Abstract
The evolution of 6th generation (6G) wireless technology has become imperative due to the exponential growth of wireless devices and applications. In a macro-cellular scenario, the 6 GHz electromagnetic spectrum is projected to be the framework for 6G commencement. However, the main obstacles that inhibit their potential at the physical layer are the fabrication intricacies entailed in comprehending high port isolation (
) and other key performance indicator (KPI) of compact printed multiple-input-multiple-output (MIMO) antennas. Subsequently, to overcome these impediments, six novel meander line-based MIMO antennas (
) have been introduced for diverse 6G use cases, including internet of things, extended reality, artificial intelligence, vehicle-to-everything, unmanned aerial vehicle, and device-to-device integrated sensing and communications. Furthermore, two unique passive metamaterial structures of square ring (
) and shorting pins (
) have been studied for attaining an electromagnetic bandgap (EBG). Their performance were investigated by means of numerical simulations and validated through measurements conducted within the anechoic chamber. Meticulous strategies for accomplishing impedance matching, circularly polarization (CP), and high 
values have been presented. Each of the proposed MIMO antennas employed dual radiators, a defected ground, and an EBG structure to exhibit 
of 
21 dB, quasi-isotropic CP, and other desirable KPI of MIMO antennas. Their assembly possessed a low-profile of 0.03 free-space wavelength (
) and an area of 1.1
1.1
, thus being preferable for cost-effective compact terminals. During the measurements, each prototype yielded one or more remarkable MIMO antenna KPI in the 6 GHz band. Particularly, 
enabled filtered bandwidth (BW) of 8.84% and modest gain (G) of 6.4 dBic, 
attained high G of 7.1 dBic and enhanced efficiency (
) of 87%, 
yielded high 
of 94%, 
established notable radiation pattern with fair G of 5.8 dBic, 
provided filtered BW of 9.69% and prominent 
of 93%, and 
featured wide axial ratio (AR-BW) of 60.63%. Furthermore, all antenna measurements demonstrated good MIMO performance with envelope correlation coefficient and diversity gain of <0.2 and 
10 dB, respectively. The novelty of this work lies in the radiator and ground designs, as well as the accomplishment of numerous KPI that surpass state-of-the art MIMO antennas.
Subject terms: Electrical and electronic engineering, Metamaterials
Introduction
The installation of cables across diverse locations for wireline communication manifests practical challenges and high expenditures1. A wireless transceiver that uses a printed antenna to convert electric current into and from EM waves represents an inexpensive alternative to a wireline system2. Therefore, the wireless technology has become essential in most of the facets of modern civilization3,4. However, the existing 5th generation (5G) network cannot accommodate the swift expansion of wireless devices and applications5, necessitating the development of 6G wireless technology6. The upper 6 GHz EM spectrum is estimated to remain utilized by both licensed and license-exempt incumbents to further 5G evolution7, and ultimately serve as the foundation for 6G commencement in a macro-cellular scenario8. Furthermore, 6G systems will initially assess the adaptability of antenna beam control by visualizing the quasi-isotropic radiation pattern (RP) at 6 GHz spectrum for channel approximation of millimeter (mm) waves9,10.
Typically, the performance of a single-input-single-output (SISO) system is significantly impacted by link disruptions induced by misaligned transmitting and receiving antennas11. The deployment of a printed antenna array with directional beam scanning operation could lead to trustworthy communication, but it often calls for a substantial amount of space, making it inappropriate for compact devices.12. An additional viable approach involves the use of a linearly polarized (LP) quasi-isotropic MIMO antenna, which increases the transmission capacity while generating nearly null-free RP in all directions13. However, the 6 GHz EM signals are susceptible to environmental factors and would eventually lead to the failure of a wireless system in many real-life circumstances14. Integrating a MIMO antenna with quasi-isotropic CP RP constitutes a realistic alternative to prevent multiple paths fading from the ionosphere, thereby making it an important aspect of a 6G system15,16.
The work efficacy of a 6G system can be determined by evaluating the KPI expressions of antenna17 and MIMO18. However, specific KPI are of critical importance depending on 6G use cases, as indicated in Fig. 1. Descriptively, the low power wide area (LPWA) technology intended for operation in the licensed narrow-spectrum to enable low data rate IoT services like environment monitoring and vehicle parking necessitates a narrow impedance bandwidth (I-BW)/AR-BW antenna with good G performance19,20. Similarly, UAV use cases that are capable of traveling far distances, such as drones, need a limited I-BW/AR-BW21. However, to sustain a prolonged flight duration with reduced power consumption, 
of a 6G UAV antenna should be maximized22. On the contrary, antennas for XR use cases, including a head-mounted display, should render reasonably wide I-BW/AR-BW with good G and 
performance23. Particularly because a 6G XR antenna would share I-BW/AR-BW requirements with other consumer electronics, and their G and 
are often affected by the vicinity of other sub-system hardware and the human head24. Furthermore, in AI-based robot communication, compliance with security standards to preserve personnel from injury constitutes a significant difficulty that can be addressed with a high 
antenna25,26. Another related use case that requires a high G antenna is V2X communication, which gathers, analyzes, and exchanges data from many infrastructures27,28. Finally, for 6G D2D ISAC, a small, low-profile antenna with high 
and wide I-BW/AR-BW will be required because these use cases entail direct communication between several handheld devices without the involvement of a base station29,30.
Figure 1.
Antenna KPI based on 6G use cases (drawn by the author: Kamal, S.).
Generally, closely-placed MIMO antennas in a compact and low-profile arrangement are susceptible to trade-off in 
values31,32. In the past, 
values between antenna elements have been manipulated by deploying defected ground33, neutralization lines34, parasitic elements35, excitation of different modes36, MTM37, or a combination of one38 or many39, which weaken, resist, or lower the surface current flow. Then again, realization of high 
values in the entire operating spectrum still remains an open challenge. Similarly, the literature presents a number of antennas to accomplish quasi-isotropic CP RP that can be grouped into two primary categories: multi-antenna and perturbation techniques. The multi-antenna solution incorporates a particular feeding architecture to excite CP RP in antennas of identical or combination types, such as cavity40, dipole41, inverted-F42, and microstrip43. These multi-antenna arrangements are capable of enabling wide AR-BW, but at the expense of considerable volumes. On the contrary, the perturbation approach generates two orthogonal RP modes of CP by constructing parasitic frameworks44, etching slots45, or slicing edges46 in the primary radiating element of an antenna. However, these perturbation-based antennas are capable of producing narrow AR-BW. Furthermore, G deviation47 and low 
48 remains important challenges in most of the quasi-isotropic CP antennas, making their straightforward integration in portable devices difficult.
Based on the above motivation, the main objective of this work was focused on the design of lucrative printed antennas for diverse 6G uses cases entailing quasi-isotropic CP coverage in the 6 GHz EM spectrum. Sequentially, the following three tasks were attempted and solved successfully. The first part was focused on the design of quasi-isotropic LP SISO antennas operating in the 6 GHz EM spectrum with enhanced performance. The second phase concentrated on converting LP waves to CP RP by integrating apertures in the ground plane, without modifying the design of the proposed SISO radiating element. The final section of the study concentrated on using passive MTM to enable improved 
values in closely-placed MIMO antenna configurations of the proposed CP SISO antenna designs. All antennas were analyzed through simulation studies, while all MIMO antennas rendering quasi-isotropic CP RP were subjected to fabrication and measurements.
Methods
Antenna configuration and analysis technique
The configuration of the proposed printed circuit board (PCB)-based MIMO antenna elements at various phases of ANSYS high frequency structure simulator (HFSS) simulation are shown in Fig. 2 and their corresponding parameter values in 
at center frequency (
) 
6 GHz are listed in Table 1. Five unique designs of SISO radiator/top copper (
) are demonstrated in Fig. 2a. Where, 
indicates circular radiator associated with 0.25
-fed rectilinear microstrip; 
denotes circular radiator amalgamated amidst 0.25
-fed meander line; 
symbolizes meandered radiator enclosed by microstrip bracket coupled to 0.25
-fed rectilinear trace; 
manifests meandered radiator within microstrip frame attached to 0.25
-fed meander line; 
exemplifies crescent-axed meandered radiator juxtaposed to 0.25
-fed meander line.
