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.     

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.     

Eighth‐mode SIW based compact high gain leaky‐wave antenna

In this article, a novel electrically small eighth‐mode substrate integrated waveguide (EMSIW) based leaky‐wave antenna (LWA) in planar environment is presented. The proposed antenna uses 1/8th mode SIW resonator which helps to improve compactness of the design while maintaining high gain and increased scanning angle. The proposed SIW cavity is excited by a 50 Ω microstrip line feeding to resonate at dominant TE₁₁₀ mode in X‐band. The dimensions of the resonators are adjusted to keep resonant mode at same frequency. The fabricated prototype is approximately 5λ₀ long. Measured results show that the proposed leaky‐wave antenna is able to operate within frequency range of 8‐10 GHz with beam scanning range of 51° and maximum gain of 13.31 dBi.     

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.     

Integrated circular polarized microstrip antenna with dual‐mode‐polarization insensitive characteristics

A dual‐mode circularly polarized compact antenna with integrated left‐hand and right‐hand circular polarization (LHCP and RHCP) is presented in this work. A multilayer arrangement of a square patch and square ring structure with an irregular transmission line is analyzed for dual‐band, dual‐CP operation. To realize dual mode propagation the proposed structure is excited using electromagnetic coupling technique. Succeeding proximity feeding with T‐stub match is analyzed, which conveys impedance bandwidth of 180 and 300 MHz within |S₁₁| < ?10 dB at 3.5 and 5.5 GHz. The designed CP elements is suitably arranged with feed line for generating two orthogonal polarization of equal amplitude and a 90° phase difference at both the resonant modes (TM₁₀ and TM₀₁). Alterable LHCP and RHCP performance is realized by altering the compensated position and peculiar angle. Having both LHCP and RHCP polarization this design shows polarization insensitive characteristic. Each LHCP and RHCP antenna element accomplished a 3‐dB AR of 70 and 120 MHz with a gain up to 6 dBi. With a low profile of 0.27λ₀ × 0.27λ₀ × 0.04λ₀, the CP antenna is fabricated, and the performance is validated through experimental analysis. With all the viable characteristics, the antenna is proposed for Wi‐MAX/WLAN communication.     

Substrate integrated low‐profile dual‐band magneto‐electric dipole antenna

In this article, a novel substrate integrated low‐profile dual‐band magneto‐electric (ME) dipole antenna is proposed. The entire antenna is constructed by four‐layer printed circuit boards (PCBs). Consequently, the height of the proposed antenna is decreased from 0.25λ₀ to 0.11λ₀ (λ₀ is the free‐space wavelength at 5.5 GHz). By introducing rectangular patches with different sizes as electric dipoles, dual operating bands are achieved. Meanwhile, for the purpose of improving the impedance matching at the lower frequency band, a pair of complementary split‐ring resonators (CSRRs) is etched on the larger rectangular patches. Moreover, the short walls composed of plated through holes operate as a magnetic dipole. The antenna is fed by an equivalent wideband microstrip‐to‐parallel stripline balun. The results show that the antenna obtains dual bandwidths of 4.31‐4.71 GHz (8.8%) and 5.07‐5.89 GHz (14.9%) with VSWR <2, which can be applied for C‐band and 5G WiFi. Over the dual operating bands, stable gain and unidirectional radiation patterns with low polarization and low back lobe are also obtained.     

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.     

Ultrawideband high‐gain dual‐polarized antenna with anti‐interference capability

In this article, a high‐gain dual‐polarized antenna with band‐rejection capability for ultrawideband (UWB) applications is proposed. Tapered dipoles are chosen as a primary radiator to achieve UWB operation and it is reflected by a metallic cavity reflector for high gain radiation. A notch at WLAN band is realized by etching a set of four bent slots in the radiating elements. The measured results demonstrate that the proposed design with overall dimensions of 0.69λ _L × 0.69λ _L × 0.16λ _L (λ _L is free‐space wavelength at the lowest operating frequency) has operating bandwidth of 95.1% (3.2‐9.0 GHz) and the rejected frequency band from 5.0 to 5.9 GHz. Additionally, good unidirectional radiation patterns with a broadside gain from 8.1 to 11.5 dBi and radiation efficiency of better than 90% are also achieved.     

A low‐profile wideband dual‐polarization patch antenna with antisymmetric feeding network

In this paper, a novel broadband dual‐polarization patch antenna is proposed. Antisymmetric Γ feeding network is applied to excite the radiating patch etched on the upper side of the horizontal substrate, which could minimize the undesired radiation from the probe and extend the impedance bandwidth. For verifying the proposed approach, a prototype is fabricated and measured, the simulated and measured results show the antenna has a wide impedance bandwidth of 48% (1.66‐2.71 GHz) for S11 < ?10 dB, as well as stable radiation gain around 9.5 dBi with low cross‐polarization. In addition, the total height of the antenna is only 0.17 λ₀ ( λ₀ is the free space wavelength of central frequency) and high port‐to‐port isolation is better than 30 dB. The characteristics of the proposed antenna illustrate it can be an indication for a micro base station in the mobile communication system.     

