A triple‐band antenna for MIMO WLAN applications

A novel triple‐band antenna element by etching parasitic slot on ground plane is presented. A three‐element antenna system for WLAN MIMO communications is fabricated by using the proposed antenna element. The triple‐band antenna element is designed for the WLAN standard frequency ranges (2.4‐2.485, 5.15‐5.35, and 5.475‐5.725 GHz). The three identical antenna elements are rotationally symmetric on the substrate, isolated by using metal‐vias cavity. The measured average peak gain within the operational bandwidth is about 2.7 dBi. The isolation between the antenna elements can achieve better than 17 dB at the lower band (2.25‐2.65 GHz), while more than 32 dB at the higher bands (5.20‐5.35 and 5.47‐5.73 GHz) is obtained.     

Design of a novel miniaturized bandstop frequency selective surface exhibiting polarization insensitivity and enhanced oblique stability

In this article, a miniaturized 2.5‐dimensional frequency selective surface (FSS) bandstop filter operating at 2.4 GHz is presented. The proposed FSS contains meander lines as well as metallic patches on top and bottom layer of FR‐4 substrate, and vertical vias are employed to connect the top and the bottom layers. The proposed configuration significantly reduced the size of unit cell to 0.040λ₀ × 0.040λ₀ (where λ₀ is the free space wavelength) at the desired frequency of 2.4 GHz. Additionally, this element arrangement assists in achieving fractional bandwidth of 140%. The measured ?10 dB bandwidth is from 1 to 4.5 GHz. The proposed FSS is polarization insensitive and highly angularly stable (up to 75°). The equivalent circuit model (ECM) of this FSS and related surface current distribution are also provided to understand its working mechanism. The design performance validation has been carried out through the construction and testing of a functional prototype. The full wave simulation, the ECM, and the measured results depict a promising agreement.     

SIW cavity‐backed 24° inclined‐slots antenna for ISM band application

In this article, a novel resonant series slot linearly polarized antenna is realized using substrate integrated waveguide (SIW) technology for industrial scientific medical radio band (ISM) at 5.8 GHz. The proposed antenna consists of two 24° inclined slots and two metallic vias to produce alternate inductive and capacitive loads. The rectangular slots are introduced at the top metallic surface at an angle of 24° from the Y‐axis to excite a hybrid mode (TE₁₁₀ + TE₁₂₀) near to the modified cavity mode TE₁₂₀. The resonant slots are excited with the help of an inset microstrip feedline which retain its planar integrability. The slots are excited to resonate in the TE₁₂₀ mode at 5.8 GHz. To enhance the bandwidth, the location of two shorting vias are optimized in proximity to the slots. These vias help to couple the hybrid mode and the cavity modes in the desired frequency band, which leads to enhancement in the bandwidth significantly. The proposed geometry is fabricated and experimentally verified. The measured and simulated results depict a good co‐relation which show measured ?10 dB fractional bandwidth of 5.2% with a maximum gain of 7.15 dBi and the front to back ratio better than 15 dB at 5.8 GHz.     

A low‐profile transmitarray antenna using square patch elements with cross dipole slots and vias

A low‐profile transmitarray antenna comprising only a single‐layer substrate and operating at X‐band is presented in this paper. The element consists of two identical metallic layers placed on the upper and lower surfaces of a single‐layer substrate and four vias connecting two metal layers through the substrate. The thickness of the element is 3 mm, corresponding to only 0.1 λ at the design frequency. Metallic layers contain square patches etched with cross dipole slots. By varying the side length of the square patch and adjusting the dimensions of cross dipole slots as well as the locations of the vias, a phase shift of 340° is achieved. A transmitarray using the novel elements is simulated, fabricated, and tested. The simulation and the measurement agree well and the gain at 10 GHz is 25.4 dBi, equivalent to an aperture efficiency of 49%. The low‐profile configuration and satisfactory behavior make this design an appealing candidate for space applications.     

