Modeling of mutual coupling of arbitrary order in coupled circuits and array antennas

This article presents a fundamental strategy for accurately modeling the mutual coupling of arbitrary order in any large‐scale electromagnetic structures and high‐density integrated chips such as antenna array elements and coupled circuit elements. The proposed method starts from the modeling of the first‐order mutual coupling, and it consists of two main steps. First of all, an equivalent circuit model describing low‐order mutual coupling (adjacent coupling) is characterized and established, of which each parametric value is accurately extracted by making use of a numerical calibration technique. Then, the circuit model for high‐order mutual coupling (crossover or crosstalk coupling) is generated from the lower order models, and it can further be used for the modeling of mutual coupling of any higher order. The accuracy and efficiency of the proposed method are demonstrated by three different kinds of structure including a linear phased array antenna, a finite periodic electromagnetic structure, and a planar low‐pass filter. This novel approach represents an easy, fast, and effective characterization of arbitrary‐order mutual coupling. It can find applications in the modeling of mutual coupling between any circuit elements and building blocks such as antennas, resonators, and even small discontinuities, and it promises to be helpful for the analysis and iterative design of microwave circuits and antenna arrays. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.     

Reconfigurable wide band circularly polarized antenna array for WiMAX,C‐Band,a and ITU‐R applications with enhanced sequentially rotated feed network

In this article, a wide‐band circularly polarized slot antenna array with reconfigurable feed‐network for WiMAX, C‐Band, and ITU‐R applications is proposed. Different novel methods are used in proposed array to improve antenna features such as impedance matching, 3 dB axial‐ratio bandwidth (ARBW), gain, and destructive coupling effects. Miniaturized dual‐feed square slot antenna, with one attached L‐shaped strip and a pair of T‐shaped strip at ground surface for improving impedance matching and circular polarization (CP) purity, is presented. For further enhancement of CP attributes, reconfigurable sequentially rotated feed network is utilized to obtain wider 3 dB ARBW. Furthermore reconfigurable property of network gives controlling Right and Left handed CPs, respectively. Finally, a special form of Electromagnetic Band gap structure is employed on top layer of substrate that provides high isolation between radiating elements and array feed network to enhance overall performance of antenna. The measured results depict 3 dB ARBW from 4.6 to 7.2 GHz, impedance bandwidth from 3.3 to 8.8 GHz for VWSR<2, and peak gain of 10 dBi at 6 GHz. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:825–833, 2015.     

Novel ultra‐compact two‐dimensional waveguide‐based metasurface for electromagnetic coupling reduction of microstrip antenna array

A novel ultracompact two‐dimensional (2D) waveguide‐based metasurface is proposed herein and applied for the first time to reduce mutual coupling in antenna array for multiple‐input multiple‐output applications. The unit cell of the proposed 2D waveguide‐based metasurface is ultracompact (8.6 mm × 4.8 mm, equal to λ₀/14.2 × λ₀/25.5) mainly due to the symmetrical spiral lines etched on the ground. The metasurface exhibits a bandgap with two transmission zeros attributing to the negative permeability in the vicinity of magnetic resonance and the negative permittivity in the vicinity of electric resonance. Taking advantage of these two features, a microstrip antenna array is then designed, fabricated, and measured by embedding an 8 × 1 array of the well‐engineered 2D waveguide‐based metasurface elements between two closely spaced (9.2 mm, equal to λ₀/13.3) H‐plane coupled rectangular patches. There is good agreement between the simulated and measured results, indicating that the metasurface effectively reduces antenna mutual coupling by more than 11.18 dB and improves forward gain. The proposed compact structure has one of the highest reported decoupling efficiencies among similar periodic structures with comparable dimensions. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:789–794, 2015.     

A tiny EBG‐based structure multiband MIMO antenna with high isolation for LTE/WLAN and C/X bands applications

This article proposes a compact multiple‐input multiple‐output (MIMO) antenna with the electromagnetic band gap (EBG) structures for mobile terminals. The proposed MIMO antenna is composed of two radiation patches in which diagonal and folded microstrip lines are utilized to control the frequency bands. The radiation patch, one EBG structure and a rectangular‐shaped ground plane are etched on both sides of the antenna. The EBG structures have been employed for reducing the mutual coupling between the antenna elements. As a result of the effect of these structures, the mutual coupling between the two elements is reduced by less than ?30 dB. The proposed antenna is implemented on an FR4 substrate with dimensions 20 × 10 × 1 mm³. According to measured results, frequency ranges of 2.2 to 3.6 GHz and 5.1 to 5.9 GHz with S₁₁ < ?10 dB and also 3.7 to 5 GHz and 8 to 12 GHz with S₂₂ < ?10 dB have been obtained. Moreover, measured S₁₂ and S₂₁ with values of less than ?30 dB for both Ports have been realized. Additionally, the envelope correlation and radiation efficiency of the purposed antenna are less than 0.09 and more than 82%, respectively.     

