Enhanced microwave absorption of rice husk‐based pyramidal microwave absorber with different lossy base layer

IET Microwaves, Antennas & PropagationVolume 14, Issue 3 p. 215-222 Research ArticleFree Access Enhanced microwave absorption of rice husk-based pyramidal microwave absorber with different lossy base layer Lee Yeng Seng, Corresponding Author Lee Yeng Seng leeyengseng@gmail.com Department of Electronic Engineering Technology, Faculty of Engineering Technology, Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia Bioelectromagnetic Research Group (BioEM), School of Computer and Communication Engineering, Universiti Malaysia Perlis, Pauh Putra Main Campus, 02600 Arau Perlis, MalaysiaSearch for more papers by this authorSoh Ping Jack, Soh Ping Jack Bioelectromagnetic Research Group (BioEM), School of Computer and Communication Engineering, Universiti Malaysia Perlis, Pauh Putra Main Campus, 02600 Arau Perlis, MalaysiaSearch for more papers by this authorYou Kok Yeow, You Kok Yeow School of Electrical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, MalaysiaSearch for more papers by this authorWee Fwen Hoon, Wee Fwen Hoon Bioelectromagnetic Research Group (BioEM), School of Computer and Communication Engineering, Universiti Malaysia Perlis, Pauh Putra Main Campus, 02600 Arau Perlis, MalaysiaSearch for more papers by this authorLee Chia Yew, Lee Chia Yew School of Electrical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, MalaysiaSearch for more papers by this authorGan Hong Seng, Gan Hong Seng Medical Engineering Technology Section, British Malaysian Institute Universiti Kuala Lumpur, 53100 Gombak Selangor Malaysia, MalaysiaSearch for more papers by this authorFareq Malek, Fareq Malek Faculty of Engineering and Information Sciences, University of Wollongong in Dubai (UOWD), UAESearch for more papers by this author Lee Yeng Seng, Corresponding Author Lee Yeng Seng leeyengseng@gmail.com Department of Electronic Engineering Technology, Faculty of Engineering Technology, Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia Bioelectromagnetic Research Group (BioEM), School of Computer and Communication Engineering, Universiti Malaysia Perlis, Pauh Putra Main Campus, 02600 Arau Perlis, MalaysiaSearch for more papers by this authorSoh Ping Jack, Soh Ping Jack Bioelectromagnetic Research Group (BioEM), School of Computer and Communication Engineering, Universiti Malaysia Perlis, Pauh Putra Main Campus, 02600 Arau Perlis, MalaysiaSearch for more papers by this authorYou Kok Yeow, You Kok Yeow School of Electrical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, MalaysiaSearch for more papers by this authorWee Fwen Hoon, Wee Fwen Hoon Bioelectromagnetic Research Group (BioEM), School of Computer and Communication Engineering, Universiti Malaysia Perlis, Pauh Putra Main Campus, 02600 Arau Perlis, MalaysiaSearch for more papers by this authorLee Chia Yew, Lee Chia Yew School of Electrical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, MalaysiaSearch for more papers by this authorGan Hong Seng, Gan Hong Seng Medical Engineering Technology Section, British Malaysian Institute Universiti Kuala Lumpur, 53100 Gombak Selangor Malaysia, MalaysiaSearch for more papers by this authorFareq Malek, Fareq Malek Faculty of Engineering and Information Sciences, University of Wollongong in Dubai (UOWD), UAESearch for more papers by this author First published: 07 January 2020 https://doi.org/10.1049/iet-map.2019.0571Citations: 9AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract The size of pyramidal microwave absorbers (PMA) is one of the main considerations for their selection for use in anechoic chambers. In this work, a new type of PMA is introduced, with rice husk (RH) as its filler. Meanwhile, a lossy base layer is also introduced in the PMA using a combined RH and carbon nanotube (CNT) composite. To do so, the dielectric properties of the RH-CNT composite are first investigated from 2–18 GHz by using a dielectric probe. Next, the microwave absorption properties of the PMA were investigated in terms of different height of the pyramidal structure and base layer. The free-space method was used to measure the microwave absorption of PMA at oblique incident angles of 0° to 60°, respectively. Results indicated good microwave absorption characteristics in the 2–6 GHz with a height of 11.5 cm the PMA structure, and 1 cm thickness of the base layer. The lossy base layer is found to increase the bandwidth of the absorber and improve the microwave absorption of PMA in the lower GHz frequencies. Further investigations concluded that the microwave absorption performance of the PMA is dependent on its dielectric properties, size and base layer. 