Miniaturization of frequency‐selective rasorber based on 2.5‐D knitted structure

Knitted structure based on through‐silicon vias is utilized to realize the miniaturization of frequency‐selective rasorber (FSR). According to equivalent circuit model analysis, additional inductance and capacitance introduced by an array of metal vias are considered, which is combined with lossy cross‐frame and lossless double square‐loop structure to realize the function of FSR. Through full wave simulation, the proposed 2.5‐D FSR demonstrates one passband between two absorption bands. The simulated results indicate a significant size reduction with P = 0.15 λ_L, where λ_L is the free‐space wavelength at the lowest frequency of ‐10 dB reflection. Moreover, an insertion loss of 0.49 dB can be observed at 3.99 GHz, the fractional bandwidth for reflection coefficients less than ‐10 dB is 100%, and the thickness of the whole structure is 0.138 λ_L, respectively. In addition, the frequency response of this miniaturized FSR is stable for incident angle up to 40° and both linear polarizations. After then, the prototype of this 2.5‐D FSR is fabricated and measured, which shows reasonable agreement with simulated results.     

A wideband reflectarray antenna using single‐layer dipole element attached with T‐shaped stubs

A wideband reflectarray antenna consisting of single‐layer dipole element attached with T‐shaped stubs is proposed. By varying the lengths of the T‐shaped stubs, the unit cell can provide a linear phase curve covering about 420°. Critical design parameters are analyzed to understand its wideband operating mechanism. Using this novel type of unit cells, a 441 element 25° offset‐fed reflectarray with grid spacing of λ/3 at 10 GHz is designed, fabricated, and measured. The experimental results show that the proposed reflectarray can achieve 1‐dB gain bandwidth of 24% and 1.5‐dB gain bandwidth of 37%. In addition, aperture efficiency of 66.6% and cross polarization level of 29 dB are obtained at 10 GHz, respectively.     

Design of a tunable omnidirectional circularly polarized antenna based on VO2

In this article, a VO₂‐based tunable omnidirectional circularly polarized (CP) antenna is designed. The proposed antenna combines copper and metamaterial VO₂. By utilizing the characteristics of insulator‐metal phase transition of VO₂, we can change the length of the resonant branches to achieve tunable working bandwidth. The proposed antenna is composed of a modified floor loaded with VO₂ and copper resonant branches, a top patch with slits, and 14 shorting vias connecting the top path and bottom floor. Different from the traditional electric controlled antennas, antennas based on metamaterial VO₂ do not need to design complicated circuit structures and can be easily tailored by the external temperature (T). The simulated results illustrate that when T ≥ 68°C (state I), the proposed antenna has a 10‐dB impedance bandwidth of 15.9% (2.09‐2.45 GHz), and a 3‐dB axial ratio (AR) bandwidth of 23.4% (2.04‐2.58 GHz). When T < 68°C (state II), it has a bandwidth of 6.5% (2.38‐2.54 GHz) with S₁₁ below ?10 dB, and a bandwidth of 19.9% (2.39‐2.92 GHz) with AR below 3 dB.     

A low‐profile circularly polarized transmitarray antenna using rotated cross dipole elements

A low profile circularly polarized (CP) transmitrarray using double‐layer cross dipole elements is presented in this letter. The proposed element exhibits an overall thickness of only 0.22λ₀, which offers the advantages of light weight, easy fabrication, and low cost compared to conventional transmitarray elements. The transmission coefficient of the element is analyzed by Ansys HFSS, and the results show that the element manifests a high transmission rate and good CP performance. Then, by using angular rotation technique, transmitarray with this type of element is designed, fabricated, and tested. The size of the transmitarray is 6.6 × 6.6λ₀ at the center frequency of 10 GHz. The measured peak gain is 22.5 dB at 9.5 GHz, resulting in an aperture efficiency of 36%. The measured 1‐dB gain and 3‐dB axial ratio bandwidths are 10.2% and 12%, respectively.     

