Engineering Science and Technology, an International Journal 28 (2022) 101015

Contents lists available at ScienceDirect

Engineering Science and Technology,

an International Journal
journal homepage: www.elsevier.com/locate/jestch

Full Length Article

Circularly polarized offset-fed DRA elements & their application in

compact MIMO antenna
Nikesh Kumar Sahu, Gourab Das, Ravi Kumar Gangwar ⇑
Department of Electronics Engineering, IIT (ISM) Dhanbad, Jharkhand 826004, India

a r t i c l e i n f o a b s t r a c t

Article history: In this work, the generation of circular polarization (CP) in two different dielectric resonator antenna
Received 29 July 2020 (DRA) elements (one element gets energized by the offset conformal-strip and another one is by the offset
Revised 30 December 2020 microstrip-slot) is precisely explored. CP radiations from the presented DRA elements are acquired by
Accepted 19 May 2021
removing a proper asymmetrical notch from the rectangular dielectric resonators (rDRs) and feeding
Available online 04 June 2021
the rDRs at an appropriate offset distance from the center. Furthermore, these two DRA elements are inte-
grated into one antenna system, which can be used for multiple-input-multiple-output (MIMO) applica-
Keywords:
tion. Owing to the use of two different excitation schemes, the dielectric resonators are mounted on two
Circular polarization (CP)
Dielectric resonator antenna (DRA)
opposite sides of the substrate and are being radiated in two opposite directions to realize pattern diver-
Pattern diversity sity compact MIMO system. So, the presented MIMO design is able to diminish the intersecting spatial
field components between the antenna elements, which in turn reduce the envelope correlation coeffi-
cient and improve the inter-port isolation. The proposed diversity/MIMO antenna can be utilized as a pro-
ficient contender in rich multipath environments to overcome the appearances of channel fading.
Experimental outcomes convey that the proposed MIMO antenna features overlapping 3-dB axial ratio
(AR) band ranging from 5.30 to 5.87 GHz and overlapping 10-dB impedance band ranging from 5.15 to
6.12 GHz with inter-port isolation of greater than 17 dB throughout the usable frequency band.
Ó 2021 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction DR and two triangular DRs with a partial ground plane) to achieve
the desired CP radiation. In [11–14], CP radiation is obtained from a
In wireless communication systems, to make a flexible orienta- rectangular DR by using the modified feed structures, such as con-
tion of source and receiver antennas with attaining stable commu- formal open half-loop [11], microstrip-coupled cross-slot [12],
nication link, circularly polarized antennas are extensively studied Archimedean spiral slot [13], and square-shaped slots [14]. By
from the past few decades. There are a variety of antennas (patch introducing the conductive coating and the notches in a rectangu-
antenna, loop antenna, horn antenna, slot antenna, and dielectric lar DR, a wideband CP response is obtained in [15].
resonator antenna) found in the literature for circular polarization In recent years, to make the antenna systems work smarter by
(CP) radiation [1–15]. Among these antennas, dielectric resonator sending and accepting multiple spatial data streams at the same
(DR) based CP antennas are paid much attention owing to their time, multiple-input-multiple-output (MIMO) antenna technology
intrinsic advantages like high radiation efficiency, and wide control is receiving enormous consideration. Basically, MIMO antenna sys-
over size and bandwidth [16]. Also, versatility in DR shape and tem is used to maximize the data speed and minimize the errors by
excitation schemes helps the antenna designers to achieve CP taking the advantages of multiple antenna elements adopted at
along with desired radiation patterns, which provides an addi- both the ends of a wireless communication link. However, it is
tional interest on exploring CP-DR based antennas. Wong et al. quite challenging and also, very important to provide sufficient
[6] used a rotated-stair DR, Hwang et al. [7] proposed an annular field and port isolation between the antenna elements adopted in
DR, Khanna et al. [8] introduced an OM-shaped DR, Khalily et al. the close vicinity. An enhancement of isolation characteristics
[9] explored inclined slits loaded square DR, and Altaf et al. [10] can effectively enhance the desired MIMO system performance.
developed a special-shaped DR (combination of one rectangular In this regard, there is an assortment of MIMO antenna designs
found in the literature [17–23]. In [17], two ultrawideband
⇑ Corresponding author. (UWB) antenna elements are arranged perpendicular to each other
E-mail address: ravi.gangwar.ece07@itbhu.ac.in (R.K. Gangwar). to establish polarization diversity and high isolation between these
Peer review under responsibility of Karabuk University. two elements. Also, to further enhance the inter-port isolation, a

https://doi.org/10.1016/j.jestch.2021.05.019
2215-0986/Ó 2021 Karabuk University. Publishing services by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Table 1
Summary of the proposed and reported CP-DRAs.