1 Introduction Microwave absorbers made using agricultural waste composites have attracted great attention in the recent few years due to their cost-effectiveness, sustainability, and custom ability [[1]–[4]]. This is also due to their relevance in both the scientific community and industrial applications [[5], [6]]. Typical microwave absorbers are essentially dependent on the intrinsic losses of the material itself and its dielectric properties and magnetic properties [[7]]. Besides material properties, microwave absorption properties in absorbers can be efficiently controlled by defining suitable geometries. Designed to achieve high-microwave absorption and broadband (absorption wavelength), fabrication simplicity of the geometrical designs need to be considered. In general, microwave absorbers can be classified into two categories: broadband absorbers and resonant absorbers [[8]]. Single- and multiple layered absorbers are considered as resonant and multi-resonant absorbers [[9], [10]]. Meanwhile, an example of broadband absorbers is the geometric transition absorbers, such as the pyramidal microwave absorbers (PMA). It is typically built using thick materials with pyramidal or gradual structures extending perpendicular to the surface in a regularly spaced pattern. PMA was developed so that the interface presents a gradual transition in impedance from air to that of the absorber, which then gradually changes the dielectric constant and loss factor [[11]]. The height and periodicity of the pyramids tend to be on the order of one wavelength. For structures with shorter or longer wavelengths, the waves are effectively met by a more abrupt change in the impedance. The multi-reflections that occur in these PMA leads to significant attenuation, and provide improved microwave absorption compared to flat absorbers. Fig. 1 illustrates the electromagnetic (EM) wave interaction with a solid PMA. Waves incident on the tip of PMA with impedance matching allow them to penetrate into the absorber. On the other hand, incident waves on other points besides their tip causes wave reflection between PMAs. Therefore, to achieve proper microwave absorption, the PMA tips are crucial in providing impedance matching to free space to obtain minimum reflection [[12]]. Fig. 1Open in figure viewerPowerPoint Wave interaction with solid PMA In the literature, only several works performed on the optimisation of agricultural absorbing materials using such base layers. A significant finding by H. Nornikman and M.N. Iqbal is that the rice husk (RH) PMA alone is unable to provide attenuation of <−20 dB in the lower-frequency range from 2 to 4 GHz [[13], [14]]. This challenge was then attempted to be solved in subsequent work. For example, a RH pyramidal microwave absorber with 13 cm height was designed with a 2 cm base layer in [[13]] and measured at different points. It was found that the best average reflection loss of these fabricated RH pyramidal absorbers is 41.969 dB over 7–13 GHz range. Besides that, the rubber tyre dust-RH pyramidal absorber with 13 cm height and 2 cm base layer studied in [[1]] reported the best average reflectivity of −27.4 dB between the frequency range of 7 and 13 GHz. Meanwhile, Iqbal et al. [[14]] investigated an 11 cm height of a hollow pyramidal microwave absorber based on RH material, indicating that such structure and material resulted in <−20 dB of reflection loss in the frequency spectrum of 5–20 GHz. The base layer made from RH is 20 mm thick. Besides, we also have published an article presenting the multiple-layered absorber designs with different stack layers of the microwave absorber that enables us to tune the microwave absorption performance [[15]]. A summary of the previous related work on RH pyramidal absorbers is presented in Table 1. Table 1. Comparison reflectivity values of pyramid absorber with previous research Type of pyramid absorber Average reflectivity, dB Operating frequency, GHz Reference RH PMA + base layer −47.613 2–18 this study rubber tyre-RH pyramidal absorber −27.40 7–13 [[1]] RH pyramidal absorber −41.969 7–13 [[13]] RH hollow pyramid −31.74 4–20 [[14]] banana leaves and coal −45.2 2–20 [[16]] RH-coal −45 0–20 [[17]] VHP-4 from Emerson & Cuming + base layer frequency selective surface −40 1–10 [[18]] sugarcane bagasse-coal −46.89 0–20 [[19]] sugarcane bagasse and rubber tire dust 1–18 [[20]] In theory, the width of the pyramidal is calculated by wavelength, λ, of the lowest frequency supported while the height