Isolation enhancement in Chinese character antenna array using simple defect ground structure

This article presents the Gui‐shaped Chinese character array antenna (GCCAA), which is loaded with the L‐shaped defect ground structure (LDGS) to improve isolation between the antennas. The GCCAA is obtained by arranging two Tu‐shaped antennas up and down. By etching a pair of LDGS on the ground, the direction of the coupling current is changed on the ground. Most importantly, the LDGS is not etched in the middle of the antenna elements, but loaded along the outer edge of the GCCAA, which not to destroy the appearance of the GCCAA and maintain the compact nature of the Chinese character. The measured results show that LDGS improves isolation from 15.4 to 37.4 dB when the antennas are placed with a 0.065 λ₀ edge‐to‐edge distance. Meanwhile, it has little influence on reflection coefficient and radiation patterns. The decoupling technique is simple and straightforward which can be easy applied in Chinese character array antenna.     

High gain flexible liquid crystal polymer based 8‐element printed antenna for millimetric wave applications

In this article, an offset fed printed dipole antenna 2‐element, 4‐element, and 8‐element arrays are developed and analyzed for millimeter wave applications. The 8‐element array antenna is of compact size with dimensions 43.6 × 25.1 × 0.25 mm³. It achieved a broad impedance bandwidth (S₁₁ < ?10 dB) of 15.7 GHz from 24.7 to 40.4 GHz. The mutual coupling between array elements is less than ?35 dB in the operating band. The antenna achieved a gain of 12.62 to 13.1 dB. The 8‐element array antenna is fabricated on liquid crystal polymer material and tested. Impedance matching, far field radiation characteristics, co‐polarized and cross‐polarized patterns and group delay are analyzed in simulation and experimental measurement. The investigated results are in good agreement and hence, the developed array antenna is attractive for wideband millimeter wave applications.     

Compact U‐array MIMO antenna designs using IWO algorithm

In this article, design of a novel compact four‐channel multiple input multiple output (MIMO) antenna is described. The antenna is composed of four U‐shaped patch elements and operates at 5.8 GHz. The single U‐shaped patch antenna, to operate at this frequency, is designed using the Invasive Weed optimization algorithm. This algorithm is then applied to design two and four‐channel MIMO antenna arrays for high degree of isolation. To measure the array performance under MIMO signaling conditions, a multiport metric is used to characterize the compact array rather than the scattering matrix characterization. The measurement and simulation results of reflection coefficient, mutual coupling, and radiation pattern are presented and discussed. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2009.     

Compact sub‐6 GHz 5G‐multiple‐input‐multiple‐output antenna system with enhanced isolation

A compact two‐element multiple‐input‐multiple‐output (MIMO) antenna system with improved impedance matching and isolation is presented for future sub‐6 GHz 5G applications. The two identical tapered microstrip line fed modified rhombus‐shaped radiating elements are placed in the same orientation at a compact substrate area of 0.24λ₀ × 0.42λ₀ (where, λ₀ at 3.6 GHz) on a shared rectangular ground. A remodeled T‐shaped ground stub is placed between a pair of radiating element to achieve improved impedance bandwidth and isolation. Further, a split U‐shaped stub connected to center of each radiating element to achieve the desired resonant frequency of 3.6 GHz. The proposed antenna covers a ?10 dB operating band of 3.34 to 3.87 GHz (530 MHz) with more than 20 dB isolation between a pair of elements. MIMO performances are also analyzed and experimentally validated. The measured performances of a prototype are found in good agreement with simulated performances. Further, the simulation study is carried out to see the effect of housing and extended ground plane on two‐element MIMO antenna for practical application. An idea of realization of 12‐element MIMO is also studied using the proposed two‐element MIMO antenna.     