Figure 2.
Proposed antenna parts. Layout: gray, 
; light gray, 
; black, 
; 
, port. (a) SISO 
; (b) SISO 
; (c) 
; (d) 
; (e) Brillouin zone; (f) MTM boundary setup; (g) MTM array; (h) 
; (i) Fabricated prototypes.
Table 1.
Parameter values of the proposed antenna elements in 
at 
6 GHz.
| Tag | Value | Description | Tag | Value | Description |
|---|---|---|---|---|---|
 |
0.03 | Substrate height |  |
0.08 | Gap between two shorting pins |
 |
0.50 | Single element substrate length |  |
0.02 | Transition slot length |
 |
0.50 | Single element substrate width |  |
0.06 | Transition slot width |
 |
0.10 | Microstrip feed length |  |
0.09 | Gap between  and substrate edge |
 |
0.06 | Microstrip feed width |  |
0.25 | Gap between  and substrate edge |
 |
0.25 | Wavelength transformer length |  |
0.18 | Gap between two transition slots |
 |
0.04 | Wavelength transformer width |  |
0.21 | Rectangular slot length |
 |
0.08 | Middle section link length |  |
0.16 | Rectangular slot width |
 |
0.14 | Meander line length |  |
0.03 | Gap between upper transition slot and 
|
 |
0.02 | Meander line width |  |
0.18 | Gap between upper  and substrate edge |
 |
0.01 | Meander line twist length |  |
0.46 | Crescent-axed microstrip diameter |
 |
0.04 | Variation in meander line twist length |  |
0.02 | Gap between  and 
|
 |
0.04 | Final meandered section length |  |
0.10 | Partial ground length |
 |
0.01 | Final meandered section width |  |
0.5 | Partial ground width |
 |
0.01 | Adjacent link length |  |
0.10 | Right-bottom cage slot length |
 |
0.46 | Outer microstrip diameter |  |
0.09 | Right-middle cage slot length |
 |
0.33 | Inner microstrip diameter |  |
0.04 | Right-top cage slot length |
 |
0.06 | Radiating parasitic element width |  |
0.13 | Left-bottom cage slot length |
 |
0.52 | Gap between  and substrate edge |
 |
0.09 | Left-middle cage slot length |
 |
0.02 | Opening in parasitic element |  |
0.01 | Left-top cage slot length |
 |
0.02 | Gap between two parasitic slot elements |  |
0.01 | Cage slot width |
 |
0.02 | Shorting pin diameter | – | – | – |
Similarly, five different layouts of SISO ground/bottom copper (
) are depicted in Fig. 2b. Where, 
illustrates full ground; 
signifies partial ground; 
expresses defected 0.25
transition points, cage slotted central segment and radiator; 
represents cage slotted radiator, defected middle section and 0.25
transition regions; 
exhibits defected 0.25
transitions, intermediate passage and radiator.
Furthermore, two distinct passive MTM-based EBG cell of square ring (
) and shorting pin (
)/via copper (
) are displayed in Fig. 2c, d, respectively. The microstrip elements and spacing between them may be specified by inductance (L) and capacitance (C), respectively, as illustrated in Fig. 2c. The Brillouin49,50 domain was applied to anticipate the characteristics of EBG framework. Where, the 
position denotes the focal point of the Brillouin zone, while the M and X locations correspond to the midpoints of an edge and a face, respectively, as depicted in Fig. 2e. The master-slave boundary setup in ANSYS HFSS eigenmode simulation setting was used to study a MTM cell, as shown in Fig. 2f. The optimum arrangement involves unit MTM cell arranged in an array, as shown in Fig. 2g.
The finalized layout of a MIMO antenna comprises of twin radiating elements, a defected ground, and a MTM-based EBG cell array. Six antenna configurations have been regarded as optimum, including 
(
; 
), 
(
; 
), 
(
; 
), 
(
; 
), 
(
; 
), and 
(
; 
). The top and bottom perspectives of one of the proposed antenna (
) as well as the equivalent circuit model of a meandering portion are depicted in Fig. 2h. Two alike radiating elements were positioned adjacent to each other in close proximity on the top surface of PCB, whereas a defected ground was incorporated onto the bottom side of PCB. The two radiating elements were fed from the identical edge of PCB and separated by an array of unit 
cell.
Figure 2i displays photographs of the six fabricated MIMO antenna prototypes, with bottom configurations shown as their respective insets. All MIMO antennas were manufactured on the flame retardant (FR-4) substrate (
4.4, 
0.02) using the photo-lithography method and fed with the SubMiniature Version A (SMA) BU-1420701851 connector. Their performance was measured with Diamond Engineering Antenna Measurement Studio and a standard horn antenna in the anechoic chamber at Barkhausen Institut, Germany. Descriptively, the values of reflection coefficient (
) and 
were obtained whilst connecting the two ports of a MIMO antenna to Keysight N5224B PNA microwave network analyzer. However, when RP, G and other MIMO antenna parameters were being measured, one of the two ports was equipped with the 50
termination resistance. Furthermore, the far-field parameters were evaluated in the direction of maximum radiation, which was near the broadside direction. The MIMO antenna under test was configured in the 
and 
directions, and RP were recorded for each of the two different alignments. The axial ratio (AR) values were equated by the G difference between the 
and 
orientations. The computation of directivity from the measured RP in the 
and 
planes formed the basis for 
evaluation. Finally, the ECC, DG, total active reflection coefficient (TARC), and channel capacity loss (CCL) values were estimated using Equations16,51,52
1, 2, 3, and 4−9, respectively. Where, 
and 
represents the ith and jth elements of RP, respectively.
 |
1 |
 |
2 |
 |
3 |
 |
4 |
 |
5 |
 |
6 |
 |
7 |
 |
8 |
 |
9 |
This research addresses all physical factors that could possibly influence the proposed antennas’ impedance and radiation characteristics. Given the substantial amount of radiator, ground, and metamaterial combinations that have been explored, each result shows the considered design on the right/left hand side bottom. Furthermore, the figure caption specifies the serial number of the top copper (
), bottom copper (
), and metamaterial (MTM) configurations under consideration as their superscript, allowing readers to easily determine which antenna data are presented.
Design formulation for impedance matching
The preliminary layout of the proposed SISO antenna was based on the hypothesis53 of a circular-patterned radiating element with radius (
/2) estimated using Equations17
10 and 11. Particularly because circular antennas have multiple advantages over other shapes, including design flexibility, wide I-BW, and enhanced quasi-isotropic G performance30. These benefits, however, depend on the formulation of the impedance matching network17. A straightforward technique to match a real load impedance to a variable source impedance constitutes the utilization of a 0.25
line, which can be expanded to multi-section configurations54. Therefore, the 0.25
line was selected as the feed mechanism of the proposed antennas. Nevertheless, direct connection of a circular radiator to a 0.25
line yields narrow I-BW55–57. Thus, the initial objective was to attain an appropriate impedance matching with enhanced I-BW in the 6 GHz EM spectrum. Subsequently, 
and AR response of a SISO antenna comprising of a circular radiator associated with 0.25
-fed rectilinear microstrip was studied, as depicted in Fig. 3a. A 
of −6 dB with AR of >3 dB was ascertained at 7.1 GHz and 8.1 GHz. The second resonance was produced by the middle segment of rectilinear line, whose width was kept narrower than the 0.25
feed line to manipulate the impedance values. Figure 3b reveals that G of 
3 dBi and 
of 
75
were perceived at 7.1 GHz, while Fig. 3c demonstrates that quasi-isotropic co-polar (co) and cross-polar (
) RP became apparent at 7.1 GHz.
Figure 3.
