Shielding effectiveness of electromagnetic energy

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Aug 1, 2018 - In RF dielectric heating application, two most applicable wave ... Most common dielectric heating applications are microwave and radio ...
An ASABE Meeting Presentation DOI: https://doi.org/10.13031/aim.201801004 Paper Number: 1801004

Shielding effectiveness of electromagnetic energy from 50-ohm radio frequency heating system for disinfestation of stored grains R.J. Macana, T. T. Moirangthem, and O.D. Baik

Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Dr., Saskatoon, SK, Canada S7N 5A9

Written for presentation at the 2018 ASABE Annual International Meeting Sponsored by ASABE Detroit, Michigan July 29-August 1, 2018 ABSTRACT. Electromagnetic radiation shielding in 50-ohm radio frequency heating systems, also known as RF shielding is significant for the safety of the workers and the equipment. The understanding of the electromagnetic radiation and behavior of waves, frequency, wavelength, energy, electric fields, magnetic fields, and power density of radio frequency heater is presented in this paper. Radio frequency leakage composed of electric and magnetic fields. High exposure to these fields can be dangerous to people because of its thermal effects to the body, especially to thermolabile parts. Therefore, RF shielding is critically important and it can be assessed through its electric and magnetic fields intensity and power density. The electric fields, magnetic fields, and power density increase when increasing the power applied to the load and decrease when increasing the distance from the RF energy source. The aluminum metal sheet is preferred as RF shielding material for industrial level applications over expanded steel metal because of its high

conductivity and low resistance. However, steel can be used as an alternative to aluminum at a certain power level. Furthermore, it is suggested that in designing an RF shielding cage for high power applications: all sides should be covered; gaps and holes should be avoided; conductor materials around/inside the cage should be removed/grounded; cage should be grounded, and in good contact with all parts (bolts, joints, doors, etc.). Current should flow smoothly at all parts of the shield cage going to the ground for a successful RF shielding. Keywords. Disinfestation, Electric fields, Electromagnetic energy, Insect pests, Magnetic fields, Microwave, Radio frequency, Radio waves, RF shielding, Stored grains

Introduction Radio frequency (RF) energy is part of the electromagnetic spectrum that sends energy in a form of waves. The electromagnetic spectrum is composed of different levels of frequencies from low energy to high-energy radiation (NASA, 2010a). Radiation occurs in the spectrum at all frequencies (low to high) and it has two categories: ionizing and non-ionizing radiation (WHO, 2017; Wang et al., 2011). The ionizing radiation includes the ultraviolet (UV), X-rays, and gamma rays. These sources of radiation belong to high energy and high-frequency parts of the electromagnetic spectrum. Whereas, the non-ionizing radiation consists the radio frequency (RF), microwaves (MW), infrared (IR), and visible light and they are part of the low energy and lowfrequency parts of the spectrum (American Cancer Society, 2016; NASA, 2010a). Speaking of radiation, people might think that it can damage the DNA inside the cells and it can cause cancer. However, according to American Cancer Society (2016), this depends on which category of radiation. For example, the ionizing radiation sources have the ability to damage the DNA because they have enough energy to remove charged particles from an atom or molecule. Unlike with non-

ionizing radiation sources, they have not enough energy to ionize an atom or molecule that can damage the DNA inside the cells. This category of radiation can only cause vibrations to an atom or molecule (American Cancer Society, 2016). Thus, it can just move the charged particles without ionizing them and there will be no damage to DNA inside the cells. It can just generate heat in the materials by ionic polarization and dipole rotation. In disinfestation using RF energy, the generated heat from movements of molecules and atoms will kill the insect pests in stored grains (Macana and Baik, 2017; Lagunas-Solar et al., 2007; Wang et al., 2007a, b). The non-ionizing radiation such as microwave and radio frequency energy have been studied for years to replace the use of the chemical in disinfestation because of its negative effect to health due to the chemical residue and the environment due to depleting the ozone layer. Macana and Baik (2017) reported that radio frequency heating is preferred for disinfestation over microwave heating because of its higher penetration depth and stronger selective heating effect (Shrestha et al., 2013; Wang et al., 2011, 2003; Guo et al., 2010; Nelson, 1996). There are two types of RF heating system that have been used in the industry: Conventional and 50-ohm RF. They offer many advantages over conventional heating methods. However, the 50-ohm has more advantages than the former and it has been successfully used in different applications (Jones and Rowley, 1996). Nevertheless, there has been none in disinfestation. Conventional RF heating has been used in drying (Wang et al., 2014; Lee et al., 2010), disinfestation (Shrestha et al., 2013; Shrestha and Baik, 2013; Lagunas-Solar et al., 2007; Wang et al., 2007a, b; Wang et al., 2001; Lagunas-Solar et al., 2007), pasteurization of food and agricultural products (Kim et al., 2012; Gao et al., 2011), thawing/tempering (Farag et al., 2011; 2010), and post-baking of cookies and snack foods (Koral, 2004). Different applications of RF energy have different designs and need dependent on the desired use. Therefore, the installation of

this RF heating system needs an RF expert to have a successful application. Thus, an RF manufacturer, Coaxial Power Limited Company, collaborates this study. One of the things to consider during the installation of this system is the RF leakage shielding. Oftentimes, this part is neglected because of its significance was not seen especially in low power application. For example, RF heater used in laboratory with low power. Another example is microwave household oven, which uses low power; electromagnetic energy leakage is not noticeable. However, in high power application for industrial level, this is significant because of large amounts of radiation will be exposed to the workers. Radio frequency (RF) leakage composed of electromagnetic fields (electric and magnetic fields).The high amount of exposure of these fields to the human body is bad because of its thermal effects, especially to heat-sensitive parts, such as brain, eyes, and testes (Zhu et al., 2014). The RF leakage was also observed in the early use of RF energy for particle board curing, plastic welding, and glue drying (Zhu et al., 2014; Eriksson and Mild, 1985; Stuchly et al., 1980; Hietanen et al., 1979). The observed values of electric and magnetic fields are above the existing safety standards (Zhu et al., 2014). Therefore, RF shielding should be assessed before implementation of the RF technology for the safety of the workers. For these reasons, this paper deals with the assessment of electric fields, magnetic fields, and power density in different shielding materials with different levels of power applied to the load, distance from the RF source, the effectiveness of expanded steel metal and aluminum sheet as RF shielding materials. Furthermore, understanding of the electromagnetic radiation and behavior of waves, frequency, wavelength, energy, electric fields, magnetic fields, and power density of radio frequency waves is presented in this paper and suggestions are given for a successful RF shielding.