S‐shaped planar antenna with 45° tilted bidirectional radiation pattern for motorcycle helmets

An S‐shaped planar antenna (SPA) with a bidirectional radiation pattern and beam tilt characteristic is proposed to achieve maximum communication distance for helmet applications. The proposed SPA is comprised of an S‐shaped radiation strip (consisting of a microstrip meander line and two main arms with two inverted L‐shaped parasitic elements) and a rectangular ground plane, where a simplified microstrip power divider is introduced by modifying the feed structure in the center of the SPA, such that the S‐shaped radiation strip works in the second resonant mode. The proposed SPA generates a tilted beam in the E‐plane with an angle of 45°, which is attributed to the obliquely staggered arms of the antenna at a distance. By introducing inverted L‐shaped parasitic elements at the end of the two arms, the directivity of the bidirectional radiation pattern can be further improved, thereby increasing the antenna gain. The working principle is analyzed theoretically, and the effects of the antenna structural parameters on the radiation pattern are also analyzed. The experimental results indicate that the 3:1 VSWR impedance bandwidth is 120 MHz, with a realized peak gain of 1.5 dBi at 2.45 GHz. The proposed antenna is designed for real‐world applications, allowing the antenna to be obliquely installed while keeping the peak gain direction horizontal for maximum communication distance.     

A new dual C‐shaped rectangular dielectric resonator based antenna for wideband circularly polarized radiation

This article presents a new dual C‐shaped rectangular dielectric resonator (DR) based antenna for generation of wideband circularly polarized (CP) radiation. The proposed antenna comprises of a pair of C‐shaped rectangular dielectric DR and a metal strip with a coaxial probe. By utilization of a metal strip at the side surface of C‐shaped rectangular DR, the wideband CP radiation is achieved from the proposed dielectric resonator antenna (DRA). Fundamental orthogonal modes (TE^x_δ11 and TE^y_1δ1) are excited using the rectangular DRA with a metal strip for the generation of CP fields. The proposed antenna with dual C‐shaped rectangular CP DRA provides the measured ?10 dB reflection coefficient bandwidth of 30.07% (3.22 GHz‐4.36 GHz) with measured 3‐dB axial ratio bandwidth of 14.81% (3.25 GHz‐3.77 GHz) at the boresight. The proposed antenna covers the useful Wi‐MAX band.     

Compact C‐shaped MIMO diversity antenna for quad band applications with hexagonal stub for isolation improvement

A compact MIMO antenna was proposed in this article. The designed antenna is compact in size with dimensions of 20 × 34 × 1.6 mm. In this proposed antenna model the patch consisting of two counter facing C‐shaped elements facing each other in which a hexagonal ring attached to a strip line which is placed in between the two C‐shaped patch acts as the stub. The novelty of the antenna elements lies isolation improvement by using the ground stub with the use of circular ring resonator. The proposed antenna operates in four bands in which 2.66 to 3.60 GHz (Wi‐Max, Wi‐Fi), 4.52 to 5.78 GHz (WLAN), 6.59 to 7.40 GHz (satellite communication), and 9.55 to 10.91 GHz and having bandwidth of 0.94, 1.26, 0.81, and 1.36 GHz at four bands. The envelope correlation coefficient is ECC ≤ 0.3 and diversity gain > 9.8 dB for the operating bands of antenna proposed. This antenna can work in the bands of Wi‐Max, Wi‐Fi, WLAN, satellite communication in X‐band and for radio location, and astronomy applications.     

Millimeter‐wave antenna with wide‐scan angle radiation characteristics for MIMO applications

A three‐element quasi Yagi‐Uda antenna array with printed metamaterial surface generated from the array of uniplanar capacitively loaded loop (CLL) unit‐cells printed on the substrate operating in the band 25‐30 GHz is proposed. The metamaterial surface is configured to provide a high‐refractive index to tilt the electromagnetic (EM) beam from the two dipole antennas placed opposite to each other. The metamaterial region focuses the rays from the dipole antenna and hence increases the gain of the individual antennas by about 5 dBi. The antenna elements are printed on a 10 mil substrate with a center to center separation of about 0.5 λ ₀ at 28 GHz. The three‐element antenna covers 25‐30 GHz band with measured return loss of 10 dB and isolation greater than 15 dB between all the three ports. The measured gain of about 11 dBi is achieved for all the antenna elements. The three antenna elements radiate in three different directions and cover a radiation scan angle of 64°.     