Compact dual wide‐band four/eight elements MIMO antenna for WLAN applications

A compact four and eight elements multiple‐input‐multiple‐output (MIMO) antenna designed for WLAN applications is presented in this article. The antenna operates in IEEE 802.11b/g WLAN (2.4 GHz), IEEE 802.11 ac/n WLAN (5.2 and 5.8 GHz) and WiMAX (5.8 GHz) bands. The resonated mode of the antenna is achieved by two unequal Reverse‐L shaped, line‐shaped slots on top and parasitic element on the ground layer. The single antenna provides wide bandwidth of about 29% (2.3‐3.1 GHz) in lower and 22% (4.9‐6.1 GHz) in the upper band. The compactness of the single element antenna is found about 95% with respect to the patch and 61% in overall dimension. Thereafter an investigation is carried out to design two, four, and eight elements MIMO antennas. All of the multi‐element structures provide compact configuration and cover entire WLAN frequency ranges (2.4‐2.48 and 5.15‐5.85 GHz). The dimension of the proposed eight element MIMO antenna is 102 × 52 × 1.6 mm³. It covers the frequency (measured) from 2.4 to 3.1 GHz and 5 to 6.1 GHz. The diversity performance of the proposed MIMO antenna is also assessed in terms of the envelope correlation coefficient (ECC), diversity gain (DG), and total active reflection co‐efficient (TARC). The ECC is found <0.5 whereas the DG >9.0 is obtained for the desired bands.     

Development of “H‐Shaped” monopole antenna for IEEE 802.11a and HIPERLAN 2 applications in the laptop computer

A very low profile and ultra‐thin “H‐Shaped” antenna for IEEE 802.11a and HIPERLAN 2 wireless applications in the laptop computer is developed. The antenna is designed using only a pure copper strip of size 17.5(L) × 4(W) mm² with thickness of only 0.035 mm. The novelty of the proposed antenna is that the antenna is designed with only one rectangular radiating strip without using any additional reactive components, vias or three dimensional structure. Furthermore, the proposed antenna does not require any additional ground plane for installing in laptops. The proposed antenna is comprised of one radiating strip, one rectangular stub, and two resonating slots, namely, “X” and “Y” of length 7.5 mm and 7 mm, respectively. The proposed structure resonates at around 5.5 GHz can cover the (5.15‐5.35/5.725‐5.825) GHz IEEE 802.11a and (5.15‐5.35/5.470‐5.725/5.725‐5.925) GHz HIPERLAN 2 bands. The fabricated prototype antenna has measured impedance bandwidth (VSWR<2) of 15% (5.10‐5.92 GHz) across the operating bands. The measured radiation patterns are nearly omnidirectional along with stable gain of 5 dBi. Moreover, the proposed antenna exhibits excellent radiation efficiency of around 90% across the operating bands. The simulated and measured results of antenna are found to be in good agreement. The very low profile and ultra‐thin structure make it an excellent candidate for wireless operations in the ultra‐thin laptop computers.     

Wideband bandpass filtering branch‐line balun with high‐isolation

The wideband bandpass filtering branch‐line balun with high isolation is presented in this paper. The proposed balun can be designed for wideband performances by choosing a proper characteristics impedance of input vertical transmission line and odd‐mode impedance of parallel‐coupled lines. The proposed balun was designed at a center frequency (f₀) of 3.5 GHz for validation. The measured results are in good agreement with the simulations. The measured power divisions are ?3.31 dB and ?3.24 dB at f₀ and ?3 ± 0.17 dB within the bandwidth of 0.95 GHz (3 GHz to 3.95 GHz). The input return loss of 24.09 is measured at f₀ and higher than 20 dB over the same bandwidth. Moreover, the measured output losses are better than 11 dB within a wide bandwidth. The isolation between output ports is 20.32 dB at f₀ and higher than 13.2 dB for a broad bandwidth from 1 GHz to 10 GHz. The phase difference and magnitude imbalance between two output ports are 180° ± 4.5° and ± 0.95 dB, respectively, for the bandwidth of 0.95 GHz.     

Novel wideband metal‐only transmitarray antenna based on 1‐bit polarization rotation element

In this article, a novel wideband metal‐only transmitarray based on 1‐bit polarization rotation element is proposed. First, a novel wideband polarization rotation element is designed, which consists of four metallic layers without any substrate layers. The element can be used to rotate polarization of the transmission wave by 90° with respect to that of the incident wave. The element and its mirror image can provide 0° and 180° phase shifts with 1‐bit phase quantization in the 9.2 to 11.2 GHz band with more than 80% polarization conversion rate. Then, by using the proposed element, a 21 × 21‐element transmitarray with a standard pyramidal horn feed is designed and fabricated. The measured results show that the transmitarray achieves 16.8% 1‐dB gain bandwidth with a peak gain of 21.6 dBi. Its cross‐polarization and side‐lobe levels are below ?20 and ?10 dB, respectively, in the operating band. The measured results agree well with the simulation ones, validating effectiveness of the transmitarray design method.     