of the pyramidal is determined by half of the lowest wavelength, λ/2. However, due to the dielectric properties of the RH material, the RH-PMA without base unable to cover the lower frequency 2–4 GHz with the proposed height of RH-PMA, 11.515 cm. Thus, there is a need to enhance the absorption performance of the RH PMA with normal incident waves from 2GHz to 4 GHz, besides extending its operating range up to 18 GHz. From the literature review, a potential solution is by using a combined dielectric lossy base layer with RH-PMA as the final structure to enhance its microwave absorption in the lower frequencies. This base layer can be fabricated using carbon-based materials such as carbon nanotubes (CNTs), graphite, and graphene due to their unique material properties such as conductivity, dielectric, physical, and mechanical properties [[15], [21]]. For instance, CNTs offer favourable EM characteristics such as good microwave absorption and high-electrical conductivity [[22]–[24]]. The high amount of carbon content in CNTs increases its dielectric properties and radiation absorption, thus decreasing the reflectivity of the microwave absorbers [[25]]. Besides being a lightweight absorbing material, its effective dielectric constant and loss factor can be combined with RHs to form a composite (denoted as RH + CNT) and optimised with a suitable geometry to form such broadband microwave absorbers. To investigate this, the dielectric properties of various types of single base layers were first measured and investigated in this study. Next, combinations of PMA and base layer aimed at broadband operation in the microwave frequencies, from 2 to 18 GHz frequency is designed and investigated. Then, the simulated and measured RH-PMA with and without the various types of base layers are compared. Owing to the difficulty of pure agricultural waste materials to provide good microwave absorption throughout a wide frequency range, the RH base layer is mixed with other absorbents, such as CNTs to result in the intended broadband microwave absorber operating with a minimum absorption of −35 dB throughout 2–18 GHz. The results indicate its promising potential to be implemented in anechoic chambers and EM compatibility test applications. The rest of the paper is organised as follows. The following section explains the preparation of the RH samples, which then will be characterised in terms of its dielectric properties. Section 2.3 describes the design process using the values obtained experimentally. Finally, the results of the evaluations are presented in Section 3, before the concluding remarks. 2 Experimental validations 2.1 Preparation of RH To prepare measurement samples, RHs were collected from a local rice mill (Klang Beras Dibuk Sdn. Bhd) with husks of 8–10 mm in length and 1.5–2.0 mm in width. Cleaning is first performed to remove dust particles which could affect the final material composition and contribute to the inaccuracy of its material properties. The collected RH is then grinded for several minutes to turn it into powder form (about 3 mm in size) using a commercial RT-34 1HP grinder machine, shown in Fig. 2. Fig. 2Open in figure viewerPowerPoint RH collected from the rice mill and RH after the grind 2.2 Fabrication of PMA and base layer Next, the fabrication of the PMA and base layer is performed using chemicals such as bonding agent, polyester, and hardening agent, methyl ethyl ketone peroxide (MEKP). Polyester is used to bond the powder form materials, whereas MEKP is used to harden the mixture. The samples were prepared according to the weight percentage (wt%) of polyester. Since polyester and MEKP are considered as transparent to EM wave due to their low loss dielectric properties, the only absorption is contributed by the RH and CNT filler. To ensure homogeneity, the mixture of 70 wt % of RH with the polyester is stirred thoroughly for 25 min using a stirring machine. Then, a 10 wt% MEKP is added into the mixture of RH and polyester and stirred for another 5 min before being poured manually into a pyramidal-shaped mould. The polymerisation takes about 2 h at room temperature at the laboratory scale before the mould is disassembled and the single fabricated pyramidal absorber is complete. Each fabricated PMA is dimensioned at 5 cm × 5 cm × 11.5 cm, as shown in Fig. 3. The same procedure is repeated to fabricate other RH-PMAs. On the other hand, a total of four different base layers were fabricated (RH, RH/CNT-2, RH/CNT-4, RH/CNT-5), with each dimensioned at 45 cm × 45 cm × 1 cm. RH/CNT-2, RH/CNT-4, and RH/CNT-5 represent 70 wt % of RH composites