The strict IEEE definition is “that portion of the near field region immediately surrounding the antenna, wherein the reactive field dominates”. Therefore, for dipole-like antennas, the energy in this region is predominantly either electric or magnetic. For electrically small antennas, the reactive near field is taken to extend to a distance of approximately R ~ λ / 2π from the antenna. In the radiating near field, the radiation fields predominate but the angular field distribution is dependent on the distance from the antenna. The strict IEEE definition is “that portion of the near field region of an antenna between the far field and the reactive portion of the near field region, wherein the angular field distribution is dependent upon distance from the antenna”. If the antenna is large, compared to a wavelength, the outer boundary of the radiating near field is taken to be R ~ 2D2 / λ. The far field region of an antenna is the region surrounding an antenna which is sufficiently far from the antenna such that only the radiating field components are significant. In other words, the far field is that region of the field of an antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region.