Proposed LP SISO antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 7.1 GHz.
 |
10 |
 |
11 |
Given that fulfillment of 
−10 dB counts for a number of use cases, further investigation of the aforementioned antenna design remained important. One of the many crucial components affecting the antenna impedance includes the dimension of the radiating element. Typically, the antenna resonance shifts to lower frequencies when the dimensions of the radiator is lengthened or shortened. However, this methodology results in an enlarged footprint when employed for switching to lower frequencies. Therefore, the conductor was folded in the form of a meander line to increase the microstrip length, shift the antenna resonance to the desired frequencies, and ensure better utilization of the available area. Particularly, a 0.25
-fed meander line was introduced as a connection to the circular radiator. Their dimensions were estimated from the characteristic impedance (
) of a transmission line with effective dielectric constant (
) Equations58
12 and 13. An appropriate impedance matching of −10 dB with AR of >3 dB was established at 6.8 GHz and 8.3 GHz, as demonstrated in Fig. 4a. Furthermore, at 6.8 GHz, G of >3.5 dBi and 
of >75
were achieved as shown in Fig. 4b, with quasi-isotropic RP as depicted in Fig. 4c.
 |
12 |
 |
13 |
The preceding simulation phases proved that integrating the middle segment contributes to generating additional resonance. Hence, the next part of the study was focused on incorporating another section to obtain supplementary resonances. Specifically, a SISO antenna consisting of a meandered radiator and a microstrip bracket connected to a 0.25
-fed rectilinear trace was studied. The length of a meander line and the gap between two meander lines were computed using Equations59
14 and 15 with permeability (
) permittivity (
) of the vacuum. The modified antenna design produced multiple resonances with AR of >3 dB, as shown in Fig. 5a. These operating frequencies included 4.3 GHz, 6.7 GHz, 7 GHz, 7.6 GHz, and 8.6 GHz. Furthermore, G of >2 dBi and 
of 
45
were attained at 8.6 GHz, as revealed in Fig. 5b. However, the responses of G and 
appeared to be subpar at other frequencies, with values ranging from 
3 to 
1 dBi and from 
10
to 
40
, respectively. Yet, reasonable quasi-isotropic RP was established at 7 GHz, as demonstrated in Fig. 5c.
 |
14 |
 |
15 |
Figure 4.
Proposed LP SISO antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 6.8 GHz.
Figure 5.
Proposed LP SISO antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 7 GHz.
To advance the last investigation, the influence of substituting the middle portion of the rectilinear line with a meander line was considered. The antenna exhibited AR of >3 dB and −10 dB 
resonances almost similar to the previous design, namely at 4.3 GHz, 6.8 GHz, 7.1 GHz, 7.7 GHz, and 8.6 GHz, as portrayed in Fig. 6a. However, relatively higher G of 
3 dBi and 
of 
60
were achieved at 8.6 GHz, as illustrated in Fig. 6b. At other frequencies, the responses of 
and G also appeared to be substantially better, with values ranging from 
20
to 
70
and 
1 dBi to 
3 dBi, respectively. Furthermore, Fig. 6c reveals that an adequate quasi-isotropic RP was accomplished at 7.1 GHz.
Figure 6.
Proposed LP SISO antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 7.1 GHz.
The radiator design was further examined to enable improved antenna performance based on the advantages of meandered microstrip found in the simulation studies outlined above. Descriptively, a meander line characterized by an irregular trend of four different parts was taken into consideration. The first segment entailed bending the microstrip in the opposite orientation to shift the trajectory of the electric current. In the subsequent section, a narrow trace was implemented in conjunction to an obese track. The third region integrated the slimmest microstrip of a circular meander structure. Finally, the circuit terminated at a crescent-shaped line. With AR of >3 dB, the antenna yielded 
−10 dB at 4.17 GHz, 4.58 GHz, 5.78 GHz, 6.08 GHz, 6.89 GHz, 7.83 GHz, and 8.39 GHz, as indicated in Fig. 7a. Their G values spanned between 
2 dBi and 
2 dBi, while their 
values ranged from 
14 to 
45
, as illustrated in Fig. 7b. Particularly, at 6.08 GHz, quasi-isotropic RP was achieved with −2 dBi G and 15
, as shown in Fig. 7c. In contrast to the other designs mentioned above, G and/or 
results seem to be less noteworthy, yet this antenna configuration was important in the realization of wideband CP EM waves that is described in the remainder of this paper.
Figure 7.
Proposed LP SISO antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 6.08 GHz.
The current distributions of all the proposed LP SISO antenna radiators have been presented in Fig. 8 to elucidate their working mechanisms. The ground plane currents that traversed to the radiating elements resisted the phase shift of field components. Hence, the antenna radiators exhibited LP behavior with >3 dB AR values. Besides, the fundamental surface currents, highlighted in red, were primarily concentrated on the feed line in the 
configuration. The integration of meandering feed line to the circular radiator contributed to a proper distribution of the surface currents throughout the entire 
configuration. Similarly, the successive evolution of distinct designs (
) resulted in equally distributed fundamental currents throughout the radiators, enabling wide I-BW. These results eventually justify the importance of the proposed feed method.
Figure 8.
Simulated current distributions of the proposed LP SISO antenna radiators: 
at 7.1 GHz, 
at 6.8 GHz, 
at 7 GHz, 
at 7.1 GHz, and 
at 6.08 GHz.
Blueprint for generating circular polarization
Typically, a CP antenna comprises of time-varying current paths on the radiator and prerequisites I-BW and CP BW that corresponds to −10 dB 
and 3 dB AR, respectively60. In order to regulate the resonant properties of the proposed LP antennas, adjust the current directions on the radiators to generate CP waves, and retain the compact profile and dimensions, the ground plane designs were meticulously studied. Initially, the partial ground plane design (
) was investigated in conjunction to 
radiator. The antenna operated from 5.7 GHz to 6.1 GHz with AR-BW of 
0.4 GHz, as shown in Fig. 9a. In the entire operating band, their G and 
values remained >2 dBic and 
70
, respectively, as demonstrated in Fig. 9b. Furthermore, quasi-isotropic CP RP was ascertained at 6 GHz, as illustrated in Fig. 9c.
Figure 9.
Proposed CP 
antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 6 GHz.
Since, the realization of CP waves was largely attributed to the introduction of apertures in the ground plane, further research was conducted to realize the additional antenna KPI necessary for various use cases. Particularly, slots were inserted near impedance transition points and radiating elements to form caged apertures in the ground plane. In contrast to the aforementioned CP antenna arrangement, the setup composed of 
and 
, enabled wide total BW (at 
) of 0.25 GHz (at 4.75 GHz), 0.02 GHz (at 5.35 GHz), 0.23 GHz (at 6.08 GHz), and 0.46 GHz (at 8.41 GHz) with AR 
3 dB, as displayed in Fig. 10a. Furthermore, their G and 
values ranged between −1.5 dBic to 3.3 dBic and 25
to 65
, respectively, as shown in Fig. 10b. At 6 GHz, quasi-isotropic CP RP was accomplished, as portrayed in Fig. 10c.
Figure 10.
Proposed CP 
antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 6 GHz.
The caged slot layout of the ground plane in the previous design increased the −10 dB I-BW, but their 3 dB CP BW did not fully match it. In order to identify the main cause of the 3 dB AR-BW limitation, the ground plane arrangement next to the meandered feed was changed to a relatively larger aperture in the subsequent study. The 
and AR curves for the new ground plane design (
) on the previously considered radiator design (
) are demonstrated in Fig. 11a. Multiple resonances including 4.55–4.78 GHz, 5.44–5.59 GHz, 6.12–6.31 GHz, and 7.64–8.64 GHz were accomplished. Furthermore, an appropriate AR of <3 dB was established in all operating bands, which led to the conclusion that this improvement may have been due to the reduction in ground plane currents’ interference with the radiating elements. Additionally, as shown in Fig. 11b, peak G and 
of 
3.5 dBic and >60
were acquired, respectively, and a quasi-omnidirectional CP RP was achieved at 6.2 GHz, as shown in Fig. 11c.
Figure 11.
Proposed CP 
antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 6.2 GHz.
Given the previous study demonstrated that increasing the ground plane aperture area close to the feed line improves the AR values, the subsequent study concentrated on assessing the effect of expanding the ground plane slot region beneath the main radiating elements on the antenna KPI. Particularly, the combination of 
and 
arrangement was explored, which showed notable improvement in the 3 dB AR values for both the −10 dB 
resonances between 6.56–6.78 GHz and 8.23–8.55 GHz, as illustrated in Fig. 12a. Similarly, Fig. 12b displays a significant improvement in G of 
4.5
dBic and 
of 
80
, while Fig. 12c indicates reasonable quasi-omnidirectional CP RP at 6.6 GHz.
Figure 12.
