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.     

A high‐gain compact circularly polarized microstrip array antenna with simplified feed network

A broadband high‐gain circularly polarized (CP) microstrip antenna operating in X band is proposed. The circular polarization property is achieved by rotating four narrow band linearly polarized (LP) microstrip patch elements in sequence. Since the conventional series‐parallel feed network is not conducive to the miniaturization of the array, a corresponding simplified feed network is designed to realize the four‐way equal power division and sequential 90° phase shift. With this feed network, the impedance bandwidth (IBW) of the CP array is greatly improved compared with that of the LP element, while maintaining a miniaturized size. Then, parasitic patches are introduced to enhance the axial ratio bandwidth (ARBW). A prototype of this antenna is fabricated and tested. The size of proposed antenna is 0.93λ₀ × 0.93λ₀ × 0.017λ₀ (λ₀ denotes the space wavelength corresponding to the center frequency 10.4 GHz). The measured 10‐dB IBW and 3‐dB ARBW are 13.6% (9.8‐11.23 GHz), 11.2% (9.9‐11.07 GHz) respectively, and peak gain in the overlapping band is 9.8 dBi.     

A low‐profile wideband dual‐polarized antenna with gain enhancement,low gain variations,and low cross polarization for 5G indoor communications

A low‐profile wideband dual‐polarized antenna with high gain, low gain variations, and low cross‐polarization for the fifth generation (5G) indoor distribution system is proposed. By using circular‐thread vase‐shaped structure, a low profile of 0.23λ₀ (λ₀ is the free‐space wavelength at the starting frequency) as well as low gain variation feature can be achieved by the vertically polarized (VP) radiating element. An eight‐way power divider network is employed to feed the horizontally polarized (HP) dipoles so that wideband performance is obtained. Here, eight pairs of arc‐shaped parasitic strips are used to broaden the bandwidth, and eight pairs of director elements are introduced to enhance the gain and reduce the gain variations. In addition, the protruded stubs that are extended from the circular ground plane will help to reduce the cross polarization in the VP direction. Measured results show that a bandwidth of 46.5% (3.3‐5.3 GHz) (S₁₁ < ?10 dB) with a gain of 0.85 ± 0.35 dBi, and another bandwidth of 85.0% (2.5‐6.2 GHz) with a gain of 4.75 ± 1.75 dBi can be realized in the HP and VP directions, respectively. Furthermore, high isolation (>27 dB) and low cross polarization (<?24 dB) can also be attained. Therefore, the proposed antenna is a good candidate for 5G indoor distributed system.     

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.     

A compact monopolar patch antenna with bandwidth‐enhancement under dual‐mode resonance

A compact monopolar microstrip patch antenna (MPA) with enhanced‐bandwidth is proposed. In order to achieve the miniaturized patch, the zeroth‐order mode of the MPA instead of its higher‐order modes is employed at first by loading the shorting pin around the center of the patch. After that, a L‐shaped microstrip line with a shorting pin is introduced at the periphery of the patch radiator to excite an additional non‐radiative mode for bandwidth enhancement. In final, the proposed MPA is fabricated and measured. The results illustrate that the antenna generates an enhanced‐bandwidth of about 4.1% ranging from 2.39 to 2.49 GHz, which is significantly larger than that of the traditional MPA around 1%. Meanwhile, the dimensions of the radiating patch are obviously decreased down due to the employment of zeroth‐order mode, which are kept as small as about 0.17 λ₀ × 0.22 λ₀ × 0.026 λ₀ (λ₀ is the free‐space wavelength).     

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 wideband circularly polarized crossed magneto‐electric dipole

A novel wideband crossed magneto‐electric (ME) dipole for circularly polarized (CP) radiation is proposed in this paper. The proposed antenna consists of a crossed dipole, four parasitic elements, and two pairs of folding metal plates (magnetic dipole). The parasitic elements and magnetic dipole are employed to enhance the axial ratio bandwidth (ARBW). The antenna size is 0.51λ₀ × 0.51λ₀ × 0.33λ₀, where λ₀ is the corresponding free‐space wavelength at the center frequency. A prototype antenna is fabricated and tested. The experiment results depict that the impedance bandwidth (IBW) for voltage standing wave ratio < 2 is 79.2% (2.5‐5.78 GHz) and the 3‐dB axial ratio bandwidth (ARBW) is 72.5% (2.7‐5.77 GHz). At the same time, good CP characteristics and stable symmetrical radiation patterns can be obtained across the operation bandwidth.     

