1 Introduction

1 Introduction A changing magnetic field will induce a changing electric field and vice-versa. These changing fields form electromagnetic waves. Electromagnetic (EM) waves differ from mechanical waves in that they do not require a medium to propagate through. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space. During the 1860’s and 1870’s, James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as “Maxwell’s Equations.” Electromagnetic waves are a complex phenomenon because they can propagate through vacuum without the need for a material medium, they simultaneously behave like waves and like particles (Dirac 1927, Einstein 1951), and they are intrinsically linked to the behaviour of the space-time continuum (Einstein 1916). It can be shown that magnetic fields appear through relativistic motion of electric fields, which is why electricity and magnetism are so closely linked (Chappell, et al. 2010). It has even been suggested that electromagnetic phenomena may be a space-time phenomenon, with gravitation being the result of space-time curvature (Einstein 1916) and electromagnetic behaviour being the result of space-time torsion (Evans 2005). An EM wave is described in terms of its: 1. Frequency (f), which is the number of waves that pass a fixed point in an inter- val of time. Frequencies are usually measured as waves per second or cycles per second, which is given the unit of Hertz (Hz); 2. Wavelength (λ), which is the distance between successive crests or troughs in the wave. If frequencies are measured in Hertz, then wavelengths are measured in metres (m); and 3. Speed (c), which is measured in metres per second and is determined by the electrical and magnetic properties of the space through which the wave travels. These three properties are related by the equation: c = lf (1.1) The speed of the electromagnetic wave is determined by: = 1 (1.2) c me where e is the electrical permittivity of the space in which wave exists and m is the magnetic permeability of the space in which the wave exists. Electromagnetic waves can be of any frequency; therefore the full range of possible frequencies is referred to as the electromagnetic spectrum. Although Maxwell’s Introduction 3 Equations do not indicate any limits on the spectrum, the known electromagnetic spectrum extends from frequencies of around f = 3 × 103 Hz (λ = 100 km) to f = 3 × 1026 Hz (λ = 10-18 m). This covers everything from ultra-long radio waves to high-energy gamma rays (International Telecommunication Union 2004). Electromagnetic waves can be harnessed to: transmit information; acquire information from a medium; or transmit energy. The first category of applications includes: terrestrial and satellite communication links; the global positioning system (GPS); mobile telephony; and so on (Commonwealth Department of Transport and Communications 1991). The second category of applications includes: radar; radio-astronomy; microwave thermography; and material permittivity measurements (Adamski and Kitlinski 2001). The third category of applications is associated with microwave heating and wireless power transmission. In these cases there is usually no signal modulation and the electromagnetic wave interacts directly with solid or liquid materials. Radio frequency (RF) is a term that refers to a portion of the electromagnetic spectrum that can be easily generated using an alternating current (AC). If an AC current is fed into a suitable structure such as an antenna, an electromagnetic (EM) field is generated. These EM fields will usually propagate through space the same as any other form of electromagnetic radiation. Many devices make use of RF fields. Cordless and mobile telecommunication, radio and television broadcast stations, satellite communications systems, and two- way radio services all operate in the RF spectrum. Some wireless devices operate at infra-red (IR) or visible-light frequencies, whose electromagnetic wavelengths are far shorter than those of RF fields. The RF spectrum is divided into several ranges, or bands. With the exception of the lowest-frequency segment, each band represents an increase of frequency corresponding to an order of magnitude (power of 10). Table 1.1 depicts the eight bands in the RF spectrum, showing frequency and bandwidth ranges. The UHF, SHF and EHF bands constitute the microwave spectrum. Table 1.1: Radio Frequency spectrum Designation Abbreviation Frequencies Free-space Wavelengths Very Low Frequency VLF 9 kHz - 30 kHz 33 km - 10 km Low Frequency LF 30 kHz - 300 kHz 10 km - 1 km Medium Frequency MF 300 kHz - 3 MHz 1 km - 100 m High Frequency HF 3 MHz - 30 MHz 100 m - 