Electromagnetic radiation Electromagnetic radiation is a source of energy that travels in waves. Everything around us is electromagnetic energy. Without this energy, the world would not exist (NASA, 2010a). Every human being is dependent on this energy. Figure 1 shows the different waves around us, such as waves from Wi-Fi, cellphone, radio (AM and FM), TV, remote control (TV and car) microwave oven, light from the sun, etc. These waves are tasteless, odorless and most of them are not noticeable to the human eye. The electromagnetic spectrum (Figure 2) shows the different source of electromagnetic energy from long waves (radio waves) to short waves (gamma rays). According to NASA (2010a), the human eye can only notice the only visible light, which is only the small part of the spectrum.

Figure 1: Electromagnetic waves around us (Modified from NASA, 2010a)

Figure 2: Electromagnetic spectrum (NASA, 2010a)

Radio frequency waves behavior Radio frequency (RF) radiation is part of the electromagnetic spectrum. It emits its energy in the form of waves. The RF waves are a combination of two waves (electric and magnetic field waves) which travel at a speed of light. The behavior of its wave is important for all applications of radio frequency radiation such as transmission of signal and heating. There are five behaviors of electromagnetic waves namely: reflection, absorption, refraction, diffraction, and scatter. The pictures of these behaviors are in Figure 3. However, maybe not all of these behaviors are applicable in RF radiation application especially in RF heating where the applicator is much smaller than the wavelength. In RF dielectric heating application, two most applicable wave behaviors are absorption and scattering. Absorption occurs when the dielectric materials absorb

the RF energy and convert it to heat. On the other hand, scattering happens in RF dielectric heating when the RF waves hit the shielding and applicator of RF energy (usually very much smaller than the wavelength) making the signal reflect and scatter. Nevertheless, in RF transmission application, the three wave behaviors (reflection, diffraction, and scattering) are predominant as shown in Figure 4. Tait Communication (2015) describes these three behaviors in the figure. Reflection happens when RF waves hit a smooth material that is much greater than the wavelength. Whereas, diffraction occurs when the dense object is greater than its wavelength blocked the wave in between the transmitter and the receiver. Lastly, scattering happens when the signal from RF hits the surface (either rough surface or an object which dimension is smaller or equal to the wavelength, causing reflected waves to scatter.

Figure 3: Reflection, scattering, and diffraction of signal (Tait Communications, 2015)

Figure 4: Electromagnetic waves behavior (NASA, 2010b)

Frequency, wavelength, and energy Most common dielectric heating applications are microwave and radio frequency heating. Microwave and radio frequency are parts of the electromagnetic spectrum and they both transmit energy in the forms of waves. However, they are different in wavelength, frequency, and energy level of heating. Frequency The electromagnetic spectrum composes different level of frequencies. Radio waves have the least number of frequencies and gamma rays has the highest number. As shown in Figure 5, the frequency is the number of a cycle per second. The unit is Hertz, named after Heinrich Hertz who established the existence of radio waves (NASA, 2010c). In dielectric heating, the US Federal

Communications Commission (FCC) has allocated the following frequencies: 915 MHz, 2.45 GHz, 5.8 GHz, and 24.125 GHz for microwave applications and 13.56 MHz, 27.12 MHz, and 40.68 MHz for radio frequency applications (Macana and Baik, 2017). One hertz is equal to 1cycle/second. This means that in microwave and radio frequency heating, the cycle per second of waves is in million to billion times. Wavelength The wavelength is the distance between the crest as shown in Figure 5. According to NASA (2010c), the shortest wavelength are just fractions of the size of an atom. Whereas, the longest wavelength can be longer than the diameter of the earth. In the electromagnetic spectrum, as the frequency increases, the wavelength decreases. Therefore, radio waves have the longest wavelength and the gamma rays have the shortest wavelength. The speed of light (299,792,458 m/s) within the electromagnetic spectrum from radio waves to gamma rays is constant. Thus, the wavelength is equal to the speed of light divided by its frequency. For example in dielectric heating, the wavelength of RF 13.56 MHz, 27.12 MHz, and 40.68 MHz are 12.11 m, 11.05 m, and 7.37 m, respectively and MW 915 MHz, 2.45 GHz, 5.8 GHz, and 24.125 GHz are 0.33 m, 0.12 m, 0.05 m, and 0.012 m, respectively. Energy Electromagnetic waves are known for its frequency and wavelength. However, it can also call for energy. The unit of measurement is electron volts (eV). It is an energy (kinetic) to move an electron over one-volt potential. In the electromagnetic spectrum as shown in Figure 6, when frequency increases, the energy in one photon increases. In contrast, the energy increases when the wavelength decreases. This is also shown in Figure 4; more energy is needed to create more waves

by pulling the rope up and down (NASA 2010c). In electromagnetic heating, the energy level is significant. The higher the energy means it has the ability to remove the charges in atom or molecule. This can damage the DNA and cause cancer. Therefore, the electromagnetic spectrum has two categories of radiation: ionizing and non-ionizing radiation. The ionizing radiation frequencies have enough energy to remove charges from an atom or molecule, thus, damage the DNA. The frequencies belong to the ionizing radiation, which are the very high frequency in the spectrum such, ultra violet, x-rays, and gamma rays. Whereas, the RF and MW radiations belong to the non-ionizing category, which means they do not have enough energy to remove charges from an atom or molecule.