Proposed CP 
antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 6.6 GHz.
Subsequently, the 
ground plane configuration was explored when integrated with the 
radiator design. As shown in Fig. 13a, the antenna exhibited appropriate impedance matching at 5.65 GHz, 6.46 GHz, and 7.28 GHz with 
0.2 GHz I-BW for each resonant frequencies. Additionally, with exceptions at a few frequencies, the AR values stayed <3 dB across a significant number of operating ranges. Besides, as Fig. 13b illustrates, good G of 
4 dBic and 
of 
70
were obtained. The CP RP curves confirm a notable improvement in Fig. 13c.
Figure 13.
Proposed CP 
antenna. Layout: 
; 
. Simulated results: (a) AR, 
; (b) 
, G; (c) RP at 6 GHz.
Figure 14 presents the current distributions on the ground planes and radiating elements of all the proposed CP SISO antennas to gain insight into their operating mechanisms. The principal currents were formed in the south-west direction at 
and the north-east direction at 
in the highlighted segments of 
(
; 
), 
(
; 
), and 
(
; 
) radiators. Similarly, the fundamental currents in the highlighted portions of the 
(
; 
) and 
(
; 
) radiators were formed at 
and 
in the north-east and south-west directions, respectively. Furthermore, other radiating elements exhibited identical responses to the primary current directions at different phase angles. Therefore, a phase shift between the two fundamental field components of 
was accomplished. The CP characteristics were rendered as the true nature of the top copper sections became apparent by altered impedance levels at multiple radiator segments and reduced coupling from the ground plane.
Figure 14.
Simulated current distributions of the proposed CP SISO antennas at 6 GHz.
Note that, out of the multiple practicable combinations of proposed radiating elements (
) and ground planes (
), only the most pertinent ones have been described, taking into account the current project requirements. However, the presented results offer ample data to estimate the effects of remaining combinations.
Road map for isolation enhancement
The ultimate goal of this work was to conclude MIMO-based antenna designs. Hence, the study of closely-placed 2-port arrangement of 
CP antenna was initially considered for the subsequent optimization phases. In particular, their surface current distribution was examined, and Fig. 15a shows that a significant amount of current traveled from port 
to port 
. Therefore, a square ring (
) shaped transmission line was studied for enabling EBG properties. Their initial dimensions were predicted from Equations61
16, 17, 18, 19, and 20. The dispersion diagram of unit 
cell with two modes (
and 
) activated is displayed in Fig. 15b. An EBG BW of 
1 GHz was yielded. Subsequently, an array of unit 
cell was positioned between the two 
radiating elements with 
ground plane. Their current distribution was then assessed, and the results showed a significant decrease in the mutual coupling between port 
and port 
, as portrayed in Fig. 15c.
Figure 15.
Proposed CP MIMO antennas. Layout: 
; 
. Simulated surface current distributions of antennas at 6 GHz: (a) without MTM; with (c) 
and (e) 
. Dispersion diagrams of (b) 
and (d) 
. Simulated 
and 
of antennas: (f) without MTM; with (g) 
and (h) 
.
Realizing a wide isolation BW constitutes one of the major objectives of this work. Although 
enabled an adequate EBG BW, another MTM design was explored to enhance it further. The research presented in the preceding sections indicate that the ground plane currents significantly affect the antenna’s overall behavior. Additionally, it has been demonstrated that periodic surfaces composed of regions with shorting pins exhibit notable EBG in their dispersion diagrams, which prevent surface waves from propagation62. Hence, a setup based on shorting pins (
) was considered was investigation. An equivalent parallel resonant LC circuit may be employed to describe architectures characterized by shorting pins63. Consequently, their initial dimensions were estimated using Eq.61
16, taking into account that C and L correspond to the patch dimension and the shorting pin length, respectively. Figure 15d illustrates the dispersion diagram of 
with 
and 
excited, which enabled an EBG BW of 
2 GHz. The two 
radiators were then separated by an array of unit 
cells with 
ground plane. When their current distribution was analyzed, relatively better reduction in the mutual coupling between ports 
and 
was noticed, as shown in Fig. 15e. Note that the current distributions of the remaining CP antennas were assessed using both 
and 
cells, which showed comparable results to those stated in Fig. 15. Furthermore, exploring this work’s prerequisites, only the most relevant MIMO and MTM combinations have been explained out of the numerous feasible possibilities.
 |
16 |
 |
17 |
 |
18 |
 |
19 |
 |
20 |
The 
and 
curves for the three scenarios–without any MTM, with 
, and with 
–have been examined in order to gain a better understanding of the isolation mechanism. In the 6 GHz band, the MIMO antenna without MTM enabled I-BW 
0.3 GHz and 
−36 dB, as portrayed in Fig. 15f. The introduction of 
realized 
−45 dB, without altering the I-BW, as depicted in Fig. 15g. Furthermore, the addition of 
contributed to improvements in both I-BW and 
values to 
0.4 GHz and 
−39 dB, respectively, as apparent in Fig. 15h.
Measurement results
The simulated and measured results of 
are portrayed in Fig. 16. The measured 
curve indicates that a filtered −10 dB I-BW from 5.5–6.1 GHz was established, as shown in Fig. 16a. The measured results presented in Fig. 16b affirm that 
of 
40 dB was yielded, representing high isolation between ports 
and 
. The measured 3 dB AR-BW and −10 dB I-BW corresponds to each other with relative BW of 8.84%, as displayed in Fig. 16c. A reasonable peak G of 6.4 dBic was noticed during measurements with average G of 
4 dBic across the operational BW, as apparent in Fig. 16d. The measured RP of port 
in 
and 
planes at 6 GHz are depicted in Fig. 16e, f, respectively. In both planes, stable quasi-omnidirectional RPs were confirmed. 
’s suitability under CP standards was guaranteed by 
20 dB difference between co-polar, right hand (RHCP) and 
-polar, left hand (LHCP) fields. Figure 16g exemplifies that the measured total power radiated within the operation BW ranged from 50 to 65%. The performance of 
was further supported by the measurements of ECC and DG. Figure 16h demonstrates that ECC of <0.2 was measured, indicating that there was nearly no correlation between the two antennas. Furthermore, the measured DG of 
is plotted in Fig. 16i. In MIMO antennas, DG constitutes an important measure that relies on a number of factors, for instance elevation angle, frequency, and spatial separation, among others. With DG of 
10 dB over the operating BW, 
demonstrates the device’s outstanding reliability. The general decline in I-BW was caused by the restricted bandwidth of square ring 
and the reduced strength of primary currents on the main radiating segments due to the partial ground plane. The aforementioned along with notable G values may be regarded as important contributions of 
because these antenna KPI are essential for IoT use cases.
Figure 16.
Proposed CP 
antenna. Layout: 
; 
; 
. Simulated and measured results: (a) 
; (b) 
; (c) AR; (d) G; (e) 
and (f) 
RP at 6 GHz; (g) 
; (h) ECC; (i) DG.
Figure 17 shows the simulated and measured results of 
. Referring to the measured 
trajectory, as illustrated in Fig. 17a, −10 dB I-BW of 33.01% was established. A peak 
of −56 dB was obtained, indicating low mutual coupling between ports 
and 
, according to the measured results shown in Fig. 17b. The measured values of AR that remained <3 dB are provided in Fig. 17c. The measurements revealed a high peak G of 7.1 dBic, as indicated in Fig. 17d. The measured RP of port 
in 
and 
planes at 6 GHz are manifested in Fig. 17e, f, respectively. The appropriateness of 
under CP regulations was proven by steady quasi-omnidirectional RPs in both planes, exhibiting a 
20 dB difference between RHCP and LHCP RPs. There was a slight overlap between RHCP and LHCP RPs near 
and 
because only one shorting pin could be soldered to the partial ground plane, and the substrate’s remaining holes were left unfilled, resulting in reflection and diffraction. The maximum measured 
was 87%, as shown in Fig. 17g. The measured ECC and DG performance are displayed in Fig. 17h, i, respectively. The remarkable reliability of 
was demonstrated by the recording of ECC of <0.2 and DG of 
10 dB over the operating BW. Although 
had similar radiating elements to 
, their I-BW, G, and 
performances were greatly improved. This was due to the shorting pin 
configuration, which featured wide EBG characteristics. Furthermore, because of the partial ground arrangement, only one shorting pin could be integrated, resulting in ground plane currents flowing towards the main radiator. Ultimately, the overall signal strength of the radiating components was improved. Thus, 
remains an excellent choice for XR use cases that need high G and enhanced 
performance of antennas.