with 2, 4, and 5 wt% of CNT, respectively. To fabricate them, RH and CNT composites are mixed with the polyester bonding agent and stirred for 30 min. Next, a 10 wt % of hardening agent (MEKP) is added into the mixture and stirred for another 10 min. This composite is then poured into a standard steel square mould dimensioned at 45 cm × 45 cm × 1 cm, and left for 6 h at room temperature (25°C). Fig. 3Open in figure viewerPowerPoint Single RH PMAs 2.3 Dielectric characterisation To characterise the dielectric properties of the RH-PMA in this study, an Agilent E8362B P-series vector network analyser (VNA), dielectric probe, and 85070E material characterisation software were used [[26]]. A coaxial cable is used to connect one of the VNA ports and the dielectric probe before measurements, as shown in Fig. 4. The open end of the probe must contact the flat surface of the absorber during measurements for it to collect the reflected signal from the absorber. These reflected signals from the absorber are converted into dielectric constant, ɛ′r and loss factor, ɛ″r by the 85070E software [[25]]. Besides ensuring that the surface of the dielectric probe and absorber is fully in contact with each other, care must be considered to ensure that this contacting surface area has to be as flat as possible to avoid the existence of the air gap. Each fabricated absorber was measured at five different locations for five times, and their averaged values are taken as the final dielectric properties. Fig. 4Open in figure viewerPowerPoint Dielectric properties measurement of the experimental setup 2.4 Design and simulation In this study, CST Microwave Studio (CST-MWS) was used to design PMA and base layer. It is one of the popular software to analyse microwave absorber [[27], [28]]. A RH hollow PMA was designed in [[14]], where the measured dielectric properties are defined in using CST-MWS to optimise the absorber's geometry. To perform three-dimensional (3D) microwave absorber simulations, the transient solver based on the finite difference time domain numerical technique was used, with their transverse EM mode boundary conditions shown in Fig. 5. A waveguide port was used to excite the signal in the far-field region of the absorber to emulate the free space reflectivity evaluation setup. The x-axis is set as E-field = 0; the y-axis is set as H-field = 0, and the positive z-axis is set as the open boundary with the far-field excitation from port 1 to allow EM wave propagation towards the negative z-axis direction. The PMA is arranged in a 9 × 9 array and the base layer's thickness is studied using the parameter sweep available in the software over the 2–18 GHz frequency range. Reflectivity of the combined structure is then obtained from the S-parameter results in dB. Fig. 5Open in figure viewerPowerPoint Setup of the absorber model, transient solver with waveguide port (red) and boundary conditions 2.5 Free-Space reflectivity measurement Reflectivity is defined as the reflected power from an absorbent material, and for microwaveabsorbers, is typically measured using the free space reflectivity method,similar to bistatic and radar cross-section measurements. Such a measurementsetup is ideal as it is contactless and non-destructive. Microwave absorbersinstalled in an anechoic chamber are backed by a metallic wall to minimiseunwanted EM waves from interfering with the measurements [[29]]. Furthermore, this metallic wall prevents the waves inthe chamber to be transmitted beyond the chamber. Similarly, the reflectivitymeasurement setup for the PMA and base layer performed with metallic plate asbacking [[30]]. Measurements from 2 GHz to18 GHz are taken using a pair of A-INFO's standard gain horn antennas connectedto the VNA. The PMA and base layer are placed on top of a metal plate, and thissetup is surrounded by the commercial absorbers to reduce the unwantedreflection and interference. During measurements, the horn antennas are directedtowards the centre of the PMA and base layer to emulate waves at a normalincidence. Port 1 of the VNA is set as the transmitter port whereas thereflected waves are collected via the horn to Port 2. Fig. 6 illustrates this free spacereflectivity measurement setup. Fig. 6Open in figure viewerPowerPoint Free space reflectivity measurement setup To obtain reflectivity, S21, which is the ratio of the transmitted energy (from port 1) and the received energy after reflected energy from the metal plate/absorber (at port 2) is obtained. The reduction in power is compared to a 'perfect' reflection, best approximated by the reflection of a flat metallic