Figure 5: Wavelength, frequency, and energy (NASA, 2010c)

Figure 6: Electromagnetic spectrum showing the energy of one photon, the frequency, and wavelength (Anonymous, 2017)

Electric and magnetic fields Radio frequency energy travels in a form of waves (electric field and magnetic field waves). The waves are created because of the oscillating electric fields and magnetic fields. The direction of the waves is perpendicular to the direction of currents. Similarly, the direction of magnetic fields is at right angles to the electric fields as shown in Figure 7. When current and voltage are applied, electric fields and magnetic fields are created. Electric fields are produced when there are differences in voltage. Whereas, magnetic fields are created when there is current flowing (WHO, 2017). Electric fields can exist even though there is no current. As long as, positive

and negative charges are present, the electric fields are developed because of the attractions of opposite charges. Unlike with magnetic fields, they can only create when the current is flowing. In moving electric fields, magnetic fields are created and in the same way, the moving magnetic fields create an electric field. According to WHO (2017), the higher the current, the higher the strength of the magnetic fields. However, the strength of electric fields is dependent on voltage difference, charge, and area of the one plate of the capacitor as shown in equations (1) and (2). The smaller the distance between the two parallel plates (capacitor) is the higher the electric field strength. This means that the higher the voltage difference, the higher the electric field strength. Furthermore, when there are more charges in the capacitors, and smaller area of one plate of the capacitor also result in higher the electric field strength.

𝐸= 𝐸=

𝑉

(1)

𝑑 𝑄 𝐴 Ԑ0

(2)

Where: V is the voltage, d is the distance between the two plates, Q is the charge, A is the are of the one plate of the capacitor, and Ԑ0 is the permittivity of free space.

Figure 7: Electric fields and magnetic fields in waves (National Weather Service, 2010)

Material and methods The 50-ohm RF heating system was used for this study. There are four parts of the 50-ohm RF heating system: RF generator, 50-ohm coaxial cable, automatic matching network and controller, and applicator. The Coaxial Power Systems Company as our partner industry supplied the first three parts (RF generator, 50-ohm coaxial cable, automatic matching network and controller) and the authors designed the last part (applicator) depending on the needs of the application. For this case, the desired application of RF energy is disinfestation of insect pests in stored grains. The operating frequency of the system is 27.12 MHz and the power output of the generator is in the range of 0 kilowatts to 15 kilowatts depending on the materials in the applicator. Figure 8 presents the design of the applicator. The applicator has two electrodes (parallel plates), the top one (ground) and the bottom one (hot). Between the two electrodes is the tubular channel, (a)

(b)

(c)

(d)

Figure 8: Designed applicator with the auger system: (a) left view (b) right view (c) front view (d) top view

which is made of polypropylene and has a diameter of 30 cm. The materials (grains) are in the channel and act as an insulator during the RF heating.

RF shielding One major problem in using RF energy for industrial application is RF leakage from the applicator. The RF leakage is a combination of electric field and magnetic field energy. However, dielectric heating only used electric field because the materials are dielectrics. The dielectric materials act as an insulator during RF heating. The ions and dipoles of the material follow the movement of the electric fields in millions of times per second (27,120,000 cycles per second) result in its heating. This means that the materials absorb the electric field energy. However, some of the electric and magnetic fields are coming out from the applicator. Thus, RF shielding is significant to prevent the leaking of electromagnetic energy to the surrounding. There are four RF shielding methods in this study to assess the RF energy leakage. RF shielding one is expanded steel metal on all sides except for the bottom side (Figure 9). The bottom side is made of concrete flooring. Whereas, RF shielding 2 is aluminum metal sheets added in sides A and B horizontally (Figure 10). Sides A, C, top are still expanded steel metal. However, some parts of the sides B and D are still expanded steel metal and the bottom side is still concrete flooring. The aluminum sheets in sides B and D just block the RF energy from the applicator. While, RF shielding three is aluminum metal sheets in sides A, B, C, and D (Figure 11). Top and bottom sides remain expanded steel metal and concrete, consecutively. In addition, the RF shielding four is shielding all sides including the bottom (Figure 12). Aluminum sheets cover all sides (A, B, C, and D). Whereas, expanded steel covers some parts of the top side as shown in Figure 11 and aluminum sheets cover some parts of the bottom side and aluminum foil tape covers the other parts of the bottom side that

are away from the applicator (Figure 13). The aluminum foil just reflects back the scattered RF energy.

Figure 9: RF shielding using expanded steel metal

Figure 10: RF shielding using aluminum sheets in sides A and B

Figure 11: RF shielding using aluminum sheets in sides A, B, C, and D

Figure 12: RF shielding in all sides including the bottom

Figure 13: Aluminum sheet and foil tape at the bottom side

Electric and magnetic field’s measurement, and calculation of power density Measurement of electromagnetic fields outside the RF shielding is significant to determine whether the system is safe for the workers (Sirav et al., 2010). The HI-3804 (Figure 14) is an RF measurement device to determine strength of electric and magnetic fields around the 50-ohm RF heating system. It is portable, battery operated device, and it has been used for assessing electromagnetic fields leakage in different acceptable radio frequencies for industrial application (13.56 MHz, 27.12 MHz, and 40.68 MHz). The HI-3804 has two parts: read out and the probe. When measuring electromagnetic field for more accurate readings, one hand will hold the readout and the other for the probe. The arm that holds the probe should point to the source of electromagnetic energy and it should be away from the body to avoid errors.

The power density is commonly used to assist the allowable leakage of RF radiation around the RF systems. The power density was computed using the measured electric and magnetic field as shown in equations (3) and (4) (Zhu et al., 2014; ILO, 1998). The relationship of electric and magnetic fields at distance greater than the wavelength is fixed as shown in equation 5 (ILO, 1998). In RF heating where the applicator is much smaller than the wavelength, the relationship of the electric and magnetic fields is not constant (ILO, 1998). Therefore, the power density calculation based on the measured electromagnetic fields does not really provide a perfect assessment. However, it can be used as rough estimation and basis for shielding the RF radiation.

𝐸2

𝑆=

𝑍0

𝑆 = 𝐻2 𝑍0 𝐸

𝑍0 = 𝐻 = μ0 𝐶0

(3) (4) (5)

Where S is the power density (W/m2 ), E is the electric field strength (V/m), H is the magnetic field strength, μ0 is the magnetic constant (4π x 10−7 H/m), 𝐶0 is the speed of light (3 x 108 m/s), and 𝑍0 is the impedance of free space (377 ohms)

Figure 14: RF Measuring device

Electric and magnetic field’s measurement locations and spacing between points This section presents the location for measuring both electric and magnetic fields. The RF shielding cage has six sides (Sides A, B, C, D, E/top, and bottom). In this experiment, each of the four sides excluding the top and bottom is divided into 9 points (Sides A and C) and 12 points (Sides B and D) for electromagnetic measurements. Figures 15 (Sides A and C) and 16 (Sides B and D) show the locations with spacing dimensions for measuring E and H. The dimensions of the cage are length = 240 cm, width = 120 cm, and height = 180 cm.