Antipodal tapered slot antenna array using broadband decoupling structure for mutual coupling reduction

In this article, an effective method to reduce the mutual coupling between the antipodal tapered slot antenna (ATSA) array is proposed. This method is mainly implemented by loading a set of decoupling structures (DS) perpendicular to the dielectric substrate between two antenna elements. The proposed DS can provide transmission forbidden band which can effectively prevent leaked electromagnetic waves. DS can operate in most frequency bands within 4 to 17.5 GHz. It can enhance about 23 dB isolation between the ATSA array without affecting bandwidth and radiation characteristics. The proposed ATSA arrays are fabricated and tested. The measured results can verify its excellent properties. The proposed broadband decoupling method is a suitable candidate for restrain mutual coupling of ultra‐wideband planar end‐fire antennas. This design sheds new light on broadband decoupling.     

Wide‐angle scanning circular polarization phased array based on polarization rotation technology

A wide‐beam circular polarization (CP) antenna and a wide‐angle scanning phased array based on novel polarization rotation reflective surface (PRRS) are proposed. The CP wide‐beam pattern is obtained by the combination of the radiation wave from the patch antenna and the orthogonal reflected wave from the PRRS with a 90° phase difference. The proposed CP wide‐beam antenna obtains the patterns with the 3‐dB beamwidth more than 136° and the axial ratio (AR) beamwidth more than 132° in the xoz‐plane. Furthermore, an eight‐element phased array based on the wide‐beam CP antenna element is also developed. The measured results show that the main beam of the array can scan from ?65° to 65° with a gain fluctuation less than 3 dB and the ARs at every scanning angle less than 3 dB.     

A triangular MIMO array antenna with a double negative metamaterial superstrate to enhance bandwidth and gain

Multiple‐input‐multiple‐output (MIMO) array antenna integrated with the double negative metamaterial superstrate is presented. The triangular metamaterial unit cell is designed by combining two triangular elements positioned in complementary on the same plane at different sizes. Such design with more gaps is used to excite rooms for more capacitance effects to shift the resonance frequency thus enlarging the bandwidth of the MIMO antenna. The unit cell is arranged in 7 × 7 periodic array created a superstrate metamaterial plane where the C_stray exists in parallel between the two consecutive cells. It is found that the existence of C_stray and gaps for each unit cells significantly influenced the bandwidth of the MIMO antenna. The higher value of the capacitance will lead to the negativity of permittivity. The superstrate plane is then located on top of the 4 × 2 MIMO with a gap of 5 mm. The integration resulted in improving the bandwidth to 12.45% (5.65‐6.4GHz) compared to only 3.49% bandwidth (5.91‐6.12GHz) of the MIMO antenna itself. Moreover, the negative permeability characteristic is created by a strong magnetic field between the complementary unit cells to have 14.05‐dBi peak gain. Besides that, the proposed antenna managed to minimize the mutual coupling and improve the mean effective gain, envelope correlation coefficient, and multiplexing efficiency.     

An SIW filtering antenna array with quasi‐elliptic gain response based on intercavity bypass coupling

In this article, a filtering antenna array based on substrate integrated waveguide (SIW) is proposed with quasi‐elliptic gain responses for the first time. Two radiation nulls in the gain responses at two sides of the frequency band edges are designed by applying a novel intercavity bypass coupling scheme. First, by carefully analyzing the bypass coupling between the in‐band and out‐of‐band modes in a single oversized TE₁₀₃ mode cavity resonator, quasi‐elliptic filtering responses are achieved for a two‐output filter. Afterward, those cavity resonators coupled with the two outputs are replaced by cavity‐backed slot antennas to achieve the proposed filtering cavity‐backed slot antenna array. Only one cavity is required in our design to achieve the two transmission zeros and the function of power divider. As a result, the complexity of the proposed filtering antenna array is reduced. A prototype operating at Ka band is designed, fabricated and measured with a center frequency of 28.5 GHz and fractional bandwidth of 1.25%. Similar characteristics can be observed between the gain response of the proposed filtering antenna and the transmission responses of the two‐output filter. The proposed SIW filtering antenna array has great potential to be integrated into millimeter‐wave transceiver modules. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:191–198, 2016.     