plate over the frequency of interest. Before measurements, the setup connected to the VNA is calibrated using the 85052B calibration kit to minimise cable losses. The measured reflectivity of the metal plate is calculated as (1) where S21,metal, is the reflectivity of the metal, Pr is the power of the reflected waves received at port 2, and Pin is the power incident from the source port. The material under test is then placed on the plate, and the reflected signal is measured. This is repeated and measurements obtained are denoted as S21,sample. The total reflectivity of microwave absorbing materials in dB can be calculated as: (2) For reflectivity measurements with angles off-normal incidence, the reflection is dependent on the polarisation of the incident wave. The absorption percentage of the microwave absorbers under normal incidence can be calculated using A (%) = [1 − |Stotal|2] × 100%, where A and |Stotal|2 are the absorbance and reflectivity of the absorber, respectively. Fig. 7 illustrates the free-space reflectivity measurement setup. Fig. 7Open in figure viewerPowerPoint Illustration of the free-space reflectivity measurement method (a) Reflected signal, Pr, from the metal plate, (b) Reflected signal, Pr, from the absorber with a metal plate 3 Results 3.1 Dielectric properties In general, the dielectric constant, ɛ′r of material correlates with the amount of polarisation contained in it and represents its electrical energy storage ability. Meanwhile, the dielectric loss factor, ɛ″r denotes the dissipated electric energies. In dielectric materials, their magnetic properties can be assumed to be magnetic constant, µ′r = 1 and magnetic loss factor µ″r = 0. In the microwave range, dielectric losses are mainly contributed by the dipolar polarisation and interfacial polarisation. Table 2 shows the average measured dielectric property values of the RH and RH/CNT composites from 2 to 18 GHz. As shown in Fig. 8, the dielectric properties of the RH/CNT composite are dependent on the CNT filler loading. Specifically, the values of dielectric constant and loss factor increase with the increase of CNT filler loading from 2 to 5 wt %. The increment of dielectric properties can be attributed to enhanced storage capability and dissipation ability. Loss tangent, tan δ, can be defined as the ratio of loss factor and dielectric constant [[31], [32]] (3) Table 2. Averaged measured dielectric properties of the RH and RH/CNT composites from 2 to 18 GHz Sample Dielectric constant, ɛ′r Loss factor, ɛ″r Loss tangent, tan δ RH 2.986 0.287 0.097 RH/CNT-2 5.761 1.143 0.199 RH/CNT-4 7.736 2.157 0.279 RH/CNT-5 9.245 3.005 0.325 Fig. 8Open in figure viewerPowerPoint Measured dielectric properties of the RH and RH/CNT composites (a) Dielectric constant, ɛ′r, (b) Loss factor, ɛ″r, (c) Loss tangent, tan δ 3.2 Microwave absorber evaluations The simulated reflectivity for the RH PMA with various heights of the pyramid structure (h = 11.5, 15, and 18 cm) without the base layer are first investigated under normal incidence using CST-MWS and are compared in Fig. 9. It can be observed that the RH PMA with heights of 15 and 18 cm has lower reflectivity (<−20 dB) at frequencies from 6.5 to 18 GHz compared to the RH PMA with the height of 11.5 cm. This also indicates that the dielectric properties of the RH-PMA are unable to provide satisfactory absorption performance from 2 to 6.49 GHz when designed with a height between 11.5 and 15 cm. Fig. 9Open in figure viewerPowerPoint Simulated reflectivity with various heights of the RH PMA without the base layer Next, the RH-PMA reflectivity is simulated with a fixed height of 11.5 cm with different types of RH/CNT base layers. Based on the simulation results, it is expected that materials with high permittivity values will produce low reflectivity, according to Fresnel's wave theory. From Figs. 10a–c, it is observed that the reflectivity of RH-PMA with base layer thicknesses of 0.5, 1, and 1.5 cm and different types of material compositions is <−20 dB (with 99% absorption) from 8 to 18 GHz. Fig. 10Open in figure viewerPowerPoint Reflectivity comparison between different thicknesses of base layers on the PMA with (a) 0.5 cm, (b) 1 cm, (c) 1.5 cm Moreover, the reflectivity of other types of base layers such as RH/CNT-2, RH/CNT-4, and RH/CNT-5 was improved between – 40 and – 80 dB when operating from 2 to 12 GHz. Obvious reflectivity improvements are seen when a 1 cm thick RH ash (RHA)/CNT-4 is used as the base layer for RH-PMA with a reflectivity of <−40 dB (99.99% absorption) between 2 and 18 GHz. This indicates that the reflectivity