This paper presents the development and implementation of a 4-port MIMO antenna based on a combination of a Hilbert curve fractal and a reactive load implemented via an open-circuited stub. The integration of these techniques results in a highly compact array with a wide operating bandwidth, extending from 2.3 GHz to 5.8 GHz. This covers the 2.4 GHz ISM band and the 5G New Radio (NR) bands N41, N77, N78, and N79. The total array dimensions are 25 mm & times; 36 mm, equivalent to 0.18 lambda 0 & times; 0.26 lambda 0 at the lower cutoff frequency. The minimum separation between elements is 3 mm, approximately 0.02 lambda 0, which is a very small distance between radiators. However, by incorporating an electromagnetic barrier and giving galvanic continuity, also implemented with a Hilbert curve on the ground plane, an isolation ranging from 15 dB to over 20 dB is achieved across the entire bandwidth, with a peak gain of 1.8 dB. Furthermore, the use of the open-circuited stub stabilizes the antenna's impedance behavior, causing the reactive component to approach 0 Omega, while the real part stabilizes within a threshold close to 50 Ohms, significantly increasing the bandwidth. The final result is a compact 4-port MIMO antenna featuring high isolation, an Envelope Correlation Coefficient (ECC) below 0.05 across the entire range, a Diversity Gain (DG) close to 10, and a stable Total Active Reflection Coefficient (TARC) below -10 dB throughout the operating bandwidth.

This paper presents the design of a compact four-element MIMO antenna based on a metamaterial structure and a reactive load generated by an open-circuit stub. The radiator array, arranged in an axial symmetry configuration, provides high inter-element isolation despite a sub-millimeter separation. The design is optimized for 5G n77/n78 band applications and employs a metamaterial structure composed of embedded octagonal split-ring resonators (SRRs) integrated on a Duroid RT5880 0500 (ϵr=2.2,h=1.27 mm) substrate. This configuration achieves high miniaturization, with individual radiators of 19×9.53 mm2. Furthermore, through a stub-loading technique, the array is enhanced in two significant aspects: (a) it exhibits an increased impedance bandwidth, rising from a 23% fractional bandwidth in the stub-less design to 39% in the final architecture; and (b) a shift of the lower cut-off frequency toward lower values is obtained, resulting in a reduction of the radiator's electrical length, which translates into physical size diminution. The total array has a size of only 28.8×28.8 mm2 (0.24λ0×0.24λ0, considering the lower cut-off frequency). Despite the proximity between radiators and the absence of electromagnetic decoupling structures, the design ensures inter-element isolation exceeding 15 dB in the lower band and reaching values above 20 dB in the mid and upper bands. Diversity metric analysis confirms high performance, yielding an Envelope Correlation Coefficient (ECC) ≪0.005, Diversity Gain (DG) close to the ideal value (≥9.9), Total Active Reflection Coefficient (TARC) below -10 dB (converging in random phase analysis), and a Channel Capacity Loss (CCL) of less than 0.4 bits/s/Hz. Therefore, the proposed antenna stands as an ideal design for compact 5G communication devices.

This paper presents a 4-port ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna that is implemented in a very compact footprint of 28 x 28 mm2, the most compact size reported so far in the literature. This antenna is also uniplanar in order to reduce costs and facilitate its integration into flexible substrates as a conformal antenna. The combination of very compact size and uniplanar characteristic makes this antenna unique among recent developments. Despite its small size, the antenna performance is acceptable with a low measured Envelope Correlation Coefficient (ECC) of less than 0.013, and more than 15 dB of isolation between MIMO elements over most of the bandwidth from 2.75 to more than 14 GHz, using a slotted defected ground plane that not only reduces coupling but also creates an extra resonance in the lower part of the bandwidth that allow us to reduce the size in a novel use of the decoupling slots.