Antenna Feeding Technique DR Shape erd 3-dB AR Band (GHz)

Proposed conformal-strip fed DRA Conformal-strip Rectangular DR with a notch 9.8 5.31–6.60
Proposed microstrip-slot fed DRA Simple slot Rectangular DR with a notch 9.8 5.35–5.73
[13] Archimedean spiral slot Rectangular DR 12 1.95–2.52
[11] Concentric open half-loops Rectangular DR 9.2 3.07–3.55
[10] Vertical-strip Combination of two triangular DR with one rectangular DR 10 3.35–5.25
[9] Tapered-strip Inclined slits loaded square DR 10 2.86–4.43
[7] Vertical coaxial probe Semi-eccentric annular DR 10 10.37–10.98

where, erd is the relative permittivity of the DR.

Table 2
Comparison with the reported MIMO-DRAs.

Antenna Size (mm2) BWIMP (GHz) CP Isolation Technique Isolation (dB)

Proposed MIMO-DRA 40
60 5.15–6.12 Yes  ˃17
[25] 80
80 5.71–8.20 Yes  ˃15
[24] 95
49.7 3.15–3.93 Yes EBG structure ˃26
[21] 50
40 2.5 – 11.0 No Carbon black film ˃15
[19] 35
33 3.1–5.0 No Neutralization line ˃22
[18] 25
30 3.1–10.6 No Reflector ˃20
[17] 38.5
38.5 3.08–11.8 (Except 5.03–5.97) No Parasitic T-shaped strip ˃15

where, BWIMP denotes the overlapping 10-dB impedance band.

Fig. 1. Proposed offset conformal-strip fed DRA element. (a) Top view, (b) 3-D view. [LS = 40 mm, WS = 40 mm, W = 20 mm, L = 17 mm, H = 7 mm, HS = 1.6 mm, r = 2 mm,
a = 18 mm, b = 7 mm, d1 = 8.7 mm, WF = 2.6 mm, HF = 10 mm].

Fig. 2. Evolution of the proposed offset conformal-strip fed DRA element.

T-shaped strip is being utilized as a decoupling structure for this is developed in [19] to diminish the mutual coupling caused
MIMO configuration. In [18], a metal strip is placed in between between the closely placed UWB elements. A dual-band dual-
the two coplanar stripline-fed staircase-shaped printed radiating element MIMO slot antenna is studied in [20], where two quarter
elements to achieve high isolation. A wideband neutralization line wavelength slots are used in each antenna element to obtain

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Fig. 3. Simulated orthogonal electric far-field components showing the conditions for the generation of CP. (a) Conf-I, (b) Conf-II, (c) Conf-III. [Towards bore-sight direction
(h = 0°)].

dual-band characteristics. In this case, the desired isolation perfor- these two CP-DRA elements are used together to realize a MIMO/-
mance is achieved by etching one wide and a pair of narrow slots diversity antenna system. The antenna elements are intended to
from the ground plane. In [21], to reduce mutual coupling between radiate in two opposite directions, which yield low inter-
the antenna elements, a carbon black film is used for absorbing the element correlation. Thus, despite of very closely placed antenna
interfering electromagnetic signals. A MIMO antenna based on two elements and also, without implementing any special decoupling
dual-band single-feed patch element is reported in [22]. The isola- mechanism, the proposed compact MIMO structure able to deli-
tions in the lower and upper bands are achieved by employing the ver the desired system performance. The pattern diversity feature
modified array antenna decoupling surface (MADS) and H-shaped of the proposed MIMO antenna model can be typically utilized in
defected ground structure, respectively. Abdel-Rahman et al. [23] rich multipath environments, where the angular power spectrum
proposed a three-port MIMO dielectric resonator antenna (DRA), of the incoming or outgoing signal is about uniform. A prototype
where three mutually decoupled and near-degenerate modes are of the proposed MIMO antenna is fabricated and tested. In Table 2,
excited to suppress the correlation between the antenna elements. a comparison between the proposed MIMO-DRA and some
But till now, the integration of CP radiators in a single antenna sys- reported MIMO-DRAs is shown. Compared with the antennas
tem has not yet been broadly explored for MIMO applications. In addressed in [24] and [25], the proposed antenna has a more
[24], Park et al. explored a two-element DRA array and in [25], compact size. Also, the proposed antenna has high isolation than
Varshney et al. implemented a two-port MIMO-DRA of having CP that in [17] and [21]. Although the compactness, as well as the
radiations. isolation reported in [18] and [19], is better than that in the pro-
In this paper, two simple offset-fed DRA elements are pre- posed work, the antennas in [18] and [19] have linearly polarized
sented for the CP radiation. By carving out a proper asymmetrical radiation.
notch from the rectangular DR and feeding the DR at a particular The detailed design and analysis of the CP-DRA elements are
offset position, it is possible to fulfill the conditions for the CP discussed in section 2. The proposed MIMO antenna structure
radiation. A detailed summary of the proposed DRA elements and its performance evaluation are studied in section 3. At last,
and some reported CP-DRAs is given in Table 1. Furthermore, the conclusions are stated in section 4.