performance in the lower frequencies (from 2 to 18 GHz) can be improved for a RH-PMA by placing a specific base layer between the pyramidal absorber and metal plate backing. On the other hand, a 0.5 mm thick base layer works the best with RHA/CNT-5 with a minimum reflectivity (RL) of up to −80 dB at 5 GHz. Besides reflectivity performance, it is expected that impedance matching can be tuned using the combined optimisation of the RH/CNT material composition and the thickness of the base layer. The average simulated reflectivity values of 11.5 cm height PMA with different types of base layers and thickness over 2–18 GHz are presented in Table 3. Table 3. Average simulated reflectivity values of 11.5 cm height PMA with a different type of base layer over 2 to 18 GHz RH PMA + base layer Base layer thickness, 0.5 cm, dB Base layer thickness, 1 cm, dB Base layer thickness, 1.5 cm, dB RH PMA + RH −23.96 −24.52 −25.27 RH PMA + RH/CNT-2 −27.74 −47.61 −49.10 RH PMA + RH/CNT-4 −28.32 −60.49 −106.32 RH PMA + RH/CNT-5 −47.07 −76.77 −117.69 The simulated reflectivity and measured reflectivity of the RH PMA (without the base layer) with a 1 cm thick base layer (RH and RH/CNT-2) are compared in Fig. 11. To minimise the usage of CNT, a RH/CNT-2 composite base layer is fabricated instead of RH/CNT-4 or RH/CNT-5, and combined for use with a RH PMA which is 11.5 cm in height. The reflectivity of this RH pyramidal with different types of base layers placed between the PMA and metal plate backing is then evaluated. A very good agreement between simulated and measured reflectivity is observed when studying the RH pyramidal absorber with different types of base layers. The reflectivity of the RH pyramidal absorber with 1 cm thick RH/CNT-2 base layer shows lower reflectivity (<−40 dB) compared to a RH PMA with a 1 cm thick RH base layer throughout the 2–12 GHz range. This is due to the total transmission path of the incident and reflected waves within base RH/CNT-2 is approximately equal to the nπ of wavelength, λ (resonate condition) around 4–6 GHz. Fig. 11Open in figure viewerPowerPoint Simulated and measured reflectivity of the RH PMA with and without the 1 cm thick base layer It is also noticed that there is an improved absorption (with lower reflectivity) of the RH PMA with RH/CNT-2 base layer throughout a broadband frequency range at the lower frequencies, despite a low CNT filler loading (2% CNT) in the base layer. To further understand this observation, the electric field distribution of the wave amplitudes with the RH PMA under illumination is simulated and presented in Fig. 12. A comparison of the field distribution of the RH PMA with and without the 1 cm thick RH CNT-2 base layer at 10 GHz indicates that more fields with higher amplitudes are concentrated in the middle of RH PMA without a base layer. For RH PMA with a 1 cm thick RH CNT-2 base layer has lower amplitudes concentrated in the middle point. This indicates that the use of the RH CNT-2 base layer lowered the level of overall reflectivity, and thus, enabling higher absorption by this structure. Fig. 12Open in figure viewerPowerPoint Simulated amplitude distributions of the electric field at 10 GHz for the RH-PMA (a) Without the 1 cm thick RH/CNT-2 base layer, (b) With the 1 cm thick RH/CNT-2 base layer To further evaluate the RH-PMA with a 1 cm thick RH/CNT-2 base layer, simulations and measurements of its operation under oblique wave incidences are assessed. Simulations were performed using the CST software with the angle of incidence set at 0°–30°, 45°, and 60°. Meanwhile, in measurements, a bistatic evaluation technique was set up with the use of a pair of horns. The transmitting horn was rotated from 0° to 30°, 45°, and 60° illuminating the PMA, and the measured reflectivity is collected by a receiving horn. The results of this evaluation shown in Fig. 13 show that the oblique angles of incidence affected the overall reflectivity performance of the proposed RH-PMA. At both the polarisations, the best reflectivity performance was observed at the incident angle of 0° compared to other angles of oblique incidence. However, RH-PMA with 1 cm thick RH/CNT-2 base resulted in <−24 dB (99.6% attenuation) even at the most extreme angle of 60°. For the case of transverse electric (TE) polarisation, it is observed that the values of the reflectivity increase with the increase in the angle of incidence from 0° to 60° over the range of 2–18 GHz. Similarly, for the case of transverse magnetic (TM) polarisation, the values of reflectivity increase in the same trend, with a slight shift of the reflectivity curve