Figure 17.
Proposed CP 
antenna. Layout: 
; 
; 
. Simulated and measured results: (a) 
; (b) 
; (c) AR; (d) G; (e) 
and (f) 
RP at 6 GHz; (g) 
; (h) ECC; (i) DG.
The simulated and measured results of 
are provided in Fig. 18. The measured 
versus frequency curves displayed in Fig. 18a proves that an appropriate −10 dB impedance matching at 6 GHz with I-BW of 31.64% was realized. Figure 18b shows the measured 
results, which confirms that a peak 
of −48 dB was established, signifying reasonable isolation between port 
and port 
. The variations in AR against frequency are shown in Fig. 18c. Within the operating spectrum, the measured AR was 
3 dB. Furthermore, a peak measured G of 4.7 dBic was yielded, as indicated in Fig. 18d. The quasi-omnidirectional operation of 
at 6 GHz has been substantiated by a comparison of simulated and measured RP of port 
in 
and 
planes, as shown in Fig. 18e, f, respectively. The direction of maximum radiation was tilted from the broadside direction to 
orientation in 
plane by the caged slot layouts on the ground plane close to both radiating segments. Furthermore, in 
plane, a minor overlap of LHCP and RHCP fields was noticed near 
and 
angles. Similar effects were observed in 
plane, with subtle distortion in RP at a few angles. These impacts have been associated to the coupling of the ground currents towards the radiators. However, at remaining angles, the difference between LHCP and RHCP values were well within the range of 
20 dB, indicating noteworthy CP functionality. This ultimately confirms the feasibility of tolerable radiation performance. The measured 
, ECC, and DG performance are demonstrated in Fig. 18g–i, respectively. Excellent 
of 94%, ECC of <0.2, and DG of 
10 dB over the operating BW was corroborated. These established antenna KPI, with a main contribution in substantial 
, reveal that 
meets the requirements of AI-based robot use cases, which often require high 
antennas.
Figure 18.
Proposed CP 
antenna. Layout: 
; 
; 
. Simulated and measured results: (a) 
; (b) 
; (c) AR; (d) G; (e) 
and (f) 
RP at 6 GHz; (g) 
; (h) ECC; (i) DG.
Figure 19 demonstrates the simulated and measured results of 
. A −10 dB I-BW of 29.01% was established in the 6 GHz band, as shown by the measured 
curves in Fig. 19a. Low mutual coupling between ports 
and 
with a peak 
of −45 dB was accomplished, evidenced by the measured results in Fig. 19b. The measured values of AR stayed <3 dB, as shown in Fig. 19c. A fairly significant peak G of 5.8 dBic became apparent in the measurements, which is illustrated in Fig. 19d. The measured RP of port 
in 
and 
planes at 6 GHz are conveyed in Fig. 19e, f, respectively. Stable quasi-omnidirectional RPs across the two fundamental planes with 
20 dB difference between RHCP and LHCP fields demonstrated the suitability of 
according to CP norms. The highest measured 
was 76%, as Fig. 19g exemplifies. The measured ECC and DG performance are revelead in Fig. 19h, i, respectively. The measurement of ECC of <0.2 and DG of 
10 dB across the operational BW verified the outstanding reliability of 
. High G antennas are required for V2X use cases, and 
makes for a suitable candidate. Particularly because reasonably high G with notable CP RP performance was rendered with 
. While 
offers a relatively higher G, their CP RP does not constitute a great option for V2X use cases, where a variety of environmental conditions affect the antenna’s performance and attaining minimal overlap between RHCP and LHCP fields remain crucial.
Figure 19.
Proposed CP 
antenna. Layout: 
; 
; 
. Simulated and measured results: (a) 
; (b) 
; (c) AR; (d) G; (e) 
and (f) 
RP at 6 GHz; (g) 
; (h) ECC; (i) DG.
The simulated and measured results of 
are shown in Fig. 20. The measured −10 dB I-BW in the 6 GHz band was 9.69%, as portrayed in Fig. 20a. In the operating spectrum, the measured 
between ports 
and 
extended from −25 dB to −40 dB, as illustrated in Fig. 20b. The measured 3 dB AR-BW agreed with −10 dB I-BW, as displayed in Fig. 20c. In the functional BW, stable G of 4.7−5.8 dBic was measured, as revealed in Fig. 20d. The measured RP of port 
in 
and 
planes at 6 GHz exhibited steady quasi-omnidirectional RPs with 
20 dB difference between RHCP and LHCP patterns, as demonstrated in Fig. 20e, f, respectively. The measured 
ranged from 88 to 93%, as evidenced in Fig. 20g. The measured ECC was <0.2 and DG was 
10 dB, as presented in Fig. 20h, i, respectively. Overall, 
established an adequate MIMO antenna KPI, with high 
and narrow I-BW/AR-BW being their key characteristics, which are essential for UAV use cases.
Figure 20.
Proposed CP 
antenna. Layout: 
; 
; 
. Simulated and measured results: (a) 
; (b) 
; (c) AR; (d) G; (e) 
and (f) 
RP at 6 GHz; (g) 
; (h) ECC; (i) DG.
The simulated and measured results of 
are displayed in Fig. 21. Measurements proved −10 dB I-BW from 4.42–8.24 GHz, covering the entire 6 GHz band, as shown in Fig. 21a. Notable peak and average 
values of, respectively, −40 dB and 
20 dB were measured between ports 
and 
, as evidenced in Fig. 21b. In the operating BW, AR of <3 dB and G of 
4 dBic were measured, as shown in Fig. 21c, d, respectively. Typically, AR-BW involves intricate design elements that degrades CP antenna performance. By exploiting apertures on the ground plane at strategic locations along with the 
-based EBG structure, the current distribution on 
radiating elements were modified. Particularly, the meandered and crescent axed segments governed the stronger currents, and the disparity between the distribution of two orthogonal currents yielded a wide AR-BW of 60.63%, which represents a significant contribution of 
. The measured RP of port 
in 
and 
planes at 6 GHz are presented in Fig. 21e, f, respectively. Quasi-omnidirectional RP with 
20 dB difference between RHCP and LHCP fields were measured in both planes. The measurement of reasonable 
of 
65
, ECC of <0.2, and DG of 
10 dB in the operating BW are portrayed in Fig. 21g–i, respectively. Based on these results, 
represents a fairly competitive design that may be applied to D2D ISAC use cases.
Figure 21.
Proposed CP 
antenna. Layout: 
; 
; 
. Simulated and measured results: (a) 
; (b) 
; (c) AR; (d) G; (e) 
and (f) 
RP at 6 GHz; (g) 
; (h) ECC; (i) DG.
In order to further validate the MIMO performance of the proposed antennas, TARC and CCL responses have been illustrated in Fig. 22. All of the proposed antennas yielded reasonable TARC and CCL performance with 
0 and 
0.4 bps/Hz values in their operating bands, respectively. Besides, it is crucial to explain the reason behind the measured data being distorted, with certain curves not matching the simulated plots. The deterioration of the results was caused by a number of factors, including misalignment, undesirable radiations from the feed cable, SMA connector tolerances, 50
termination resistance, hand soldering of shorting pins, and changes in the physical parameters throughout the fabrication process. Finally, the authors would like to highlight that an insignificant portion of 
antenna was presented at a high-impact scientific conference in Leuven, Belgium, in 202464.
Figure 22.
Simulated TARC and CCL results of the proposed CP MIMO antennas: (a) 
; (b) 
; (c) 
; (d) 
; (e) 
; (f) 
.
Benchmarking
A summary of the proposed antenna elements has been presented for the ease of understanding the design methodology in Fig. 23. Furthermore, the proposed CP MIMO antennas (
) have been compared with state-of-the-art CP MIMO antennas in Table 2, and discussed below to highlight their novelty. The cited works substantially boost the overall antenna and MIMO performance, but they have a number of limitations that have been addressed in this work. The novelty and importance of the proposed research has been addressed in the ensuing points.