This work presents the use of a novel impedance coupling technique and electrical length increase by using stub loading placed from the radiator to the ground plane. This method is applied to the design of a small four-element ultrawideband (UWB) MIMO antenna arranged in axial symmetry to achieve a compact array size while obtaining a bandwidth starting from a very low cutoff frequency compared to a conventional radiator operating at the same frequency. The four-element MIMO antenna, with an operational bandwidth of 1.9 GHz to 30 GHz, is based on a wideband monopole with a semicircular geometry, fed by a coplanar structure and an L-shaped half-ground plane section. To increase the electrical length of the structure and achieve a compact antenna design, reactive stub loading is introduced, placing it on the backside of the substrate, located orthogonally between the radiator and the L-shaped ground plane, obtaining a small-sized configuration. The axial symmetry is employed to increase the antennas’ isolation by taking advantage of the orthogonal positioning and making the radiated fields have a low correlation. The antenna array footprint measures 48 mm × 48 mm, corresponding to 0.3λ0 × 0.3λ0 at the lower cutoff frequency. The array exhibits a low envelope correlation coefficient (ECC) of around 0.033 at 2 GHz, and less than 0.001 at the rest of the bandwidth; a diversity gain (DG) of approximately 10; a stable total active reflection coefficient (TARC) below −10 dB; interport isolation between 20 and 40 dB; and an average gain of 2.8 dBi.

A detailed statistical analysis of the total active reflection coefficient (TARC) is carried out in this paper for three 4-port MIMO antennas featuring different levels of isolation across its ports. This analysis is very useful to determine the most likely performance of a MIMO antenna in a real communications scenario. The TARC parameter is commonly evaluated for only several combinations of the random phase with which a signal reaches every input port of a MIMO antenna. By contrast, we have evaluated a million combinations to obtain the probability density function of the TARC, using frequency as its parameter. In this way, an expected value of the TARC is obtained for each frequency, as well as a confidence interval (ΔCITARC) where the TARC values occur with 90% probability. Additionally, we have introduced the term “TARC shadow”, a visual representation of the TARC as a function of the frequency where the probability function is projected into this 2D graphic with different colors to identify the most likely values of the TARC. To demonstrate these concepts, a full TARC evaluation was performed for three 4-port MIMO antennas with increasing isolation of 12.9 dB, 25.4 dB, and 37 dB between elements, and different values of the Snn and Snm parameters, with n and m= 1 to 4. From this study, the importance of the isolation among ports and its comparison with the return losses becomes evident in achieving a MIMO antenna array insensitive to random phase variations occurring in the communication channel.

This work presents a compact four-port MIMO antenna with each radiator consisting of a conventional two-monopole array fed at a single point by a coplanar line and reactively loaded with a stub. The incorporation of a T-stub-loaded tuning technique significantly improves the radiating element’s impedance, leading to deeper port coupling, a broader bandwidth, and an increased electrical length. Consequently, the operating frequency is substantially lower compared to a standalone radiator. By implementing this configuration with two monopoles of different lengths fed at the same end, an ultra-wideband effect is achieved. By placing four of these stub-loaded monopole arrays in an axial symmetric configuration, a MIMO antenna array is formed. The proposed MIMO array operates from 2.89 GHz to 12 GHz, exhibiting a TARC of less than −10 dB, an ECC of less than 0.002, an average diversity gain of 9.999, and port isolations are within a threshold from −18 dB to −50 dB over the entire bandwidth. The array’s footprint is 32 × 32 mm2, equivalent to 0.083λ02 at the lower cutoff frequency.

In this paper, a very small 4-port MIMO antenna is designed, based on a metamaterial structure composed of embedded octagonal Split-Ring Resonators (SSRs).The antenna array shows an axial symmetry configuration with dimensions of 32 × 32 mm 2 , corresponding to 0.157λ 2 , approximately, related to a center frequency of 3.5 GHz, with a high electromagnetic isolation despite the radiators' closeness, reaching values bigger than 26 dB for adjacent antennas, and more than 28 dB for opposite antennas.The antenna is built on a substrate with dielectric permittivity of 2.2 and 1.27 mm thick.The Total Active Reflection Coefficient (TARC) presents a steady behavior for different random phases at the incoming signals, keeping a system bandwidth of 0.9 GHz for a -10 dB value.On the other hand, the Envelope Correlations Coefficient (ECC) reports values lower than 0.001 in all the antenna relationships, achieving a very uncorrelated performance of the electric fields in each element.The radiation pattern is quasi-omnidirectional, obtaining a low gain around -2 dBi, a trade-off that is considering the size reduction of the MIMO antenna.

