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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 interval 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:
c=
1
me
(1.2)
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
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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 twoway 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
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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.
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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
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