towards the higher frequencies. Fig. 13Open in figure viewerPowerPoint Measured reflectivity of the RH PMA with 1 cm thick RH/CNT-2 base layer at oblique incident angles of 0°, 30°, 45°, and 60° in (a) TE polarisation, (b) TM polarisation This can be explained by bistatic angle, β (or incidence angle), as: (4) where L is the linear distance between both horns and λ is corresponding to the wavelength, as shown in Fig. 14. Thus, when the bistatic angle, β, increases, the resonate condition is shifted to a higher frequency as shown in Fig. 13. Based on (4), the shifted resonate frequency, f2, can be estimated as (5), once the initial resonate frequency, f1, initial bistatic angle, β1, and shifted bistatic angle, β2, are known (5) For instance, the cases of β1 = 30° and β2 = 60° cause the resonate frequency shifted from ∼5 to ∼ 6 GHz. When the bistatic angle, β, is increased, the bistatic radar is gradually like a forward scatter radar. Hence, the reflectivity (received signal) is also higher for a high value of β as shown in Fig. 13. Despite this, the reflectivity of this structure is still <−25 dB (99.7% attenuation) throughout the 2–18 GHz range. Fig. 14Open in figure viewerPowerPoint Bistatic free-space measurement set-up 4 Conclusion PMAs with various RH/CNT composites have been studied and investigated via simulations and measurements. The work starts with the characterisation of the dielectric properties of RH, RH/CNT-2, RH/CNT-4, and RH/CNT-5 throughout a broad operating band. Next, the absorption performance of RH PMA and the combination RH PMA with different heights and compositions of base layers are investigated. The study shows that the PMA with a base layer made using RH/CNT-2 composite is capable of improving its absorption throughout a broad operating band, with an average reflectivity of −47.613 dB from 2 to 18 GHz. In comparison, the performance of such RH PMA without any base layers at normal incidence only operated with an average reflectivity of −21.384 from 2 to 18 GHz. Results obtained throughout this study also indicated a satisfactory agreement between simulations and measurements. It can be concluded that the additional base layer placed between the PMA and the metal plate backing is a simple and effective solution to enhance absorption within a broad operating band, especially at the lower frequencies. Besides suitable values of dielectric properties, the base layer thickness can be used to optimise the performance of the PMA, especially to facilitate impedance matching. The finding from this study also indicates the huge potential in using these material properties to enable dielectric tunability, and consequently, tunable absorption in the microwave regime. Moreover, the base layer can be replaced using a metamaterial-based absorber for further absorption in RH PMAs. 5 References [1]Malek, M.F.B.A., Cheng, E.M., Nadiah, O., et al.: 'Rubber tire dust-rice husk pyramidal microwave absorber', Prog. Electromagn. Res., 2011, 117, pp. 449– 477 [2]Zahid, L., Malek, M.F.A., Meng, C.E., et al.: 'Performance of sugarcane bagasse and rubber tire dust microwave absorber in Ku band frequency', Theory Appl. Appl. Electromagn., 2015, 344, pp. 207– 214 [3]Iqbal, M.N., Malek, M.F.B.A., Ronald, S.H., et al.: 'A study of the EMC performance of a graded-impedance, microwave, rice-husk absorber', Prog. Electromagn. Res., 2012, 131, pp. 19– 44 [4]Zahid, L., Malek, F., Nornikman, H., et al.: 'Development of pyramidal microwave absorber using sugar cane bagasse (SCB)', Prog. Electromagn. Res., 2013, 137, pp. 687– 702 [5]Méjean, C., Badard, M., Benzerga, R., et al.: 'Rigid composite materials for anechoic chamber application', Mater. Res. Bull., 2017, 96, pp. 94– 99 [6]Qin, F., Brosseau, C.: 'A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles', J. Appl. Phys., 2012, 111 [7]Ogawa, S., Kimata, M.: 'Metal–insulator–metal-based plasmonic metamaterial absorbers at visible and infrared wavelengths: a review', Materials, 2018, 11, (3), pp. 1– 18 [8]Saville, P.: ' Review of Radar Absorbing Materials', ( Defence Research & Development Canada, Ottawa, Canada, 2005) [9]Cao, M., Zhu, J., Yuan, J., et al.: 'Computation design and performance prediction towards a multi-layer microwave absorber', Mater. Des., 2002, 23, (6), pp. 557– 564 [10]Gogoi, J.P., Bhattacharyya, N.S., Bhattacharyya, S.: 'Single layer microwave absorber based on expanded graphite–novolac phenolic resin composite for X-band applications', Compos. B, Eng., 2014, 58, pp. 518– 523 [11]Chung, Y.-C., Kim, B.-W., Park, D.