A compact widebeam circularly polarized antenna with wide impedance and axial ratio bandwidths

A compact widebeam circularly polarized antenna with wide impedance and axial ratio (AR) bandwidths is proposed in this study. The antenna is composed of a pair of crossed dipoles, four sequentially rotated parasitic elements, a slotted ground plane with four vertical plates, and four inverted L‐shaped structures. A prototype is fabricated and measured. An impedance bandwidth (S11 < ?10 dB) of 102.4% and a 3‐dB AR bandwidth of 83.5% are measured. And half‐power beamwidth is more than 120° over the whole operating band. Radiation patterns are symmetrical and identical in both principal planes. The compact size of antenna is 0.42λ₀ × 0.42λ₀ × 0.16λ₀.     

A wideband circularly polarized slot antenna with antipodal strips for WLAN and C‐band applications

A single‐fed circularly polarized square shaped wide slot antenna with modified ground plane and microstrip feed has been presented. The field in the slot is perturbed by introducing an antipodal strips section attached with a microstrip line to produce circular polarization in a wide band of frequencies. The antipodal strip section consists of a group of four strips of unequal length and separation. The presence of asymmetric perturbations in the slot is mainly responsible for exciting two orthogonal modes in the slot having equal magnitude and 90° phase difference which results in circular polarization. A wide bandwidth of 3.3 GHz (4.4 GHz‐7.7 GHz) has been achieved for an axial ratio value AR < 3 dB with the minimum axial ratio value being 0.3 dB. The impedance bandwidth for |S₁₁| < ?10 dB ranges from 4.3 GHz to 8 GHz, and therefore covers most of the C‐band communication systems. The antenna exhibits stable radiation patterns throughout the circular polarization bandwidth with a gain around 6 dBi in entire operational bandwidth. A prototype of antenna was fabricated and measured. The antenna has a planar size 0.40λ₀ × 0.40λ₀ and thickness of 0.02λ₀ where λ₀ is the wavelength in free space at the lowest frequency. With its compact size and low profile, the antenna is a favorable choice for WLAN (5.15‐5.85 GHz) and a wide variety of C‐band wireless applications.     

Spoof surface plasmon polariton‐fed circularly polarized leaky‐wave antenna with suppressed side‐lobe levels

A wide‐angle scanning circularly polarized (CP) leaky‐wave antenna (LWA) with suppressed side‐lobe levels (SLLs) is proposed, which can be a good candidate for future radar and wireless communication systems. The LWA consists of 12 cross slotted elliptical patch elements, which are fed by a microstrip spoof surface plasmon polariton (SSPP) line. Two fundamental modes of the patch array with two orthogonal polarizations can be excited by the electromagnetic coupling between the array and the SSPP line. By optimizing the elliptical eccentricity e and etching cross slots on the elliptical patch array, a 90° phase difference is introduced, and then, the CP radiation is realized. A tapered aperture field distribution is also realized by adjusting coupling intensities between the patch elements and the SSPP line, which is beneficial to reduce the SLLs. The electrical size of the LWA is 1.29λ₀ × 6.02λ₀ × 0.08λ₀, where λ₀ is air wavelength at 12.9 GHz (broadside direction). Both the simulated and measured results indicate that the CP operating band is 12.0 to 15.0 GHz. The proposed CP LWA scans continuously from ?14° to 38°. In the whole operating band, the axial ratios are less than 3 dB, and the SLLs are less than ?20 dB as well.     

An ultrawideband unidirectional modified foursquare antenna

This study introduces an ultrawideband unidirectional modified foursquare antenna. The antenna consists of two radiating loops and two truncated parasitic patches. A microstrip‐to‐stripline transition is used as a balun to feed the two radiating loops. A square cavity works as a reflector to realise a unidirectional radiation. This antenna has a total size of 0.56λ_L × 0.56λ_L × 0.15λ_L (λ_L: free space wavelength at lowest operating frequency). The measured results show that a broad operating bandwidth of 103.8% (2.67–8.44 GHz) for return loss being higher than 10 dB is achieved. Meanwhile, measured stable unidirectional radiation patterns with a gain (6.9–11.3 dBi) in +z direction, a front‐to‐back ratio better than 14.5 dB and low cross‐polarization level (相似文献   

A miniaturized circularly polarized multi‐dipole antenna with wide axial‐ratio beamwidth via loading loop resonators