Figures 17 to 19 show the locations of E and H measurements with different levels of shielding. Figure 17 shows the locations of measuring E and H in RF shielding using expanded steel metal. Whereas, Figure 18 presents the positions of E and H measurements in RF shielding using aluminum sheets added in sides A and B. Lastly, Figure 19 illustrates the points of E and H measurements in RF shielding using aluminum sheets added in sides A, B, C, and D.

Figure 15: Electric and magnetic fields measurements locations (Sides A and C)

Figure 16: Electric and magnetic fields measurements locations (Sides B and D)

Figure 17: Locations of measuring E and H in RF shielding using expanded steel metal

Figure 18: Locations of measuring E and H in RF shielding using aluminum sheets added in sides A and B

Figure 19: Locations of measuring E and H in RF shielding using aluminum sheets added in sides A, B, C, and D

Results and discussions Shielding the RF energy is significant during installation and implementation of radio frequency technology at different applications. Understanding the behavior of electromagnetic energy is important to shield the RF radiation. The RF radiation travels in waves as discussed earlier in this paper. Both electric and magnetic fields, which move perpendicular to each other, create waves because of their oscillations. The RF generator produced this oscillating electric and magnetic fields and connected to the matching network and applicator by the 50-ohm coaxial cable. This oscillation of the RF fields heats the material between the electrodes. The dipole molecules of water and ions in material follow the oscillation. This happens because of the movement of charges. Opposite charges attract and similar charges repel. This means that material between the electrodes that generate alternating charges, absorb RF energy. However, electric and magnetic fields are also present outside the two electrodes. Nevertheless, the strength is lesser than in the material, but the electric and magnetic fields exposure is harmful to the workers to a certain level as discussed earlier. Therefore, this section presents the results of the effect of four RF shielding methods, power levels supplied to the load, and distance from the RF source on electric fields, magnetic fields, and power density. In addition, this section presents the effect of expanded steel metal and aluminum sheets on shielding the RF electric and magnetic fields Effect of RF shielding methods on electric fields, magnetic fields, and power density Table 1 shows the result of the shielding effectiveness of the four methods presented in this paper. The maximum electric fields outside the RF shielding cage is measured at the four sides (Sides A, B, C, and D) at different points. The result shows that RF shielding one to three do not block the electric and magnetic fields effectively. RF shielding one has the maximum electric fields and power density value range of 1 V/m to 33 V/m and 0 W/m2 to 2.89 W/m2 consecutively, at

side A; 3 V/m to 35 V/m and 0.02 W/m2 to 3.25 W/m2 consecutively, at side B; 0 V/m to 11 V/m and 0 W/m2 to 0.32 W/m2 consecutively, at side C; and 4 V/m to 34 V/m and 0.04 W/m2 to 3.07 W/m2 consecutively, at side D. Whereas, RF shielding two has the maximum electric fields and power density value range of 0.4 V/m to 20 V/m and 0 W/m2 to 1.06 W/m2 consecutively, at side A; 4.7 V/m to 25 V/m and 0.06 W/m2 to 1.66 W/m2 consecutively, at side B; 0 V/m to 17 V/m and 0 W/m2 to 0.77 W/m2 consecutively, at side C; and 0 V/m to 28 V/m and 0.04 W/m2 to 2 W/m2 consecutively, at side D. While, in RF shielding three, it has the maximum electric fields and power density value range of 9.5 V/m to 25 V/m and 0.24 W/m2 to 1.66 W/m2 consecutively, at side A; 6 V/m to 40 V/m and 0.1 W/m2 to 4.24 W/m2 consecutively, at side B; 1 V/m to 35 V/m and 0 W/m2 to 3.25 W/m2 consecutively, at side C; and 3 V/m to 32 V/m and 0.02 W/m2 to 2.72 W/m2 consecutively, at side D. However, in RF shielding four, it shields the RF electric and magnetic fields effectively with 0 values of electric fields and power density at 1 kW applied to the load. The output of different shielding method is also similar to the magnetic fields. RF shielding four shields the magnetic fields. These results mean that shielding the RF energy is effective when shielding all the sides of the cage including the top and bottom. The electric fields values measured on different sides of the cage in RF shielding 1 to 3 is from the electric and magnetic fields leaked on the bottom side. The concrete flooring at the bottom in RF shielding does not block the RF energy. The waves penetrated on the concrete and went out to the cage. Effect of power applied to the load on electric fields Most of the experiments about RF energy applications is in low power. However, in applications for industrial level, higher power is required. Macana and Baik (2017) reported the relationship between heating time and power. The higher the power for heating the insect pests in stored grains is the faster the heating time. Therefore, an assessment of the effect of power level

to the intensity of electric fields and magnetic fields is essential. Table 2 shows the effect of two power levels on the intensity of electric fields in heating grains using 50-ohm RF technology at 27.12 MHz. For the three RF shielding methods tested (RF shielding 2, 3, 4), increasing the power level applied to the load increases the electric fields strength at all points and sides of the cage (A, B, C, and D). For example, RF shielding two has the highest electric fields intensity of 28 V/m at one kilowatt and 100 V/m at five kilowatts. This happens also to RF shielding 3 and 4. RF shielding 3 has the highest electric fields value of 40 V/m at 1 kW and 100 V/m at 5 kW. Whereas, in RF shielding 4, the electric field strength at 1 kW is 0 V/m and 9 V/m for 5 kW. Similarly in magnetic fields strength, the higher power level, the higher the intensity of magnetic energy (Zhu et al., 2014; Sirav et al., 2010; ILO, 1998). Therefore, this means that in industrial application, shielding the RF energy is one of the top priority. The allowable limit for electric field at frequencies 10 to 100 MHz for human body exposure is 61 V/m and for magnetic fields is 0.16 A/m (ILO, 1998). Effect of power applied to the load on power density Power density is one of the parameters measured to assess the human exposure to RF radiation. This is based on the standard of the Institute of Electrical and Electronics Engineers (IEEE) C95.1 for assessing the safe level of radiation around the RF system for human exposure. The allowable maximum level of power density for safe exposure to human at 27.12 MHz is 12.2 W/m2 for 6 minutes (Zhu et al., 2010; IEEE, 2006). Therefore, it is important to determine the effect of power applied to the load to the electric power density. This will set as a basis for using the RF energy for the industrial level. Industrial applications use the high power of RF energy. Table 3 presents the result of power density at 1 kW and 5 kW at different shielding methods. The result shows that in increasing the power applied to the load increases the power density. For instance, RF shielding 2 has the highest power density value of 2 W/m2 at 1 kW and 26.53 W/m2