-C.: 'Range of validity of transmission line approximations for design of electromagnetic absorbers', IEEE Trans. Magn., 2000, 36, pp. 1188– 1192 [12]Álvarez, H.F., De Cos Gómez, M.E., Las-Heras, F.: 'A thin C-band polarization and incidence angle-insensitive metamaterial perfect absorber', Materials, 2015, 8, pp. 1666– 1681 [13]Nornikman, H., Malek, M.F.B.A., Ahmed, M., et al.: 'Setup and results of pyramidal microwave absorbers using rice husks', Prog. Electromagn. Res., 2011, 111, pp. 141– 161 [14]Iqbal, M.N., Malek, M.F., Lee, Y.S., et al.: 'A study of the anechoic performance of rice husk-based, geometrically tapered, hollow absorbers', Int. J. Antennas Propag., 2014, 2014, pp. 1– 9 [15]Nam, I.W., Lee, H.K., Jang, J.H.: 'Electromagnetic interference shielding/absorbing characteristics of CNT-embedded epoxy composites', Compos. A, Appl. Sci. Manuf., 2011, 42, (9), pp. 1110– 1118 [16]Kaur, R., Aul, G.D., Chawla, V.: 'Improved reflection loss performance of dried banana leaves pyramidal microwave absorbers by coal for application in anechoic chambers', Prog. Electromagn. Res. M, 2015, 43, pp. 157– 164 [17]Kaur, H., Aul, G.D., Chawla, V.: 'Enhanced reflection loss performance of square based pyramidal microwave absorber using rice husk-coal', Prog. Electromagn. Res. M, 2015, 43, pp. 165– 173 [18]Holtby, D.G., Ford, K.L., Chambers, B.: 'Genetic algorithm optimisation of dual polarised pyramidal absorbers loaded with a binary FSS'. Loughborough Antennas and Propagation Conf. (LAPC 2009), Loughborough, UK, November 2009, pp. 217– 220 [19]Duggal, S., Aul, G.D., Chawla, V.: 'Investigation of absorption properties of sugarcane bagasse-coal pyramidal microwave absorber'. 2016 Asia-Pacific Microwave Conf. (APMC), New Delhi, India, December 2016, pp. 1– 10 [20]Zahid, L., Jusoh, M., Badlishah Ahmad, R., et al.: 'FEMC performance of pyramidal microwave absorber using sugarcane baggasse and rubber tire dust at 1 GHz to 18 GHz frequencies', Appl. Comput. Electromagn. Soc. J., 2019, 34, pp. 162– 171 [21]Dai, Q., Butt, H., Rajasekharan, R., et al.: 'Fabrication of carbon nanotubes on inter-digitated metal electrode for switchable nanophotonic devices', Prog. Electromagn. Res., 2012, 127, pp. 65– 77 [22]Cao, M.-S., Yang, J., Song, W.-L., et al.: 'Ferroferric oxide/multiwalled carbon nanotube vs polyaniline/ferroferric oxide/multiwalled carbon nanotube multiheterostructures for highly effective microwave absorption', ACS Appl. Mater. Interfaces, 2012, 4, (12), pp. 6949– 6956 [23]Wen, B., Cao, M.-S., Hou, Z.-L., et al.: 'Temperature dependent microwave attenuation behavior for carbon-nanotube/silica composites', Carbon, 2013, 65, pp. 124– 139 [24]Wen, F., Zhang, F., Liu, Z.: 'Investigation on microwave absorption properties for multiwalled carbon nanotubes/Fe/Co/Ni nanopowders as lightweight absorbers', J. Phys. Chem. C, 2011, 115, (29), pp. 14025– 14030 [25]Seng, L.Y., Wee, F.H., Rahim, H.A., et al.: 'Design of multiple-layer microwave absorbing structure based on rice husk and carbon nanotubes', Appl. Phys. A, Mater. Sci. Process., 2017, 123, (1), p. 73 [26]Pometcu, L., Méjean, C., Benzerga, R., et al.: 'On the choice of the dielectric characterization method for foam composite absorber material', Mater. Res. Bull., 2017, 96, pp. 107– 114 [27]Pometcu, L., Benzerga, R., Sharaiha, A., et al.: 'Combination of artificial materials with conventional pyramidal absorbers for microwave absorption improvement', Mater. Res. Bull., 2017, 96, pp. 86– 93 [28]Cheng, Y.Z., Cheng, Z.Z., Mao, X.S., et al.: 'Ultra-thin multi-band polarization-insensitive microwave metamaterial absorber based on multiple-order responses using a single resonator structure', Materials, 2017, 10, (11), pp. 1– 12 [29]Drake, E.A., Rajamani, V., Bunting, C.F., et al.: 'Extension and verification of absorbing material effectiveness on reducing electromagnetic emissions'. 2015 IEEE Symp. on Electromagnetic Compatibility and Signal Integrity (EMCSI 2015), Santa Clara, CA, USA, March 2015, pp. 226– 230 [30]Fan, D., Shen, H., Huang, L., et al.: 'Microwave-absorbing properties of rice starch', Polymers, 2015, 7, pp. 1895– 1904 [31]Lee, Y.S.Y., Malek, M.M.F.B.A., Cheng, E.M.E.E.M., et al.: 'Experimental determination of the performance of rice husk-carbon nanotube composites for absorbing microwave signals in the frequency range of 12.4–18 GHz', Prog. Electromagn. Res., 2013, 140, pp. 795– 812 [32]Iqbal, M.N., Malek, M.F.B.A., Lee, Y.S., et al.: 'A simple technique for improving the anechoic performance of a pyramidal absorber', Prog. Electromagn. Res. M, 2013, 32, pp. 129– 143 Citing Literature Volume14, Issue3February 2020Pages 215-222 FiguresReferencesRelatedInformation