A WLAN band‐notched compact four element UWB MIMO antenna

A compact four‐element multiple‐input‐multiple‐output (MIMO) antenna for ultra‐wideband (UWB) applications with WLAN band‐notched characteristics is proposed here. The proposed antenna has been designed to operate from 2 to 12 GHz while reject the frequencies between 4.9 to 6.4 GHz. The four antenna elements are placed orthogonal to attain the polarization diversity and high isolation. A thin stub connected to the ground plane is deployed as a LC notch filter to accomplish the rejected WLAN band in each antenna element. The mutual coupling between the adjacent elements is at least 17 dB while it has low indoor and outdoor envelop correlation (<0.45) and high gain with compact size of two boards, each measuring 50 × 25 mm². To validate the concept, the prototype antenna is manufactured and measured. The comparison of the simulation results showed good agreement with the measured results. The low‐profile design and compact size of the proposed MIMO antenna make it a good candidate for diversity applications desired in portable devices operating in the UWB region.     

Broadband compact CPW‐fed metasurface antenna for SAR‐based portable imaging system

A broadband and compact coplanar waveguide (CPW) coupled‐fed metasurface (MS)‐based antenna for C‐band synthetic aperture radar (SAR) imaging application is proposed in this article, which is consisted of 16 uniform periodic square patches performed as radiators. The CPW feeding structure gives two following functions: (1) It excites an aperture coupling slot structure underneath the center of MS patch array. (2) It acts as a ground plane for the metasurface patch units. Different slots were investigated and eventually an hourglass‐shaped slot is applied to enhance bandwidth for imaging applications. A prototype with a dimension of 60 × 60 × 1.524 mm³ (1.1λ₀ × 1.1λ₀ × 0.03λ₀) operating at the center frequency 5.5 GHz (f₀) has been fabricated and measured to verify the design principle. This antenna has a measured impedance bandwidth of 12.4% from 5.14 to 5.82 GHz, a peak gain of 9.2 dBi and averaged gain of 7.2 dBi at broadside radiation. Microwave imaging experiments using the proposed antenna have been carried out and a good performance is achieved.     

Compact circularly polarized beam‐switching wireless power transfer system for ambient energy harvesting applications

In this work, we propose a circularly polarized (CP) beam‐switching wireless power transfer system for ambient energy harvesting applications operating at 2.4 GHz. Beam‐switching is achieved using a low profile, electrically small CP antenna array with four elements and a novel miniaturized 4× 4 butler matrix. The CP antenna is designed with an e‐shaped slot and four antennas. The CP antenna measures 0.32 λ₀× 0.32 λ₀× 0.006 λ₀ at 2.4 GHz. The antenna has a gain of 3 dBic and an axial ratio less than 3‐dB at 2.4 GHz. A linear antenna array consisting of four elements is designed with the CP antenna element with an inter‐element distance of 0.29 λ₀ . A 4× 4 butler matrix with miniaturized couplers and crossovers are used to feed the four antenna array elements. Based on the input port of excitation, the main beam of the antenna array is demonstrated to be switched to four directions: ?5°, 65°, ?55°, and 20°. A CP rectenna is used to demonstrate the wireless power transfer capability of the combination of the butler matrix and the CP‐antenna array. The rectenna consists of a Teo‐shaped CP antenna and a rectifier. The open circuit voltage at the output of the rectenna is found to peak value of 30 mV at ?3°, 61°, ?53°, and 17°. Thus a complete system for CP wireless power transfer including the power transmission system as well as the RF energy harvesting sensor is designed and experimentally verified.     

Broadband T‐bar fed slot antenna array with stable horizontally polarized omnidirectional radiation

A broadband horizontally polarized omnidirectional antenna array is proposed, which consists of a circular array of four identical broadband T‐bar fed cavity‐backed slot antenna elements and a 1‐to‐4 power divider. The proposed omnidirectional antenna array has a compact diameter of only 0.44λ₀, a broad bandwidth of 75.9% (450‐1000 MHz) and a favorable omnidirectional radiation pattern in the azimuth plane with a gain variation below 3 dB in the operating band. Moreover, the cavity‐backed structure makes the proposed antenna array hardly affected by metal environment and the all metal construction allows for high‐power applications, and the reserved cable channel behind the cavities of the antenna elements ensures the extensionality and stability of the proposed array when longitudinal array expansion is needed. Design procedures of the proposed antenna array have been described in detail, simulations and measurements of the proposed antenna array have also been carried out to validate its performance in this article.