at 5 kW. This means that in shielding 2, the power density is above the safe level. Exposure to this level at 6 minutes or longer should be avoided. The trend increasing density when increasing the power to the load also happens in RF shielding 3 and 4. RF shielding 3 has the highest power density value of 4.24 W/m2 at one kilowatt and 2.53 W/m2 at five kilowatts. Similarly, RF shielding 4 has the greatest power density value of 0 W/m2 at 1 kW and 0.21 W/m2 at 5 kW. Therefore, increasing the power applied to the load significantly increases the power density. This means that proper shielding is needed for the industrial application and for this case, RF shielding 4 has the effective shielding of RF energy with higher power level.

Table 1: Electric fields at different RF shielding methods Side

Point

A A A A A A A A A B B B B B B B B B B B B C C C C C C C C C D D D D D D D D D D D D

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12

RF shielding 1 E 2 (V/m) S (W/m ) 6 0.1 12 0.38 14 0.52 4 0.04 9 0.21 26 1.79 1 0 11 0.32 33 2.89 25 1.66 20 1.06 11 0.32 5 0.07 35 3.25 22 1.28 11 0.32 3 0.02 28 2.08 22 1.28 15 0.6 5 0.07 0 0 1 0 7 0.13 0 0 2 0.01 9 0.21 0 0 2 0.01 11 0.32 22 1.28 12 0.38 8 0.17 4 0.04 34 3.07 15 0.6 7 0.13 15 0.6 27 1.93 30 2.39 18 0.86 16 0.68

RF shielding 2

RF shielding 3

RF shielding 4

E (V/m)

S (W/m2 )

E (V/m)

S (W/m2 )

E (V/m)

S (W/m2 )

2.6 6.5 13 2.7 8.3 15.6 0.4 7.3 20 13.5 13.5 9 4.7 25 21.5 12 5.8 20 20 12.5 7 0 0 5.5 0 0 5.5 1.5 0.6 17 20 10 5 0 28 17 5 0 26 24 15 0

0.02 0.11 0.45 0.02 0.18 0.65 0 0.14 1.06 0.48 0.48 0.21 0.06 1.66 1.23 0.38 0.09 1.06 1.06 0.41 0.13 0 0 0.08 0 0 0.08 0.01 0 0.77 1.06 0.27 0.07 0 2 1 0.07 0 1.79 1.53 0.6 0

9.5 18.5 19 15 19 25 22 15 20 11 12 7 6 20 18 9 6 40 30 21 8 1 3 6 2 9 9 14 12 35 11 9 32 3 13 9 8 20 22 19 26 18

0.24 0.91 0.96 0.6 0.96 1.66 1.28 0.6 1.06 0.32 0.38 0.13 0.1 1.06 0.86 0.21 0.1 4.24 2.39 1.17 0.17 0 0.02 0.1 0.01 0.21 0.21 0.52 0.38 3.25 0.32 0.21 2.72 0.02 0.45 0.21 0.17 1.06 1.28 0.96 1.79 0.86

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Table 2: Effect of power levels of the generator on the electric fields Side

Point

A A A A A A A A A B B B B B B B B B B B B C C C C C C C C C D D D D D D D D D D D D

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12

RF shielding 2

RF shielding 3

RF shielding 4

E at 1 kW

E at 5 kW

E at 1 kW

E at 5 kW

E at 1 kW

E at 5 kW

2.6 6.5 13 2.7 8.3 15.6 0.4 7.3 20 13.5 13.5 9 4.7 25 21.5 12 5.8 20 20 12.5 7 0 0 5.5 0 0 5.5 1.5 0.6 17 20 10 5 0 28 17 5 0 26 24 15 0

16 28.5 70 21 55 82 7 47 82 86 85 59 36 100 100 75 35 83 100 80 38 2 4.2 40 3.6 5 31 5.2 8.4 50 83 70 35 3.9 99 79 36 3.1 93 91 58 3.3

9.5 18.5 19 15 19 25 22 15 20 11 12 7 6 20 18 9 6 40 30 21 8 1 3 6 2 9 9 14 12 35 11 9 32 3 13 9 8 20 22 19 26 18

51 98 98 74 77 100 96 56 60 94 80 63 51 66 100 56 38 92 60 81 43 11 8 27 14 28 34 32 28 36 100 27 50 37 97 43 24 63 81 78 78 79

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

9 1 1 9 5 1 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.2 0 0 0 1 0 0 0 1

Table 3: Effect of power levels of the generator on the power density outside the shielding cage

Side

Point

A A A A A A A A A B B B B B B B B B B B B C C C C C C C C C D D D D D D D D D D D D

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12

RF shielding 2

RF shielding 3

RF shielding 4

1 (kW)

5 (kW)

1 (kW)

5 (kW)

1 (kW)

5 (kW)

0.02 0.11 0.45 0.02 0.18 0.65 0 0.14 1.06 0.48 0.48 0.21 0.06 1.66 1.23 0.38 0.09 1.06 1.06 0.41 0.13 0 0 0.08 0 0 0.08 0.01 0 0.77 1.06 0.27 0.07 0 2 1 0.07 0 1.79 1.53 0.6 0

0.68 2.15 13 1.17 8.02 17.84 0.13 5.86 17.84 19.62 19.16 9.23 3.44 26.53 26.53 14.92 3.25 18.27 26.53 16.98 3.83 0.01 0.05 4.24 0.03 0.07 2.55 0.07 0.19 6.63 18.27 13 3.25 0 26 17 3.44 0.03 22.94 21.97 8.92 0.03