Figure 23.
A summary of the proposed antenna elements.
Table 2.
Performance summary of the (a) proposed and (b) state-of-the-art quasi-isotropic antennas.
|
|
|
|
|
|
|
|---|---|---|---|---|---|---|
| (a) | ||||||
| Layout |
|
|
|
|
|
|
Size [ ] |
1.1 1.1 |
1.1 1.1 |
1.1 1.1 |
1.1 1.1 |
1.1 1.1 |
1.1 1.1 |
Profile [ ] |
0.03 | 0.03 | 0.03 | 0.03 | 0.03 | 0.03 |
| Layers | 2 | 2 | 2 | 2 | 2 | 2 |
 [GHz] |
6.0 | 6.0 | 6.0 | 6.0 | 6.0 | 6.0 |
| I-BW [%] | 8.84 | 33.01 | 31.64 | 29.01 | 9.69 | 60.63 |
| AR-BW [%] | 8.84 | 33.01 | 31.64 | 29.01 | 9.69 | 60.63 |
 [dB] |
40−58 | 31−56 | 24−48 | 21−45 | 25−40 | 21−40 |
| G [dBic] | 3.6−6.4 | 3.9−7.1 | 1.2−4.7 | 2.8−5.8 | 4.7−5.8 | 1.9−4.2 |
 [%] |
50−65 | 55−87 | 56−94 | 45−76 | 88−93 | 60−68 |
|
202572 | 202473 | 2024 74 | 2023 75 | 2022 76 | 2020 77 |
|---|---|---|---|---|---|---|
| (b) | ||||||
| Layout |
|
|
|
|
|
|
Size [ ] |
0.1 0.2 |
0.2 0.2 |
0.6 0.5 |
0.1 0.1 |
1.9 1.9 |
2.0 2.0 |
Profile [ ] |
0.03 | 0.0005 | 0.03 | 0.02 | 0.11 | 0.04 |
| Layers | 2 | 2 | 2 | 2 | >2 | >2 |
 [GHz] |
8.9 | 2.5 | 5.6 | 3.5 | 5.8 | 6.2 |
| I-BW [%] | 125.8 | 18.9 | 12.5 | 146 | 8.5 | 6.4 |
| AR-BW [%] | – | – | 10.8 | 47 | 0.04 | 6.4 |
 [dB] |
– | – | 21−25 | 23−31 | 40−67 | 38−52 |
| G [dBi] |
 2.2 |
 2.2 |
3.9−4.9 | – | 7.6−10.5 | 6.0−8.0 |
 [%] |
 85 |
 95 |
94−95 | 79−95 | – | – |
Antenna layout. The polarization of antennas, LP and CP, is anticipated to play a vital role in enabling current and future applications, including but not limited to ISAC. The radiator and ground configurations presented in two patents65,66 are based on the meander lines and/or defected ground. However, both these patents described their arrangements for generating LP waves only, whereas the proposed antenna designs are suitable for realizing both LP as well as CP waves. Furthermore, the integration of varied dimensions of the meander lines (obese and thin tracks) represents a uniqueness of the proposed layouts. Particularly because the arrangement modifies the antenna’s impedance values and contributes to enabling wide AR-BW with compact size. On the other hand, the cited patents utilized meander lines with similar outlines. Besides, defected ground has been utilized to reduce the front-to-back ratio of the antenna featuring directional radiation pattern in one patent66. However, there are two significant differences between the proposed geometries and the relevant patent. First, the proposed patterns of the ground plane produces radiations that eventually lead to the formation of quasi-omnidirectional RP with notable antenna KPI, which are required for densely populated applications. Second, the proposed designs of the ground plane as well as the radiators are entirely novel in terms of configurations. Scholarly papers over the years have been explored to further explain the novelty of the proposed antenna designs. Numerous antennas with meander lines adhering to a particular impedance and/or defected ground have been researched67–71. However, the benefits and drawbacks of transforming the meander line impedance have never been fully established. Therefore, based on existing patents and scholarly papers, the present research offers the first precise comprehension into the impact of altering meander line impedance on overall antenna performance.
Antenna performance pros. All of the proposed antennas feature a low-profile with two planar layers, signifying feasibility for mass production with minimal fabrication complexities. Four papers72–75 rendered the same, but at the cost of I-BW and/or AR-BW performance. The other two articles76,77 exhibited reasonably good G and
values, however at the expense of complex design besides from having relatively poor I-BW and AR-BW results. Overall, the proposed antennas delivers significant 
-BW, unlike previous efforts that only exhibited notable 
for limited BW. Another important achievement of this research (
) is the realization of AR-BW of 60.63%, which to the best of the authors’ knowledge, is the widest AR-BW ever accomplished. Finally, note that this work has been presented based on the specific requirements of the project and practical scenarios, and the proposed antennas are interchangeable for diverse use cases.Antenna performance cons. Typically, the antenna design stages include the process of striking the ideal balance between competing attributes to realize the optimum performance for a particular application. Correspondingly, the proposed antennas have design and performance trade-offs that are addressed as follows. Firstly, the entire array of unit EBG design does not follow the same pattern in
antenna layout, eventually resulting in an imbalanced electric field around the primary radiating elements and increased cross polarization magnitudes. Secondarily, the radiation from the asymmetrical defected ground design altered the cross polarization magnitudes of the primary radiators in the 
antenna arrangement. Finally, the G deviation has been noted in a few of the proposed MIMO antennas, which may be attributed to the substantially different designs of the radiating parts, including the ground plane that operates as a radiator and the top copper elements. However, all of these drawbacks are justified because the higher cross polarization magnitudes exist for extremely limited angles, and the ratio of G divergence was 
3 dBic, making all of the proposed antennas viable for real-world applications.
Conclusion
This paper introduced six novel and lucrative printed MIMO antennas featuring quasi-isotropic CP for six different 6G uses cases–IoT, XR, AI, V2X, UAV, and D2D ISAC–in the 6 GHz band. Systematic mechanisms for concluding impedance matching, realizing CP, and reducing 
values have been described. The measurement results points strongly to the benefits of every proposed MIMO antennas. Their main highlights include notable 
-BW, wide AR-BW, and simple construction. The overall study indicates potentially scalable approaches for mm waves.
Acknowledgements
This work was developed within the “Perceptive communication networks with integrated sensors for 6th generation mobile communications (KOMSENS-6G)” project, supported by the Federal Ministry of Education and Research (BMBF), Germany under the funding code 16KISK122. The responsibility for the content of this publication lies with the authors.
Author contributions
Conceptualization, SK; Data curation, SK and PS; Formal analysis, SK; Funding acquisition, PS; Visualization, SK; Writing—original draft, SK; Writing—review and editing, SK and PS; Software and Resources, SK and PS; Supervision, PS.
Data availability
The data supporting the reported results in this study are not available due to confidentiality restrictions. However, the data generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. Contact person: shahanawaz.kamal@barkhauseninstitut.org or shahanawazkamal@gmail.com.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
Publisher's note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
References
- 1.Lu, H.-H. et al. A combined fibre/free-space-optical communication system for long-haul wireline/wireless transmission at millimetre-wave/sub-thz frequencies. Commun. Eng.2, 18 (2023). [Google Scholar]
- 2.Parker, M. A 640 Gb s–1 transceiver. Nat. Electron.7, 1–1 (2024).
- 3.Giordani, M., Polese, M., Mezzavilla, M., Rangan, S. & Zorzi, M. Toward 6g networks: use cases and technologies. IEEE Commun. Mag.58, 55–61 (2020). [Google Scholar]
- 4.Huang, V. K., Pang, Z., Chen, C.-J.A. & Tsang, K. F. New trends in the practical deployment of industrial wireless: from noncritical to critical use cases. IEEE Ind. Electron. Mag.12, 50–58 (2018). [Google Scholar]
- 5.Robles-Carrillo, M. European union policy on 5g: context, scope and limits. Telecommun. Policy45, 102216 (2021). [Google Scholar]
- 6.Latva-Aho, M., Leppänen, K. et al. Key drivers and research challenges for 6g ubiquitous wireless intelligence. In White Paper (6G Flagship, University of Oulu, 2019).