A miniaturized, loop resonators (LRs)‐loaded, circularly polarized (CP) multi‐dipole antenna with wide axial‐ratio (AR) beamwidth is proposed and demonstrated in this article. The radiator of this CP antenna consists of two pairs of parallel dipoles loaded with four LRs at their corners. By increasing the length of the LR, or decreasing the coupling space between the dipole and the LR, the effective length of the dipole can be lengthened significantly and thus the working frequency can be reduced without increasing the whole aperture size. As a result, a miniaturized radiator structure is completed. A feeding network consisting of a T‐shaped coupling feeding structure and four coplanar striplines having different lengths are specially designed to feed these dipoles with approximately the same magnitude and 90° phase quadrature. What is more, a cavity reflector is employed to achieve a unidirectional radiation with wide axial‐ratio beamwidth. The radiator of the proposed CP antenna has a small aperture size of only 0.34λ₀ × 0.34λ₀, where λ₀ is the free space wavelength at the working frequency. Measured results are in a good agreement with the corresponding simulated counterparts. Especially, the experimental results show that the antenna has achieved a wide AR beamwidth of 182° and 174° at the center frequency in the xoz and yoz planes, respectively.     

Dual‐functional QM SIW cavity integrating two‐element MIMO antenna and second‐order bandpass filter

A quarter‐mode (QM) substrate‐integrated‐waveguide (SIW) cavity is designed as a dual‐functional component. By etching three slots, four sub‐cavities are formed and then two of them with the same size are individually fed by a coaxial port. Three resonating frequencies are excited in the single QM SIW cavity. One of them can radiate cavity energy input by these ports into free space, implying a two‐element multiple‐input‐multiple‐output (MIMO) antenna, whereas the other two can transmit energy from one port to the other port, indicating a second‐order bandpass filter. Moreover, antenna isolation and filter bandwidth can be adjusted to a certain degree. A prototype with the overall size of 0.40λ₀ × 0.40λ₀ × 0.02λ₀ has been fabricated. The integrated bandpass filter demonstrates the measured center frequency of 3.8 GHz and operating bandwidth of 32 MHz while the integrated MIMO antenna exhibits the frequency of 3.4 GHz, bandwidth of 67 MHz, port isolation of 18.0 dB, radiation gain of 4.0 dBi, and envelope correlation coefficient of 0.25.     

A metasurface‐based slit‐loaded wideband circularly polarized crossed dipole antenna

A metasurface‐based low‐profile crossed dipole antenna with wide circularly polarized bandwidth for 2.45 GHz ISM band wireless communications is proposed and fabricated in this article. Consisting of four slit‐loaded rectangular patches, the double‐sided printing crossed dipoles are fed by a pair of vacant‐quarter printed rings which circularly polarized (CP) radiation could be generated. With slits loaded, by properly combining the fundamental mode of the two inverted L‐shaped dipole, the slot mode and extra resonance generated by the AMC surface, a wideband circularly polarized operation can be obtained. After optimization, the final design with an overall size of 0.44λ₀ × 0.44λ₀ × 0.1448λ₀ at 2.4 GHz had measured a 31.6% (2–2.75 GHz) impedance bandwidth and 3 dB axial ratio bandwidths of measured were 23.2% (2.1–2.65 GHz), respectively. In addition, the antenna performed a small gain variation (7.0–7.5 dBic) and a front‐to‐back ratio (FBR) of over 25 dB across the whole CP region.     

A dual‐patch polarization rotation reflective surface and its application to wideband wide‐beam low‐profile circularly polarized patch antennas

This study investigates the use of a polarization rotation reflective surface (PRRS) to construct a wideband, wide‐beam, low‐profile circularly polarized (CP) patch antenna. The device is composed of a feeding monopole antenna and a novel PRRS‐based dual‐patch artificial magnetic conductor (AMC) cell structure. The PRRS has two polarization rotation (PR) frequency points, generated by properly adjusting the width between square and L‐shaped metallic patches. A large PR band of 35.5% (5.1‐7.3 GHz) was achieved by combining two adjacent PR frequency points. The PRRS‐based patch antenna impedance bandwidth was measured to be 28.6% (5.1‐6.35 GHz), with a 3 dB axial ratio (AR) bandwidth of 21.8% (4.8‐6.4 GHz) and a profile of 0.045λ₀. Additionally, the proposed antenna exhibited the largest AR beamwidth (to our knowledge) of 175° and 128° in the xoz and yoz planes, respectively. It also produced a high broadside gain of 6.7 dBic within the operational bandwidth.     