0.24 0.91 0.96 0.6 0.96 1.66 1.28 0.6 1.06 0.32 0.38 0.13 0.1 1.06 0.86 0.21 0.1 4.24 2.39 1.17 0.17 0 0.02 0.1 0.01 0.21 0.21 0.52 0.38 3.25 0.32 0.21 2.72 0.02 0.45 0.21 0.17 1.06 1.28 0.96 1.79 0.86

6.9 25.47 25.47 14.53 15.73 26.53 24.45 8.32 9.55 23.44 16.98 10.53 6.9 11.55 26.53 8.32 3.83 22.45 9.55 17.4 4.9 0.32 0.17 1.93 0.52 2.08 3.07 2.72 2.08 3.44 26.53 1.93 6.63 3.63 24.96 4.9 1.53 10.53 17.4 16.14 16.14 16.55

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0.21 0 0 0.21 0.07 0 0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Effect of distance on the electric field and power density The RF energy travels in waves (oscillating electric and magnetic fields) along the space. This is the behavior of all frequency in all parts of the electromagnetic spectrum. Radio frequency has the highest wavelength in the spectrum as discussed earlier in this paper. The speed of its travel is similar to the speed of light.

The distance that the RF waves can travel is dependent on its

behavior. In dielectric heating, it can travel as far it can from the generator to the coaxial cable, matching network, electrodes, and outside the applicator. Once, it travels to the dielectric materials (grains) between the two electrodes (hot and ground), the energy decreases. Some energy travels to the outside of the applicator. Therefore, this is where RF shielding comes in. The RF shielding blocks and reflects the waves that come out from the applicator. If there is no shielding, the radiation exposes to the surrounding. The waves stop until something block, reflect, and absorb it. If there is none, it continues to travel but the energy attenuates along the way. Table 4 shows the effect of distance from the three RF shielding methods cage on electric fields and power density. The results show that increasing the distance from the cage of the three RF shielding methods decreases the strength of electric field and power density. For example, RF shielding 2 has the highest value of electric field and power density of 63 V/m and 10.53 W/m2 at 60 cm, 13 V/m and 0.45 W/m2 at 120 cm, and 1 V/m and 0 W/m2 at 180 cm away from the cage, consecutively. A similar trend also happens in RF shielding 3. However, in RF shielding 4, the value of electric field and power density is 0 V/m and 0 W/m2 , consecutively. This means that the travel of the RF waves stops at the cage. Therefore, increasing the distance from the RF source decreases the RF energy (electric fields, magnetic fields, and power density). It is also important to consider that RF energy stops when RF shielding is effective.

Table 4: Effect of distance from the RF shielding cage on electric fields and power density

RF shielding 2 at side B Point 1 2 3 4 5 6 7 8 9 10 11 12 RF shielding 3 at side B 1 2 3 4 5 6 7 8 9 10 11 12 RF shielding 4 at side B 1 2 3 4 5 6 7 8 9 10 11 12

60 cm E (V/m) S (W/m2 ) 27 1.93 48 6.11 54 7.73 41 4.46 34 3.07 63 10.53 45 5.37 42 4.68 21 1.17 42 4.68 31 2.55 15 0.60

120 cm E (V/m) S (W/m2 ) 6.6 0.12 9 0.21 13 0.45 10 0.27 5 0.07 6 0.10 3.5 0.03 0.1 0.00 3.1 0.03 6.4 0.11 8.6 0.20 3.3 0.03

180 cm E (V/m) S (W/m2 ) 0 0.00 0 0.00 1 0.00 1 0.00 0 0.00 0 0.00 0 0.00 0 0.00 0 0.00 0 0.00 0 0.00 0 0.00

16 24 29 20 24 41 36 27 8 19 17 8

0.68 1.53 2.23 1.06 1.53 4.46 3.44 1.93 0.17 0.96 0.77 0.17

9 15 17 11 5 16 18 9 1 5 4 2

0.21 0.60 0.77 0.32 0.07 0.68 0.86 0.21 0.00 0.07 0.04 0.01

0 3 3 3 0 0 1 0 0 0 0 0

0.00 0.02 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0 0 0 0 0 0 0 0 0 0 0 0

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0 0 0 0 0 0 0 0 0 0 0 0

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0 0 0 0 0 0 0 0 0 0 0 0

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

RF shielding effectiveness of expanded metal and aluminum sheet metal RF shielding is significant to prevent the leakage of RF energy outside the applicator. The effectiveness of shielding materials is dependent on many factors: grounding, contacts of metals (bolts, joints), and conductivity, skin depth, resistance, and slots (openings) of the materials. All of these factors can be summarized in one statement: current should flow smoothly at all parts of the shield going to the ground (ILO, 1998). This means that all the materials of the shielding have high conductivity, less resistance, the thickness is higher than the skin depth, fewer openings, good connection, and connected to the ground. The most common RF shielding materials are silver, copper, aluminum, and steel. Silver and copper are the best shielding materials because of their high conductivity and low resistance. However, they are expensive and not practical for industrial shielding. Nevertheless, Steel and aluminum are less expensive shielding materials. Table 5 shows the effectiveness of expanded steel metal and a combination of expanded steel metal and aluminum sheet for shield RF energy at 27.12 MHz and at two power levels. The results show that both materials shield effectively at 1 kW with 0 value of electric fields, magnetic fields, power density. However, in higher power level (5 kW), only the combination of expanded steel metal and aluminum has 0 RF leakage. Nevertheless, the values of RF leakage are below the standard. For example, highest electric field intensity, magnetic field strength, and power density recorded in Table 5 are 2.8 V/m, 0.0024 A/m, and 2.17 W/m2 , sequentially. The values are much lower than the standard. Nonetheless, the values are still significant to note because with increasing power especially for industrial applications increases the strength of RF leakage.