10 m Very High Frequency VHF 30 MHz - 300 MHz 10 m - 1 m Ultra High Frequency UHF 300 MHz - 3 GHz 1 m - 100 mm Super High Frequency SHF 3 GHz - 30 GHz 100 mm - 10 mm Extremely High Frequency EHF 30 GHz - 300 GHz 10 mm - 1 mm 4 Introduction Microwave frequencies occupy portions of the electromagnetic spectrum between 300 MHz to 300 GHz. Because microwaves are also used in the communication, navigation and defence industries, their use in thermal heating is restricted to a small subset of the available frequency bands. A small number of frequencies have been set aside for Industrial, Scientific and Medical (ISM) applications (Table 1.2). All these frequencies interact to some degree with moist materials. All interactions between electromagnetic waves and the media that they encounter can be described by Maxwell’s equations for electro-magnetism. Table 1.2: ISM Frequency allocations (International Telecommunication Union 2004). Frequency Availability 6.78 MHz ± 15 kHz Subject to local acceptance 13.56 MHz ± 7 kHz World wide 27.12 MHz ± 163 kHz World wide 40.68 MHz ± 20 kHz World wide 433.92 MHz ± 870 kHz Region 1 only and subject to local acceptance 915.00 MHz ± 13 MHz Region 2 only with some exceptions 2.45 GHz ± 50 MHz World wide 5.8 GHz ± 75 MHz World wide 24.125 GHz ± 125 MHz World wide 61.25 GHz ± 250 MHz Subject to local acceptance 122.5 GHz ± 500 MHz Subject to local acceptance 245.0 GHz ± 1.0 GHz Subject to local acceptance The regions defined by the International Telecommunications Union(2004) are: – Region 1: Europe, Africa, the Middle East west of the Persian Gulf including Iraq, Russia, and Mongolia; – Region 2: The Americas, Greenland, and some of the Pacific Islands; – Region 3: Most of non-Russian Asia and most of Oceania. Some microwave and radio frequency technologies such as RFID, wireless sensor networks, microwave tempering of frozen meat, electromagnetic surveys of soil properties, GPS guidance of farm machinery, and radar terrain imaging have been “standard practice” in many agricultural industries for some time. Many new technologies are currently being explored in research institutions around the world. Knowledge of these technologies is critical for agriculturalists and engineers alike. References 5 Although RF and microwave technologies are becoming ubiquitous in most agricultural industries, there appears to be no single text that comprehensively covers the practice and theory behind these critical technologies. This book will provide a review of microwave and radio-frequency applications that have been considered for use in agriculture, and point out the advantages of some of the key applications. The principal purpose of the book is to bring to the attention of students and practitioners in the electrical, microwave/radio-frequency and agricultural industries those applications that have been studied so that practical use may be realised. This book is subdivided into four sections, with each section consisting of several individual chapters. The earlier chapters will provide an overview of innovations in agriculture and an introduction to electromagnetism. The second section will indicate how RF and microwave energy can be used to characterise agricultural and forestry materials. Other sections will focus on heating applications. Another section will explore how wireless systems can be used in agricultural systems. Some chapters will focus on the applications, while others will necessarily be more theoretical to provide the necessary background to the technology. References Adamski, W. and Kitlinski, M. 2001. On measurements applied in scientific researches of microwave heating processes. Measurement Science Review. 1(1): 199-203. Chappell, J. M., Iqbal, A. and Abbott, D. 2010. A simplified approach to electromagnetism using geometric algebra. Commonwealth Department of Transport and Communications. 1991. Australian Radio Frequency Spectrum Allocations. Commonwealth Department of Transport and Communications Dirac, P. A. M. 1927. The quantum theory of the emission and absorption of radiation. Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character. 114(767): 243-265. Einstein, A. 1916, Relativity: the Special and General Theory, Methuen & Co Ltd. Einstein, A. 1951. The Advent of the Quantum Theory. Science. 113(2926): 82-84. Evans, M. W. 2005. The Spinning and Curving of Spacetime: The Electromagnetic and Gravitational Fields in the Evans Field Theory. Foundations of Physics Letters. 18(5): 431-454. International Telecommunication Union. 2004. Spectrum Management for a Converging World: Case Study on Australia. International Telecommunication Union.

Load more