Compact three‐antenna MIMO system based on triangular half‐mode substrate‐integrated‐waveguide cavity

A compact three‐antenna MIMO system based on a triangular half‐mode SIW cavity is proposed. Two isosceles‐right cavity edges are shorted by metallic vias while a hypotenuse is opened to radiate cavity energy into the air. By etching two T‐shaped slots and adopting coaxial feedings, three antennas are formed. The same operating frequencies are achieved by adjusting the position of these slots and high isolation is obtained by optimizing their length. The proposed design has attractive features of simple configuration and compact size, which is completely printed on a single‐layered substrate without external circuitries. A prototype with the overall size of 0.53λ₀ × 0.53λ₀ has been fabricated. Measured results exhibit the operating frequencies of about 3.51 GHz, high isolation of 16.0 dB, moderate gain of around 4.12 dBi, good radiation efficiency of 81.22%, and low envelope correlation coefficient of 0.16.     

Compact tri‐band MIMO antenna based on quarter‐mode slotted substrate‐integrated‐waveguide cavity

A compact tri‐band multiple‐input‐multiple‐output (MIMO) antenna based on a quarter‐mode slotted substrate‐integrated‐waveguide (SIW) cavity is proposed. By etching a wide slot, a single SIW cavity is divided into two sub‐cavities with the same size. They are fed by coaxial ports to form two MIMO elements and high antenna isolation can be achieved by this slot. To obtain multi‐band operations, two narrow slots are cut in the upper sub‐cavity and the other two slots are etched in the lower sub‐cavity. Three eighth‐mode resonances with different areas can simultaneously occur in these antenna elements. A prototype with the overall size of 0.34λ₀ × 0.34λ₀ has been fabricated. The measured center frequencies of three operating bands are 2.31, 2.91, and 3.35 GHz, respectively. The measured gain at above frequencies is 4.52, 4.29, and 4.57 dBi, respectively. Moreover, the measured isolation is higher than 16.7 dB within the frequency of interest.     

A compact and grounded comb‐shaped microstrip antenna: A key to realize enhanced radiation performance

A compact grounded comb‐shaped single‐element microstrip antenna is proposed with wide beam width and symmetrical radiation pattern in both principal and diagonal planes. The proposed antenna exhibits excellent isolation of around 35 dB between co‐polarized and cross‐polarized radiation while the same for a conventional patch is only 15 dB. Around 105°‐110° of 3 dB beamwidth is achieved in both principal and diagonal (skew) planes at the center frequency. Around 1.5 dB of front‐to‐back radiation isolation is found from the proposed patch. Most importantly, the proposed antenna produces 65% and 35% broader beam in H‐ and E‐planes, respectively, than that of a classical microstrip antenna. Furthermore, in all principal and diagonal planes, the radiation pattern is found to be symmetrical in a wide angular region (?125° to +125°).     

A dual‐polarized low‐profile microstrip patch antenna with U‐ or M‐shaped feed network

In this article, a dual‐polarized low‐profile microstrip patch antenna with U‐ or M‐shaped feed network is presented. The U‐ or M‐shaped feed network is printed on the same layer, which can achieve dual bands (5.3 and 5.8 GHz) and low profile (0.06 λ_g). Dual polarizations and high isolation are realized by making use of a quasi‐cross‐shaped slot feeding. Moreover, the port isolation is better than 25 dB, and the antenna gain is above 8.4 dBi for the two ports. And the cross‐polarization levels in both E and H planes are better than ‐30 dB for the two polarization ports, respectively. The design is suitable for array application in MIMO system. Details of the proposed design and experimental results are presented and well agreed.     

A substrate integrated cavity‐fed dual‐polarized filtering patch antenna with high isolation

This article presents a dual‐polarized filtering patch antenna, which uses two orthogonal modes (TE₂₁₀/TE₁₂₀) of the substrate integrated cavity (SIC) to couple with two orthogonal modes (TM₁₀/TM₀₁) of the patch by the cross slot, respectively. The second‐order filtering response on dual polarizations can be achieved by using just one SIC resonator and one slotted square patch, which display simple structure of the proposed antenna. The slotted square patch provides a new way to obtain same external quality factor of the radiator on dual polarization, which makes the performances on two polarizations agree well with each other when changing the bandwidth. High isolation can be achieved by controlling the space of the vias of the SIC. Radiation nulls can be produced by connecting the coupled lines with the feeding lines in parallel. A prototype with the entire height of 0.019 λ₀ (λ₀ is the free‐space wavelength at center frequency) achieves a 10‐dB bandwidth of 1.6%, the gain of 4.9 dBi at the center frequency, the port isolation of 43 dB, and the out‐of‐band rejection level of 25 dB.