Design and implementation of a low profile polarization‐reconfigurable transmitarray with linear polarized feed

A novel polarization‐reconfigurable transmitarray with linear polarized feed is presented in this letter. Each element in the proposed transmitarray consists of two cascaded cross dipoles to achieve more than 310° of linear phase with transmission magnitude equal to or better than 0.8 dB. The proposed unit cell has a thickness of 0.22λ₀, which leads to a low profile transmitarray compared with those conventional designs. The element behavior of low transmission loss has resulted in higher antenna efficiency. By properly rotating the feed antenna around the center and selecting the x‐ and y‐dimension of each element, the proposed transmitarray can generate radiation patterns with different polarization states, including left‐handed circular polarization (LHCP), right‐handed circular polarization (RHCP), and linear polarization(LP). For the purpose of validation, a double‐layer center‐fed 11 × 11‐element transmitarray prototype is manufactured and tested at X‐band. Measurement results demonstrate the designed transmitarray realize both high gain performance and multiple polarization operation.     

A high‐isolation dual‐polarized quad‐patch antenna fed by stacked substrate integrated waveguides for millimeter‐wave application

A high‐isolation dual‐polarized quad‐patch antenna fed by stacked substrate integrated waveguide (SIW) that is suitable for millimeter‐wave band is proposed in this paper. The antenna consists of a quad‐patch radiator, a two‐layer SIW feeding structure and two feeding ports for horizontal and vertical polarization. The two‐layer stacked SIW feeding structure achieves the high isolation between the two feeding ports (|S₂₁| ≤ ?45 dB). Based on the proposed element, a 1 × 4 antenna array with a simple series‐fed network is also designed and investigated. A prototype working at the frequency band from 38 to 40 GHz is fabricated and tested. The results indicate that the proposed antenna has good radiation performance at 38 GHz that covers future 5G applications.     

Circularly polarized high efficiency wideband reflectarray using rectangle‐shaped patch element

A single‐layer circularly polarized reflectarray using rectangle‐shaped elements in X‐band is presented in this article. The designed element is analyzed and optimized with parametrical studies to obtain good performance at the operating frequency. The phase shifts can be controlled by varying the dimensions in two orthogonal directions of the element for circular polarization. A reflectarray antenna with 27 × 27 elements has been designed, manufactured and measured. The measured results show that the proposed reflectarray antenna provides a 1‐dB gain bandwidth of 18% and a 3‐dB axial ratio of 13%. The measured peak gain at 10 GHz is 26.1 dBi, which corresponds to the high aperture efficiency of 40.3%.     

Wideband bandpass filter based on U‐slotted SW‐HMSIW cavities

A wideband bandpass filter (BPF) is designed based on U‐slotted slow wave half mode substrate integrated waveguide (SW‐HMSIW) cavities. Similar to the substrate integrated waveguide (SIW), the SW‐HMSIW can also achieve a highpass characteristic while the lateral dimensions can be reduced by about 50%. By etching a U‐shape slot on the SW‐HMSIW cavity, a multiple‐mode resonator (MMR) can be realized, which can achieve a wide passband response and make the overall dimension of the filter much more compact. A wide passband, covering from 6.0 GHz to 10.65 GHz with a FBW about 58.13% is achieved. The measured minimum insertion losses including the losses from SMA connectors are 1.1 dB and return losses are better than 10 dB. Besides, the group delay varies between 0.2 and 0.5 ns within the passband. To validate its practicability, a wideband SW‐HMSIW BPF fabricated on a double‐layer printed circuit board (PCB) is designed and examined. The proposed filter has a more than 54% size reduction compared to the other designs reported in open literatures. The measured results have a good agreement with the simulated results. The effective size of the fabricated filter is about 27 mm × 8.55 mm.     