Table 5: RF shielding effectiveness of expanded steel metal and aluminum sheet metal at two different power levels Expanded metal Side

Aluminum sheet added

Point E (V/m)

SE (W/m^2)

H (mA/m)

SH (W/m^2)

E (V/m)

SE (W/m^2)

H (mA/m)

SH (W/m^2)

At 1 Kw power supplied to the load A

1

A

2

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

4

0 0 0 0

A

3

A

0

0

0

0

0

0

0

A

5

0

0

0

0

0

0

0

0

A

6

0

0

0

0

0

0

0

0

A

7

0

0

0

0

0

0

0

0

A

8

0

0

0

0

0

0

0

0

A

9

0

0

0

0

0

0

0

0

C

1

0

0

0

0

0

0

0

0

C

2

0

0

0

0

0

0

0

0

C

3

0

0

0

0

0

0

0

0

C

4

0

0

0

0

0

0

0

0

C

5

0

0

0

0

0

0

0

0

C

6

0

0

0

0

0

0

0

0

C

7

0

0

0

0

0

0

0

0

C

8

0

0

0

0

0

0

0

0

C

9

0

0

0

0

0

0

0

0

At 5 kW power supplied to the load A

1

0.015279

0.32

0.0386

0

0

0

0

0.007939

2.4

2.1715

0

0

0

0

0.020647

0.4

0.6032

0

0

0

0

4

2.4 1.73 2.79 0.66

A

2

A

3

A A

0.001155

0.32

0.0386

0

0

0

0

5

2.8

0.020796

2.4

2.1715

0

0

0

0

A

6

1.36

0.004906

0.37

0.0516

0

0

0

0

A

7

0.56

0.000832

1.37

0.7075

0

0

0

0

A

8

0.4

0.000424

0.67

0.1692

0

0

0

0

A

9

0.89

0.002101

0.42

0.0665

0

0

0

0

C

1

1.39

0.005125

0

0

0

0

0

0

C

2

1.23

0.004013

0

0

0

0

0

0

C

3

1.2

0.00382

0

0

0

0

0

0

C

4

0.9

0.002149

0

0

0

0

0

0

C

5

0.27

0.000193

0

0

0

0

0

0

C

6

0

0

0.58

0.1268

0

0

0

0

C

7

0

0

2.23

1.8747

0

0

0

0

C

8

0

0

0.56

0.1182

0

0

0

0

C

9

0

0

1.82

1.2487

0

0

0

0

The cause of the leakage in RF shielding using expanded steel metal is mainly on the holes of the metal and its conductive properties. Holes in the shielding materials hinder the flow of current. In addition, steel has less conductive material than aluminum, which means it has a higher resistance than aluminum. The conductivity and resistance per unit surface area of steel are 0.602 x 107 mho/m and 3.435x10−2 , consecutively. Whereas, aluminum has a conductivity of 3.067 x 107 and resistance per unit surface area of 6.201x10−4 (ILO, 1998). Therefore, aluminum metal sheet is preferred for shielding high power RF heater.

Conclusions and recommendations Radio frequency energy is non-ionizing radiation which emits its energy in a form of waves (electric and magnetic field waves). RF waves travel at a speed of light and can be described also as frequency, wavelength, and energy. RF waves compose of oscillating electric and magnetic fields. The high amount of intensity exposure to these fields is bad to the workers because of its thermal effects to the body, especially to heat sensitive parts. An RF shielding is, therefore essential and one of the ways to assess the effectiveness of its shielding is to measure the electric and magnetic fields intensity and power density. The results show that the electric fields, magnetic fields, and power density decreased when decreasing the power applied to the load and increased when decreasing the distance from the RF energy source. It is also important to conclude that the aluminum metal sheet is preferred as RF shielding material for industrial scale applications over expanded steel metal because of its high conductivity and low resistance. Expanded steel metal, however, can be used as an alternative to aluminum at a certain power level. Furthermore, shielding cage with any non-conductive side is not possible for shielding RF energy at 27.12 MHz even at low power. Thus, it is suggested that in designing an RF shielding cage for low to high power

applications: all sides should be covered and gaps and holes are avoided. Other suggestions for a successful RF shielding is the current should flow with less resistance to the shielding cage going to the ground. Hence, all parts of the shielding cage (bolts, joints, doors, etc.) should have a good contact and shielding cage should be grounded.

Acknowledgments The authors are grateful for the financial support of this study of Saskatchewan Ministry of Agriculture and the Western Grains Research Foundation, Saskatchewan, Canada through the Agriculture Development Fund program (ADF #20130219). The authors gratefully acknowledge also the contributions of the following persons: Rlee Prokopishyn, the technician of University of Saskatchewan Chemical and Biological Engineering Department for helping us during the design and installations; Blair Cole and Daniel Vessey from University of Saskatchewan Engineering Shops for helping us with the fabrication; Ian Armer, the manager of Grain Quality Control Division, Viterra Inc. for providing us grains; and Oliver Broad from Coaxial Power Company Limited for providing as technical assistance.

References Anonymous. (2017). Image Acquisition (Introduction to video and image processing) Part 1. Date accessed on Decemeber 21, 2017 at http://what-when-how.com/introduction-to-video-andimage-processing/image-acquisition-introduction-to-video-and-image-processing-part-1/ Eriksson, A., & Mild, K. H. (1985). Radiofrequency electromagnetic leakage fields from plastic welding machines. Microwave Power Electrom. Ener., 20(2), 95-107. Farag, K. W., Marra, F., Lyng, J. G., Morgan, D. J., & Cronin, D. A. (2010). Temperature changes and power consumption during radio frequency tempering of beef lean/fat formulations. Food Bioprocess Tech., 3(5), 732-740. Farag, K., Lyng, J., Morgan, D., & Cronin, D. (2011). A comparison of conventional and radio frequency thawing of beef meats: Effects on product temperature distribution. Food Bioprocess Tech., 4(7), 1128-1136. Gao, M., Tang, J., Villa-Rojas, R., Wang, Y., & Wang, S. (2011). Pasteurization process development for controlling Salmonella in in-shell almonds using radio frequency energy. J. Food Eng., 104(2), 299-306. Guo, W.; Wang, S.; Tiwari, G.; Johnson, J.; Tang, J. (2010) Temperature and Moisture Dependent Dielectric Properties of Legume Flour Associated with Dielectric Heating. LWT Food Sci. Technol., 43, 193–201. Hietanen, M., Kalliomäki, K., Kalliomäki, P. L., & Lindfors, P. (1979). Measurements of strengths of electric and magnetic fields near industrial and radio-frequency heaters. Radio Sci., 14(6S), 31-33. Institute of Electrical and Electronics Engineers (IEEE). (2006). IEEE Std C95.1-2005. IEEE standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 300 GHz. New York, N.Y. Date accessed on Decemeber, 2017 at http://standards.ieee.org/findstds/standard/C95.1-2005.html International Commission on Non-Ionizing Radiation Protection (ICNIRP). (1998). Guidelines for limiting exposure to time varying electric, magnetic, and electromagnetic fields (up to 300 GHz). Health Phys., 74(4), 494-522. International Labour Organization (ILO). (1998). Safety in the use of radiofrequency dielectric heaters and sealers. Geneva, Switzerland. Jones, P.; Rowley, A. (1996). Dielectric Drying. Drying Technol., 14, 1063–1098. Kim, S.-Y., Sagong, H.-G., Choi, S. H., Ryu, S., & Kang, D.H. (2012). Radio-frequency heating to inactivate Salmonella Typhimurium and Escherichia coli O157:H7 on black and red pepper spice. Int. J. Food Microbiol., 153(1-2), 171-175.