- 7.Zhang, J., Miao, H., Tang, P., Tian, L. & Liu, G. New mid-band for 6g: Several considerations from the channel propagation characteristics perspective. IEEE Commun. Mag.63, 175–180 (2024).
- 8.Holma, H., Viswanathan, H. & Mogensen, P. Extreme massive MIMO for macro cell capacity boost in 5g-advanced and 6g. In White paper (Nokia Bell Labs, 2021).
- 9.Meneghello, F., Chen, C., Cordeiro, C. & Restuccia, F. Toward integrated sensing and communications in IEEE 802.11 bf Wi-Fi networks. IEEE Commun. Mag.61, 128–133 (2023). [Google Scholar]
- 10.Ali, A., González-Prelcic, N. & Heath, R. W. Millimeter wave beam-selection using out-of-band spatial information. IEEE Trans. Wirel. Commun.17, 1038–1052 (2017). [Google Scholar]
- 11.Faisal, F., Zada, M., Basir, A., Chaker, M. & Djerafi, T. Cost-effective 3d printable water-based MIMO and siso antennas for high-data-rate biomedical implantable devices. IEEE Trans. Antennas Propag.72, 6177–6186 (2024).
- 12.Afzal, M. U., Matekovits, L., Esselle, K. P. & Lalbakhsh, A. Beam-scanning antenna based on near-electric field phase transformation and refraction of electromagnetic wave through dielectric structures. IEEE Access8, 199242–199253 (2020). [Google Scholar]
- 13.Gao, R. B. et al. A wideband co-linearly polarized composite antenna with high isolation. IEEE Antennas Wirel. Propag. Lett.23, 419–423 (2023).
- 14.Zhang, G. et al. Passive human localization with the aid of reconfigurable intelligent surface. In 2023 IEEE Wireless Communications and Networking Conference, 1–6 (2023).
- 15.Hajiyat, Z. R., Ismail, A., Sali, A. & Hamidon, M. N. Antenna in 6g wireless communication system: specifications, challenges, and research directions. Optik231, 166415 (2021). [Google Scholar]
- 16.Kamal, S. & Sen, P. Microstrip-ministered proximity-coupled stacked dual-port antenna for 6g applications. IEEE Access12, 2817–2829 (2024). [Google Scholar]
- 17.Balanis, C. A. Antenna Theory: Analysis and Design (Wiley, 2016).
- 18.Blanch, S., Romeu, J. & Corbella, I. Exact representation of antenna system diversity performance from input parameter description. Electron. Lett.39, 705–707 (2003). [Google Scholar]
- 19.IoT, T. 3gpp low power wide area technologies. GSMA White Paper downloaded Mar11, 49 (2018).
- 20.Chen, M., Miao, Y., Hao, Y. & Hwang, K. Narrow band internet of things. IEEE Access5, 20557–20577 (2017). [Google Scholar]
- 21.Mahfuz, M. H. & Park, C.-W. Review of patch antennas used in drone applications. IEEE Access11, 58367–58388 (2023). [Google Scholar]
- 22.Amodu, O. A., Jarray, C., Busari, S. A. & Othman, M. THz-enabled UAV communications: motivations, results, applications, challenges, and future considerations. Ad Hoc Netw.140, 103073 (2023). [Google Scholar]
- 23.Gapeyenko, M., Petrov, V., Paris, S., Marcano, A. & Pedersen, K. I. Standardization of extended reality (xr) over 5g and 5g-advanced 3g pp new radio. IEEE Netw.37, 22–28 (2023). [Google Scholar]
- 24.Su, W., Prasannakumar, P. V., Li, Y., Ye, G. & Zhu, J. Wearable antennas for virtual reality cross-body links. IEEE Open J. Antennas Propag.4, 207–215 (2023). [Google Scholar]
- 25.Geiger, M. & Waldschmidt, C. 160-GHz radar proximity sensor with distributed and flexible antennas for collaborative robots. IEEE Access7, 14977–14984 (2019). [Google Scholar]
- 26.Alsamhi, S. H., Ma, O. & Ansari, M. S. Survey on artificial intelligence based techniques for emerging robotic communication. Telecommun. Syst.72, 483–503 (2019). [Google Scholar]
- 27.Su, H., Wu, L. L., Zhang, Y., Xu, H. L. & Zhang, X. Y. Analysis and design of filtering series-feed omni-directional antenna for v2x application. IEEE Trans. Antennas Propag.70, 11485–11495 (2022). [Google Scholar]
- 28.Awan, W. A., Hussain, N., Park, S. G. & Kim, N. Intelligent metasurface based antenna with pattern and beam reconfigurability for internet of things applications. Alex. Eng. J.92, 50–62 (2024). [Google Scholar]
- 29.Li, P. et al. Wideband widebeam circular-polarized antenna using asymmetrical tri-dipoles for direct satellite-to-handset communication. IEEE Trans. Antennas Propag.72, 6270–6277 (2024).
- 30.Kamal, S. et al. A low-profile quasi-loop magneto-electric dipole antenna featuring a wide bandwidth and circular polarization for 5g mmwave device-to-device communication. J. Electromagn. Eng. Sci.22, 459–471 (2022). [Google Scholar]
- 31.Sen, P., Harutyunyan, A., Umar, M. & Kamal, S. Joint communication and radar sensing: Rf hardware opportunities and challenges-a circuits and systems perspective. Sensors23, 7673 (2023). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 32.Yalcinkaya, M. T., Sen, P. & Fettweis, G. Comparative analysis of antenna isolation characteristic with & without self-interference reduction techniques towards in-band full-duplex operation. IET Microwaves Antennas Propag17, 329–342 (2023). [Google Scholar]
- 33.Kamal, S., Ain, M. F., Ullah, U. & Omar, M. F. M. Enabling MIMO antenna miniaturization and wide circular polarization coverage by amalgamation of a dielectric strip between meandered traces and slotted ground. IEEE Antennas Wirel. Propag. Lett.21, 1901–1905 (2022). [Google Scholar]
- 34.Wu, T. & Wang, J.-W. Neutralization-line-based decoupling for miniaturized MIMO antenna array. Microw. Opt. Technol. Lett.65, 685–689 (2023). [Google Scholar]
- 35.Banerjee, J., Gorai, A. & Ghatak, R. A compact UWB MIMO antenna augmented with isolation improvement structures in situ with ground stubs and slots. Int. J. Microwave Wirel. Technol.16, 1–12 (2024). [Google Scholar]
- 36.Khan, R. et al. Enhancing gain and isolation of a quad-element MIMO antenna array design for 5g sub-6 GHz applications assisted with characteristic mode analysis. Sci. Rep.14, 11111 (2024). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 37.Esmail, B. A. & Koziel, S. Design and optimization of metamaterial-based highly-isolated MIMO antenna with high gain and beam tilting ability for 5g millimeter wave applications. Sci. Rep.14, 3203 (2024). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 38.Ma, L., Shao, Z., Lai, J., Gu, C. & Mao, J. A compact dual-decoupling scheme for aperture-coupled and probe-fed closely-spaced wideband microstrip antennas. IEEE Trans. Antennas Propag.71, 9072–9077 (2023).
- 39.Palanisamy, S., Rubini, S. S., Khalaf, O. I. & Hamam, H. Multi-objective hybrid split-ring resonator and electromagnetic bandgap structure-based fractal antennas using hybrid metaheuristic framework for wireless applications. Sci. Rep.14, 3288 (2024). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 40.Zhang, W.-H., Lu, W.-J. & Tam, K.-W. A planar end-fire circularly polarized complementary antenna with beam in parallel with its plane. IEEE Trans. Antennas Propag.64, 1146–1152 (2016). [Google Scholar]
- 41.Nguyen, T. K., Tran, H. H. & Nguyen-Trong, N. A wideband dual-cavity-backed circularly polarized crossed dipole antenna. IEEE Antennas Wirel. Propag. Lett.16, 3135–3138 (2017). [Google Scholar]
- 42.Wang, Z., Liu, Y. & Dong, Y. Novel miniaturized circularly polarized inverted-f antenna with planar configuration. IEEE Antennas Wirel. Propag. Lett.23, 1005–1009 (2023).