Design of a compact orthogonal fed self‐diplexing bowtie‐ring slot antenna based on substrate integrated waveguide

In this article, a novel design of compact cavity‐backed slot antenna based on substrate integrated waveguide (SIW) technology is presented for dual‐frequency communication services. A single layer printed circuit board is applied to implement the proposed antenna. The bowtie‐ring slot engraved on the SIW square cavity is excited using two orthogonal microstrip feed lines to operate at two distinct frequencies (6.62 GHz and 11.18 GHz). The proposed antenna allows each of these frequencies to be designed independently. A prototype of the proposed cavity‐backed antenna that radiates at both 6.62 GHz and 11.18 GHz is fabricated and measured. The port isolation better than 29.3 dB is achieved by utilizing the transmission zeros (TZs), which are produced due to the orthogonal feed lines, TE₁₁₀ mode and coupling between the TE₁₂₀ and TE₂₁₀ modes. The measured peak gains of the proposed diplexing antenna are 5.77 dBi and 5.81 dBi at lower and upper resonating frequencies, respectively. The proposed dual‐frequency antenna exhibits the front‐to‐back‐ratio (FTBR) and cross‐polarization level greater than 26 dB and 21 dB, respectively, at both resonating frequencies.     

Low‐cost and wideband circularly polarized slot antenna array

A slot antenna with wideband circular polarization (CP) array, which operates on millimeter waves band, is proposed. A four‐direction sequential rotation technique is used in the feed network to feed the 2 × 2 slot element based on waveguide. The shot element resonates at both the fundamental mode and the high‐order mode. The slot element is studied in high order mode, and the radiation lobe can be redirected by changing the size of the slot element, thus improving the multi‐lobe problem. A strong single lobe is formed in the +z‐direction by using the ground edge diffraction characteristics of the slot element in the waveguide. The designed broadband characteristics are obtained through the appropriate combination of the feed network and CP antenna. The prototype of the antenna with an overall size of 137 mm × 137 mm × 0.6 mm³ is processed and verified by experiments. The prototype of the slot array is processed and examined. The test results display that the device has good performance of |S₁₁| < ?10 dB bandwidth of 3.72 to 6.56 GHz (2.84 GHZ, 55.25%), a 3 dB axial ratio bandwidth of approximately 4.39 to 5.43 GHz (21.18%).     

Isolation enhancement of rectangular dielectric resonator antennas using wideband double slit complementary split ring resonators

A wideband epsilon‐negative structure is employed as one‐layer and two‐layer isolators to reduce mutual coupling in multiple‐input multiple‐output systems composed of two E‐coupled rectangular dielectric resonator antennas. The proposed unit cell with ?15 dB bandwidth for S₂₁ extending from 1970 to 3317 MHz, is a double slit complementary split ring resonator etched on the ground plane of a stripline. Each layer is composed of a 2 × 3 array of the suggested unit cell. Reduction in isolation of more than 11 dB for the one‐layer case and higher than 20 dB for the two‐layer case are measured within the frequency range of 2.604 to 2.64 GHz which includes WiMAX. The highest isolation level of 36 dB is realized at 2.868 GHz. The impedance matching, gain, radiation efficiency, and envelope correlation are improved compared to the original case. A prototype is designed, fabricated, and tested. Simulation data and measurement results are in good agreement.     

Ultrathin dual‐layer triple‐band flexible microwave metamaterial absorber for energy harvesting applications

An ultrathin dual‐layer flexible metamaterial absorber with triple‐band for RF energy harvesting applications has been reported in this article. The sub‐wavelength unit cell of the proposed absorber is composed of six distinct concentric annular having outer circumference of ring and octagonal inner circumference. The metallic resonators are constructed from copper foil self‐adhesive tape which are affixed on flexible neoprene rubber sheet terminated by metal ground plate. The proposed absorber prototype is ultrathin and compact with the thickness less than 0.037λ₀ and cell size less than 0.2λ₀ at the lower absorption frequency of 1.75 GHz. Flexible dual‐layer absorber exhibits triple absorption peaks of 96.91%, 96.41% and 90.12% at 1.75 GHz, 2.17 GHz and 2.6 GHz with full width at half maximum (FWHM) bandwidth of about ~6.5%. The RF performance of proposed absorber is numerically computed for different polarization and incidence angle variations. The absorption value is above 76% for the oblique incidence angle up to 45° in TE mode operation, whereas the absorption value is 94% for oblique incidence angle up to 60° in TM mode operation. The measured outcomes are in agreement with the numerically calculated results. The energy harvesting potential of the proposed absorber structure is numerically confirmed by the resulting improved RF absorption value in dependence to different resistive loading of the polarization insensitive unit cells.