Koral, T. (2004). Radio frequency heating and post-baking. Biscuit World Iss., 7(4), 1-6. Lagunas-Solar, M.; Pan, Z.; Zeng, N.; Truong, T.; Khir, R.; Amaratunga, K. (2007). Application of radiofrequency power for non-chemical disinfestation of rough rice with full retention of quality attributes. Applied Eng. in Agric., 23(5), 647-654. Lee, N.-H., Li, C., Zhao, X.-F., & Park, M.J. (2010). Effect of pretreatment with high temperature and low humidity on drying time and prevention of checking during radiofrequency/vacuum drying of Japanese cedar pillar. J. Wood Sci., 56(1), 19-24. Macana, R.J.; Baik, O. D. (2017). Disinfestation of insect pests in stored agricultural materials using microwave and radio frequency heating: A review, Food Reviews International, 34:5, 483-510, DOI: 10.1080/87559129.2017.1359840 Nagel, J. (2013). The beauty of electromagnetics. Accessed on December 12, 2017 at http://www.drjamesnagel.com/EM_Beauty.htm National Aeronautics and Space Administration, Science Mission Directorate. (2010a). Introduction to the Electromagnetic Spectrum. Retrieved December 3, 2017 from NASA Science website: http://science.nasa.gov/ems/01_intro National Aeronautics and Space Administration, Science Mission Directorate. (2010b). Wave Behaviors. Retrieved December 3, 2017 from NASA Science website: http://science.nasa.gov/ems/03_behaviors National Aeronautics and Space Administration, Science Mission Directorate. (2010c). Anatomy of an Electromagnetic Wave. Retrieved December 3, 2017 2016], from NASA Science website: http://science.nasa.gov/ems/02_anatomy National Weather Service. (2010). Remote sensing. 2010. Date accessed on December 28, 2017 at https://forecast.weather.gov/jetstream/remote/remote_intro.htm Nelson, S. (1996). Review and Assessment of Radio-Frequency and Microwave Energy for StoredGrain Control. Trans. ASAE, 39, 1475–1484. Shrestha, B.; Baik, O.-D. (2013). Radio Frequency Selective Heating of Stored-Grain Insects at 27.12 MHz: A Feasibility Study. Biosyst. Eng., 114, 195–204. Shrestha, B.; Yu, D.; Baik, O.D. (2013). Elimination of Crystolestes Ferrungineus in Wheat by Radio Frequency Dielectric Heating at Different Moisture Contents. Prog. Electromagn. Res., 139, 517–538.

Sirav, B.; Tuysuz, M.; Canseven, A.; Seyhan, N. (2010). Evaluation of non-ionizing radiation around the dielectric heaters and sealers: A Case Report, Electromagnetic Biology and Medicine, 29 (4), 144-153. Stuchly, M., Repacholi, H., Lecuyer, D., & Mann, R. (1980). Radiation survey of dielectric (RF) heaters in Canada. J. Microwave Power, 15(2), 113-122. Tait Communications. (2015). Channel concepts every system designer needs to understand. Accessed on December 21, 2017 at https://blog.taitradio.com/2015/09/23/9-channel-conceptsevery-system-designer-needs-to-understand/ Wang, S., Monzon, M., Johnson, J., Mitcham, E., & Tang, J. (2007a). Industrial-scale radio frequency treatments for insect control in walnuts: I: Heating uniformity and energy efficiency. Postharvest Biol. Tech., 45(2), 240-246. Wang, S., Monzon, M., Johnson, J., Mitcham, E., & Tang, J. (2007b). Industrial-scale radio frequency treatments for insect control in walnuts: II: Insect mortality and product quality. Postharvest Biol. Technol., 45(2), 247-253. Wang, S.; Ikediala, J.N.; Tang, J.; Hansen, J.D.; Mitcham, E.; Mao, R.; Swanson, B. (2001). Radio Frequency Treatments to Control Codling Moth in In-Shell Walnuts. Postharvest Biol. Technol., 22, 29 –38. Wang, S.; Tang, J.; Cavalieri, R.; Davis, D. (2003). Differential Heating of Insects in Dried Nuts and Fruits Associated with Radio Frequency and Microwave Treatments. Trans. ASAE, 46, 4, 1175–1182. Wang, Y., Zhang, L., Johnson, J., Gao, M., Tang, J., Powers, J., & Wang, S. (2014). Developing hot air-assisted radio frequency drying for in-shell Macadamia nuts. Food Bioprocess Tech., 7(1), 278-288. Wang, Y.; Li, Y.; Wang, S.; Zhang, L.; Gao, M.; Tang, J. (2011). Review of Dielectric Drying of Foods and Agricultural Products. Int. J. Agric. Biol. Eng., 4, 1 –19. World Health Organization (WHO). (2017). Electromagnetic fields. Date accessed on December 28, 2017 at http://www.who.int/peh-emf/about/WhatisEMF/en/ Zhu, H.; Yan, R.; Huang, R.; Li, S.; Wang, S. (2014). Experimental studies on leaked electromagnetic fields around radio frequency heating systems. ASABE Applied Engineering in Agriculture, 30, 601-608.