- 43.Kamal, S., Ullah, U. & Koziel, S. Excitation of circularly polarized wave via single-feed metasurface-integrated compact antenna for internet of things. AEU-Int. J. Electron. Commun.177, 155196 (2024). [Google Scholar]
- 44.Cao, Z., Chang, L., Li, Y., Wei, K. & Zhang, Z. Compact mobile terminal antenna with end-fire circularly polarized beam for satellite communication. IEEE Trans. Antennas Propag.71, 9980–9985 (2023). [Google Scholar]
- 45.Liu, N.-W. et al. A novel low-profile circularly polarized diversity patch antenna with extremely small spacing, reduced size, and low mutual coupling. IEEE Trans. Antennas Propag.70, 135–144 (2021). [Google Scholar]
- 46.Lam, K. Y., Luk, K.-M., Lee, K. F., Wong, H. & Ng, K. B. Small circularly polarized u-slot wideband patch antenna. IEEE Antennas Wirel. Propag. Lett.10, 87–90 (2011). [Google Scholar]
- 47.Wang, R., Klionovski, K. & Shamim, A. Theory and design of a 3d antenna with wideband radiation isotropy. IEEE Trans. Antennas Propag.72, 986–991 (2023). [Google Scholar]
- 48.Shah, S. I. H., Radha, S. M., Park, P. & Yoon, I.-J. Recent advancements in quasi-isotropic antennas: a review. IEEE Access9, 146296–146317 (2021). [Google Scholar]
- 49.Balanis, C. A. Advanced Engineering Electromagnetics (Wiley, 2012).
- 50.Kamal, S., Ramzan, M. & Sen, P. An EBG-driven 2-port MIMO antenna featuring enhanced isolation and circular polarization for IEEE 802.11 ad JC &S terminals. In 2025 IEEE 5th International Symposium on Joint Communications & Sensing (JC &S), 1–5 (IEEE, 2025).
- 51.Rizvi, S. N. R. et al. A closely spaced two-port MIMO antenna with a radiation null for out-of-band suppressions for 5g sub-6 GHz applications. PLoS ONE19, e0306446 (2024). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 52.Truong-Quang, N. et al. A method to design compact MIMO patch antenna using self-isolated technique. Sensors25, 2073 (2025). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 53.Gopi, D., Vadaboyina, A. R. & Dabbakuti, J. K. DGS based monopole circular-shaped patch antenna for UWB applications. SN Appl. Sci.3, 198 (2021). [Google Scholar]
- 54.Jongsuebchoke, I., Akkaraekthalin, P. & Torrungrueng, D. Theory and design of quarter-wave-like transformers implemented using conjugately characteristic-impedance transmission lines. Microw. Opt. Technol. Lett.58, 2614–2619 (2016). [Google Scholar]
- 55.Singh, M., Basu, A. & Koul, S. Circular patch antenna with quarter wave transformer feed for wireless communication. In 2006 Annual IEEE India Conference, 1–5 (IEEE, 2006).
- 56.Hussain, M., Awan, W. A., Abbas, S. M. & Zhu, Y. Design and development of low-profile polymer based broadband antenna for on-body applications. Results Eng.25, 103818 (2025). [Google Scholar]
- 57.Awan, W. A., Islam, T., Alibakhshikenari, M. & Kouhalvandi, L. A pin diode-enabled PDMS antenna for transition between UWB and filtered narrowband modes. In 2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI), 2065–2066 (IEEE, 2024).
- 58.Kaupp, H. Characteristics of microstrip transmission lines. IEEE Trans. Electron. Comput.2, 185–193 (1967). [Google Scholar]
- 59.Ericsson, A. & Sjöberg, D. Design and analysis of a multilayer meander line circular polarization selective structure. IEEE Trans. Antennas Propag.65, 4089–4101 (2017). [Google Scholar]
- 60.Ullah, U., Koziel, S., Pietrenko-Dabrowska, A. & Kamal, S. A planar-structured circularly polarized single-layer MIMO antenna for wideband millimetre-wave applications. Eng. Sci. Technol. Int. J.57, 101819 (2024). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 61.Silva, Z. J., Valenta, C. R. & Durgin, G. D. Design and characterization of meshed microstrip transmission lines. In 2019 IEEE MTT-S International Microwave Symposium (IMS), 811–814 (IEEE, 2019).
- 62.Rajo-Iglesias, E., Inclán-Sánchez, L., Vázquez-Roy, J.-L. & García-Muñoz, E. Size reduction of mushroom-type EBG surfaces by using edge-located vias. IEEE Microw. Wirel. Compon. Lett.17, 670–672 (2007). [Google Scholar]
- 63.Sievenpiper, D., Zhang, L., Broas, R. F., Alexopolous, N. G. & Yablonovitch, E. High-impedance electromagnetic surfaces with a forbidden frequency band. IEEE Trans. Microw. Theory Tech.47, 2059–2074 (1999). [Google Scholar]
- 64.Kamal, S., Adler, J., Matthé, M. & Sen, P. Circularly polarized dual antenna with crescent axed sporadic meander traces isolated by shorting pins and defected ground for Wi-Fi sensing. In 2024 IEEE 4th International Symposium on Joint Communications & Sensing (JC &S), 01–06 (IEEE, 2024).
- 65.Tayama, H. & Guan, N. Wide band antenna (2012). US Patent App. 13/240,198.
- 66.Sharawi, M. S. & Jehangir, S. S. Compact size, low profile, dual wideband, quasi-yagi, multiple-input multiple-output antenna system (2019). US Patent 10,256,549.
- 67.Jia-feng, Z. & Jiu-sheng, L. A meander line integrated rectangular tri-band antenna for 5g sub-6GHz applications. Microw. Opt. Technol. Lett.67, e70164 (2025). [Google Scholar]
- 68.Teng, X. et al. Axial mode planar meander-line antenna with high-gain endfire radiation pattern mounted on large-scale ground plane. IEEE Trans. Antennas Propag.73, 1394–1405 (2024).
- 69.Alam, T., Almutairi, A. F., Samsuzzaman, M., Cho, M. & Islam, M. T. Metamaterial array based meander line planar antenna for cube satellite communication. Sci. Rep.11, 14087 (2021). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 70.Santos, H. M., Pinho, P., Silva, R. P., Pinheiro, M. & Salgado, H. M. Meander-line monopole antenna with compact ground plane for a bluetooth system-in-package. IEEE Antennas Wirel. Propag. Lett.18, 2379–2383 (2019). [Google Scholar]
- 71.Kamal, S. & Chaudhari, A. A. Printed meander line MIMO antenna integrated with air gap, DGS and RIS: A low mutual coupling design for LTE applications. Prog. Electromagn. Res. C71, 149–159 (2017). [Google Scholar]
- 72.Hussain, M. et al. A pin diode-enabled compact size antenna for transition between UWB and notched UWB modes. Arab. J. Sci. Eng. 1–10 (2025).
- 73.Hussain, M., Abbas, A., Awan, W. A. & Naqvi, S. I. Miniaturized arrow-shaped flexible filter-embedded antenna for industrial and medical applications. Appl. Sci.14, 11004 (2024).
- 74.Kumari, P. & Das, S. A wideband circularly polarized SIW MIMO antenna based on coupled QMSIW and EMSIW resonators for sub-6ghz 5g applications. IEEE Antennas Wirel. Propag. Lett.23, 2979–2983 (2024).
- 75.Kaushik, M., Dhanoa, J. K. & Khandelwal, M. K. Partially omnidirectional and circularly polarized MIMO antenna covering sub-6 GHz band for 5g fast plan. IEEE Trans. Compon. Packag. Manuf. Technol.13, 1443–1450 (2023). [Google Scholar]
- 76.Ma, L., Lu, J., Gu, C. & Mao, J. A wideband dual-circularly polarized, simultaneous transmit and receive (star) antenna array for integrated sensing and communication in IoT. IEEE Internet Things J.10, 6367–6376 (2022). [Google Scholar]
- 77.Nikkhah, M. R., Wu, J., Luyen, H. & Behdad, N. A concurrently dual-polarized, simultaneous transmit and receive (star) antenna. IEEE Trans. Antennas Propag.68, 5935–5944 (2020). [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data supporting the reported results in this study are not available due to confidentiality restrictions. However, the data generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. Contact person: shahanawaz.kamal@barkhauseninstitut.org or shahanawazkamal@gmail.com.