In this article, a 4-port multiple input, multiple output (MIMO) antenna array is presented, based on a Vivaldi design for nonplanar applications like access points above 6 GHz. The antenna is configured with four Vivaldi antennas arranged orthogonally, one to each other, to increase the electromagnetic isolation among elements. The Vivaldi element is fed with a microstrip with an integrated balun in only one arm, and each arm is set in an antipodal position to the other, giving an easy-to-design and build prototype, with no complicated feeding techniques. Each antenna plane has dimensions of 90 x 40 mm2. The array presents a bandwidth from 5.3 GHz to beyond 20 GHz, with electromagnetic isolation above 20 dB, an envelope correlation coefficient below 0.001 for the entire bandwidth, giving a diversity gain around 20 dB, and a total active reflection coefficient below -10 dB from 7 to 20 dB, fulfilling the metrics for MIMO applications.

This article presents a symmetrical reduced-size eight-element MIMO antenna array with high electromagnetic isolation among radiators. The array utilizes easy-to-build techniques to cover the n77 and n78 new radio (NR) bands. It is based on an octagonal double-negative metamaterial split-ring resonator (SRR), which enables a size reduction of over 50% for the radiators compared to a conventional disc monopole antenna by increasing the slow-wave factor. Additionally, due to the extreme proximity between the radiating elements in the array, the modal significance (MS) method was employed to identify which propagation modes had the most impact on the electromagnetic coupling among elements. This approach aimed to mitigate their effect by using an electromagnetic barrier, thereby enhancing electromagnetic isolation. The electromagnetic barriers, implemented with strip lines, achieved isolation values exceeding 20 dB for adjacent elements (<0.023 λ) and approaching 40 dB for opposite ones (<0.23 λ) after analyzing the surface current distribution by the MS method. The elements are arranged in axial symmetry, forming an octagon with each antenna port located on a side. The array occupies an area of 0.32 λ2 at 3.5 GHz, significantly smaller than previously published works. It exhibits excellent performance for MIMO applications, demonstrating an envelope correlation coefficient (ECC) below 0.0001, a total active reflection coefficient (TARC) lower than −10 dB for various incoming signals with random phases, and a diversity gain (DG) close to 20 dB.

A reduced-size four-port antenna array with highly electromagnetic isolation based on a metamaterial structure is presented. The radiator antenna unit is created using the split ring resonator (SRR) arrangement, which has been widely demonstrated to perform as metamaterial. The metamaterial comportment allows increasing the slow-wave factor on the structure, and as a result, the array size is 42 x 42 mm(2), where each radiator presents a 52% length reduction compared to a conventional disc monopole with lower cutoff frequency of 2.5 GHz. The separation among elements is very small, achieving values of 0.045 lambda at the resonance; however, despite the proximity among elements, the electromagnetic isolation reaches values around 30 dB all over the working band. The antenna has a wide bandwidth, going from 2.5 GHz to almost 4 GHz. The array performs a very low Envelope Correlation Coefficient (ECC), achieving levels lower than 0.035 and a steady Total Active Reflection Coefficient (TARC), obtaining a system bandwidth of 1.38 GHz for a TARC <= -10 dB, resulting a suitable structure for Multiple-Input Multiple-Output (MIMO) applications.

