The design of a thin microwave absorber which exhibits a −10 dB reflectivity bandwidth of 108% at normal incidence and 16% for simultaneous suppression of TE and TM polarised waves over the angular range 0-45°is presented. The structure consists of a 3 mm-thick metal backed frequency selective surface (FSS) with four resistively loaded hexagonal loop elements in each unit cell. The surface resistivity and width of the loops are carefully chosen to maximise the bandwidth by merging the reflection nulls that are generated by the multi-resonant absorber. Measurement and simulation results are in good agreement over the broad frequency range 7.8-24 GHz.Introduction: Microwave absorbers based on resistively loaded high impedance surfaces [1] are an attractive option for providing radar cloaking where the main design drivers are weight and thickness. However, the reflectivity bandwidth of this class of absorbers is proportional to the structure thickness [2-6], which is determined by the gap between the frequency selective surface (FSS) and the ground plane. In [2], a −10 dB reflectivity bandwidth of 109% was obtained from a 5 mm-thick metal backed resistively loaded single square loop FSS array. This was designed to resonate at the two frequencies where the imaginary component of the FSS impedance is cancelled by the inductance and capacitance presented by the ground plane with gap widths of <λ/4 and >λ/4 at the lower and upper frequencies, respectively. The FSS absorber studied in this Letter is shown to give a similar bandwidth; however, the ground plane spacing is < λ/4 at all frequencies where the reflectivity is below −10 dB. Moreover, because the structure is 40% thinner, the figure of merit (FOM), which is defined as the common (TE/TM) bandwidth divided by the physical thickness normalised to the centre working frequency, is significantly larger, 670 compared with 470 in [2]. For this arrangement, radar backscatter suppression occurs at four frequencies where the complex impedances of the individual hexagonal loops cancel the inductance which is presented by the ground plane. In [3,5,[7][8][9], the simulated backscatter suppression for various resistively loaded thin FSS absorbers based on single and nested square loop elements has been reported, but only for normal or small incidence angle operation. The structure depicted in Fig. 1 which was studied in this Letter, exhibits higher FOM values except for the arrangement reported in [7], where numerical results are presented for an FSS design with different surface resistances for the five individual nested elements in the range 4-1680 Ω/sq. In contrast, our methodology enables the use of the same surface resistance for all four hexagonal loops in the unit cell which significantly simplifies the construction and manufacture of the FSS. Bi-static measured results obtained for an absorber with an FSS that was fabricated by patterning a uniform thickness film of commercially available shielding paint [10] are shown to be in good agreement with numerical predictions...

Inkjet printing is proposed as a means to create the resistively loaded elements of a Frequency Selective Surface (FSS) which suppresses radar backscatter when placed above a metal ground plane. Spectral transmission and reflection measurements from 9 to 18 GHz show that the dot density of the printed features and the volume ratio of an aqueous vehicle and nano silver ink mixture can be selected to obtain surface resistances in the range 1.2 -200 Ω/sq.Introduction: Thin microwave absorbers composed of metal backed resistively loaded FSS [1]- [3] provide an attractive solution for radar cloaking where the main design drivers are weight and thickness. The equivalent network of the structure consists of a parallel connected FSS sheet impedance, which can be described by an LCR circuit, and the inductance which is presented by the ground plane spaced a distance < λ/4 apart [1]. At resonance the two reactive components of the complex impedance cancel and the absorber can be impedance matched to free space (377 Ω) by carefully selecting the resistance which is used to represent the FSS loss. Therefore unlike conventional bandstop FSS where it is desirable to employ high conductivity metal to create the resonant elements that form the periodic array [4], [5], absorbers based on this technology require selective patterning of material which exhibits a small surface resistance. For example the authors have recently reported the design of thin FSS based microwave absorbers composed of rectangular [6] and nested hexagonal loop [7] elements with surface resistances in the range 13 -175 Ω/sq. Experimental prototypes of both arrangements were constructed using commercially available graphite and carbon based shielding paint, and the required resistive loading was realised by adjusting the thickness of the ink features [6]. However the minor variations of micron thick paint layers deposited on the stencil printed substrates resulted in measured values that were 20% [6] and 30% [7] higher than the nominal design for optimum computed backscatter response. In this letter we describe an alternative manufacturing strategy which provides a low cost, simple and repeatable means to solve this problem. By employing an ink-jet printer to simultaneously pattern the FSS elements on the substrate and digitally control the dot density of a novel solution composed of an aqueous vehicle and nano silver ink mixture, we show that it is possible to obtain surface resistances that are much closer to the specified values for optimum absorber performance.

This paper presents an analysis of the statistical texture representation of the Local Binary Pattern (LBP) variants in the classification of wood defect images. The basic and variants of the LBP feature set that was constructed from a stage of feature extraction processes with the Basic LBP, Rotation Invariant LBP, Uniform LBP, and Rotation Invariant Uniform LBP. For significantly discriminating, the wood defect classes were further evaluated with the use of different classifiers. By comparing the results of the classification performances that had been conducted across the multiple wood species, the Uniform LBP was found to have demonstrated the highest accuracy level in the classification of the wood defects.

A frequency selective surface (FSS) is described which exhibits coincident spectral responses for TE and TM polarisation when the FSS operates at 45°incidence. The structure consists of two closely spaced arrays of ring elements with the conductor split at one or two locations to provide independent control of the resonances for the vertical and horizontal field directions. The FSS is designed to diplex two channels separated by an edge of a band ratio of 1.7:1 and yield a common − 10 dB reflection bandwidth of 10.2%. Measured and numerical results are shown to be in good agreement over the frequency range 9-12 GHz.

This paper proposes and demonstrates a compact integrated filtering antenna built on a square ring resonator coupled with a capacitors loaded microstrip line filter. A microstrip filter module is connected to feeding line of the conventional patch without adding extra space. Thus, the combined configuration possesses radiating and filtering functions simultaneously. The proposed filtenna has a fractional bandwidth (FBW) of 3% at center frequency 2.4 GHz with 2.5 dB of maximum gain. The obtained result shows that the proposed design shows good stopband gain rejection, good selectivity at band edges, and smooth passband gain. Furthermore, the introduced filtenna has advantages of a small size and a simple structure, which makes it ideal for interconnection with different wearable devices operating within 2.4 GHz wireless system range.