Single‐layer low‐profile wideband circularly polarized patch antenna surrounded by periodic metallic plates

This article presents a simple design of circularly polarized (CP) antenna with low profile and wideband operation characteristics. To achieve these desirable features, a truncated corner squared patch is chosen as primary radiating source and surrounded by periodic metallic plates for bandwidth enhancement. Notably, all the radiating elements are designed on a single layer of substrate using printed circuit techniques, which significantly reduces the design complexity. The final prototype with overall size of 0.60λ_o × 0.60λ_o × 0.05λ_o (λ_o is free‐space wavelength at the center operating frequency) was fabricated and tested. Measured results show that the proposed antenna has wide operation bandwidth of 19.7% (5.1‐6.2 GHz). Additionally, broadside gain ranging from 5.0 to 6.9 dBic is also attained within the operating band. In comparison with the other reported antennas in literature, the proposed one has the simplest design architecture with competitive operating bandwidth.     

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.     

A dipole‐type millimeter‐wave antenna with directional radiation characteristics

A dipole‐type millimeter‐wave (mm‐wave) antenna with directional radiation characteristics is presented. A radiating patch structure composed of a dipole‐type radiation patch and a rectangular‐shaped parasitic patch are initially investigated to achieve a wider bandwidth. To further improve the operating bandwidth and to realize a directional radiation characteristic, this radiating patch structure is top‐loaded above a conducting cavity‐backed ground structure, which has a low profile (thickness of 3 mm). The measured results show that the proposed mm‐wave antenna can achieve a wide 10‐dB bandwidth of 51.3% (29.6‐50.0 GHz) and stable gain across the desired frequency range. Furthermore, good directional characteristics over the entire mm‐wave frequency band with a compact antenna size of 0.64λ_40GHz × 0.91_λ40GHz × 0.43_λ40GHz are also realized. Hence, it is suitable for many small size wireless mm‐wave systems.     

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.     

A beam scanning Fabry–Pérot cavity antenna for millimeter‐wave applications

A beam scanning Fabry‐Pérot cavity antenna (FPCA) for 28 GHz‐band is presented in this article. The proposed antenna consists of a slot‐fed patch antenna and several layers of perforated superstrates with different dielectric constant. The beam of the antenna can be controlled by moving the superstrate over the antenna. By increasing the offset between the feeding antenna and the superstrate, a larger tilt angle can be obtained. The size of the antenna is 0.95λ₀ × 0.95λ₀ × 0.48λ₀ at 28.5 GHz. The results show the proposed antenna achieves an impedance bandwidth (S₁₁ < ‐10 dB) of 10.5% (27.2‐30.2 GHz), and the beam can be scanned from 0° to 14° in the yoz‐plane with the offset changed from 0 mm to 2 mm. The gain of the antenna is enhanced by 5 dBi in comparison with the feeding antenna without the superstrate, which ranges from 10.91 to 11.53 dBi with the different offset. The proposed antenna is fabricated and shows a good agreement with simulated result.     

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.     

A dual‐polarized cross‐dipole antenna with wide beam and high isolation for base station

In this paper, a dual‐polarized cross‐dipole antenna with wide beam and high isolation is designed and analyzed for base station. The proposed antenna consists of two planar cross dipoles with four square patches, two L‐shaped microstrip lines, two ground plates, four parasitic patches, and a reflector. The square patches are placed between the center of cross dipoles to couple with L‐shaped microstrip lines. By introducing the parasitic patches, the wide beam can be realized. The measured results show that the proposed antenna achieves an impedance bandwidth (|S₁₁| < ?10 dB) of about 18.7% (1.9‐2.35 GHz) and an isolation better than 30 dB. A measured gain of 5.7 dBi and a half‐power beamwidth over 120° at the center frequency are obtained. Furthermore, the size of the proposed antenna is only 0.5λ₀ × 0.5λ₀ × 0.22λ₀ (λ₀ is wavelength at the center frequency).     

Compact high gain half‐mode substrate integrated waveguide cavity antenna using hybrid mode

In this article, a compact fully planar high gain antenna based on half‐mode substrate integrated waveguide (HMSIW) cavity is presented. The design uses a novel configuration of HMSIW cavity with high length to width ratio along with tapered open edge and a pair of slot stub. The high length to width ratio of the cavity helps to excite closely spaced multiple TE^y_m10 cavity modes within comparatively smaller footprint due to use of the HMSIW cavity. These modes combine to give hybrid mode resonance in the cavity which helps to generate a narrow beam high gain radiation pattern of the antenna. The size of the proposed antenna is further reduced and a pair of slot stub is put along the sidewall of the cavity which helps to sustain similar hybrid mode field distribution within much smaller dimension. A size reduction of 76.7% is achieved in the proposed design configuration without degrading much of the gain performance. The proposed antenna resonates at 9.8 GHz with a gain of 7.9 dBi which is much higher than other reported HMSIW cavity antenna. The proposed antenna may find application in point to point communication, short range radar in X band.     

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.     

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°.