In this article, a combination of rectangular loop array and slot radiator for multiband applications is presented. The antenna is configured by arranging, concentrically, a set of rectangular loop radiators excited by electromagnetic coupling provided by a dumbbell slot. The size of the loops is calculated to obtain the desired resonant frequencies, which are almost independent of the adjacent rings. The exciting slot is designed to operate in a wideband frequency range to cover the upper desired resonance. In addition, to obtain directive radiation patterns, a reflector shaped like a box is introduced, giving a stable gain, radiation pattern shape, and port matching at the selected frequencies. The configuration presents great results, since to the authors' knowledge, even a similar configuration given in the open literature presents some disadvantages compared to this one; moreover, not just any structure can be employed as the resonating elements, obtaining multiband behavior at the same time.

In this paper, a novel ultrawideband (UWB) four ports multiple-input multiple-output (MIMO) antenna was developed. The MIMO antenna is based on four wideband circular monopoles with inscribed Fibonacci circles in order to reduce the size and stabilize the impedance over a wider bandwidth. The array shows a frequency bandwidth from 3.6 GHz to 13.8 GHz, obtaining inter-port coupling below -15 dB over the entire bandwidth. Convergent TARC curves over the entire bandwidth exhibit values less than -25 dB and high stability against phase changes in the incoming signals to the different radiators. Finally, the UWB MIMO antenna proposed generates a quasi-omnidirectional radiation pattern with orthogonal properties among radiators, demonstrated by a very small Envelope Correlation Coefficient (ECC) value, averaging 0.005.

To establish the RF power traceability up to 50 GHz in Mexico by means of a primary standard for microwave power and millimeter-wave power, a new coaxial microcalorimeter system in 2.4-mm line size along with thermoelectric-type transfer standards (TSs) were developed and their metrological characterization was performed. Coaxial microcalorimeters allow both broadband operation and power substitution through its feeding line; however, characterization and fabrication of millimeter-wave coaxial lines are technologically difficult. Thus, the design of the microcalorimeter is described, as well as the RF performance of its adiabatic lines. To overcome the lack of coaxial TSs up to 50 GHz, commercially available 2.4-mm thermoelectric power sensors are investigated to use them as low-frequency (LF) power substitution detectors. The effective efficiency is measured in the microcalorimeter by means of calorimetric measurements and the substitution of LF power; the developed measurement procedure is presented as well as the measurement results and the uncertainty analysis. The theoretical basis for the procedure employed to determine the microcalorimeter correction factor, based on the use of specially constructed open and short standards, is described with the measurement results and uncertainties are shown as well. To avoid the drift of the millimeter-wave power during the calorimetric measurements, a novel software leveling loop method for thermoelectric power sensors was devised. This method allows to keep the RF power level variations less than 100 ppm and being a straightforward solution to recent complex hardware realizations.

A multiple-input–multiple-output (MIMO) microstrip patch antenna array of four elements is proposed to cover the 5.8 GHz WLAN band. The characteristic mode analysis is carried out to design a defected ground structure that improves the MIMO performance without impacting negatively the radiation characteristics of the four-element array. Simulations and measurements demonstrate that the implemented prototype reaches a maximum mutual coupling of −32 dB and a gain-per-element of 5.3 dB with good MIMO attributes, surpassing many other proposals in the antenna literature: maximum envelope correlation coefficient of 0.0001, diversity gain very close to 10 dB, capacity loss near to 0.0023 bps/Hz at the resonant frequency, multiplexing efficiency of 0.935, and a total active reflection coefficient weakly dependent on the excitation phases. The presented design is proposed for operation on the IEEE 802.11a/n standard.

In this paper, a proposal of a four-port antenna for MIMO applications is presented with high interport isolation. The isolation among elements is implemented by limiting the surface current distribution of each radiator by placing them on different substrate layers, leading to a size optimization. The array is implemented by four drop-shaped slot radiators. The antenna performs from 1.7 to 7.2 GHz, presents a very low Envelope Correlation Coefficient (ECC), reaching values of 0.0025 tops, and a stable Total Active Reflection Coefficient (TARC) over the entire operational bandwidth. These parameters demonstrate the feasibility of the antenna for MIMO systems. The peak average gain is 5 dB, and the port isolation between adjacent radiators, S-12 and S-14, and opposite radiators, S-13 and S-24 are below -20 dB, reaching values even less than -30 dB in some bands without adding extra structures, avoiding an increment in the total area.

Several papers published by prestigious Journals on Antennas and Microwave Techniques have disseminated misconceptions about total active reflection coefficient (TARC), a meaningful metric to characterize multiple-input multiple-output antenna arrays, and the misapprehension has been spread over the years. In this article, an extrapolation to calculate TARC for N-antennas systems is derived and applied to published results for illustration purposes. Moreover, the inaccurate and correct TARC equations are put in context by means of its physic and mathematical insight, demonstrating the divergence of results when the interport RF isolation is comparable or higher than the port matching. The importance of assessing multiantenna systems using TARC is explained.

An antenna array formed by four PIFA elements located very close to each other, with low inter-element matching for MIMO applications is proposed. The antenna array consists of four F-inverted wideband radiators, with a fractional bandwidth around 56%, spaced one to each other by a very short distance (< 0.065λ0) at a centre frequency of 2.55 GHz. The operational bandwidth goes from 1.88 to 3.15 GHz considering the Sii < −10 dB at each port. Moreover, the coupling among ports reaches values below Sij < −10 dB and getting values less than −30 dB at 1.8 GHz, just by employing an uncomplicated technique implemented by a neutralization line between elements. The antenna array gain goes from 2 dB to 6 dB over the operating bandwidth. Concerning MIMO figures of merit, the radiation pattern of each element is orthogonal to each other. The Envelope Correlation Coefficient is below 0.04 at the designed frequency, reaching a peak around 0.082 at 1.8 GHz, but still achieving the requirement for MIMO operation (less than 0.5). The Total Active Reflection Coefficient (TARC) is almost convergent at the design frequency, showing low dependence on random signals at different elements, and finally, the diversity gain reaches values close to 20 dB, making the array suitable for MIMO access point applications.

In this paper four types of cross-dipoles antennas, highly compatible to be utilized in base stations of cellular systems, were thoroughly compared: (1) dipoles with sloping-cut arms, (2) dipoles with folded arms, (3) dipoles with helical shape, and (4) dipoles with helical shape and inverted branches. To carry out the comparison, a design of each type of antenna was performed with an objective bandwidth of 1710–1880 MHz and a return loss higher than 15 dB.By computer simulations, some important parameters were examined: mutual coupling, return losses, input impedance, gain stability, beamwidth invariability, cross-pol discrimination and tracking error. From the comparison, it was found that the crossed dipoles with folded arms occupy the lowest volume and have the higher beamwidth stability. On the other hand, the dipoles with helical shape have the lowest mutual coupling, meanwhile the helical dipoles with inverted branches have the higher cross polarization discrimination. To experimentally validate some of these results, a prototype of the crossed dipoles with helical shape was constructed and measured to demonstrate that the simulations are consistent with the real implementation. After all this study is clear that each antenna has advantages and limitations, and depends on the more stringent requirement that a certain application demands, to select one or another type of antenna.

In this comments, the authors assure that (9) is not appropriate to compute the total active reflection coefficient, and present the correct expression.

In this paper, a very low-profile two-port ultra-wideband antenna for multiple input-multiple output applications is proposed. The antenna is implemented by using two orthogonal quasi-circular slot dipole radiators fed by coaxial lines. The antenna performs from 2 to 10 GHz, with an average peak-gain of 4 dB, and high port isolation, with values around S-21 = -20 dB and below. The radiation patterns of both antennas are opposite to each other and hence, the diversity gain reaches values around 20 dB. The low inter-port coupling and low correlation are verified by obtaining the envelope correlation coefficient, which is lower than 0.003. These calculations were made by the S-parameter and far-field methods. The total active reflection coefficient shows that the antenna operative bandwidth does not change for different input signals with random phases, preserving the operation from 2 to 10 GHz. The antenna performance is compared to different state of the art slot configurations, showing advantages to previously published work.