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thickness ‘HS’ and the ground plane of the antenna is printed on the
backside of the substrate. The DR is fed by a conformal-strip at an
offset distance ‘d10 (along Y-direction) from the center of the DR.
To precisely explore the operational principle of the proposed
offset conformal-strip fed DRA element, three antenna variants
(denoted as Conf-I, Conf-II, and Conf-III) are designed and are illus-
trated in Fig. 2. Conf-I is the initial design, where a rectangular DRA
is center-fed by a conformal-strip. In the next evolution of antenna
(denoted as Conf-II), the feed position is shifted at an offset dis-
tance ‘d10 . Finally, the proposed DRA element is formed by remov-
ing a proper asymmetrical notch from the rectangular DR, which is
denoted as Conf-III.
For the above three antenna variants, the magnitudes of orthog-
onal electric field components (E/ and Eh) and the phase difference
between them are depicted in Fig. 3. In Conf-I, only one component
of electric field (Eh) is predominantly induced, which indicates that
the Conf-I can be used for better linearly polarized wave radiation.
For circularly polarized wave radiation, it is essential to establish
two mutually orthogonal electric field components of equal magni-
tude and 90° phase difference. In Conf-II, due to the offset feed
position, it is possible to induce both orthogonal electric field com-
ponents inside the rectangular DR, which is clearly depicted in
Fig. 3(b). So, by shifting the feed position from the center, the
excited electric fields are decomposed into two orthogonal compo-
nents. To clarify the decomposition, the electric field vectors on the
top surface of the rectangular DR are plotted in Fig. 4(a). Fig. 4(a)
displays that electric field vectors are aligned at an angle about
45° with respect to x-axis, which is attributed to the existence of
the two orthogonal electric field components of equal magnitude.
In general, the offset feed position causes mode degeneracy, which
Fig. 4. (a) Rotated electric field vectors on the top surface of the offset conformal- in turn allows to excite mutually orthogonal degenerate modes
strip fed rectangular DRA, (b) Different amount of paths covered by the different inside the DR. However, Conf-II is unable to completely fulfill the
electric field components inside the proposed offset conformal-strip fed DRA
above stated conditions for CP generation. It mainly fails to deliver
element.
the desired phase difference of 90°. So, in Conf-III, a notch is carved
out from the rectangular DR to create a phase difference between
2. CP DRA elements design the orthogonal field components. This can be clarified by plotting
the electric field distributions inside the DR, shown in Fig. 4(b).
2.1. Offset conformal-strip fed DRA element Observe that each field component covers a different amount of
distance to provide path delay, thereby introduces the phase shift
The schematic picture of the proposed offset conformal-strip between the orthogonal field components (phase shift = 2p/k
pa
fed DRA element is depicted in Fig. 1. It is composed of a rectangu- th delay). Thereafter, the phase difference is controlled by varying
lar DR (erd = 9.8) from which a proper asymmetrical rectangular the position and dimensions of the rectangular notch. Thus, by
notch with length ‘a’ and width ‘b’ is removed. The DR is mounted proper optimization, the desired conditions are nearly achieved
on the top of a commercial available FR4 substrate (ers = 4.4) with over a band of frequencies (shown by the shaded region in Fig. 3

Fig. 5. Influence of varying notch length (a) on: (a) Magnitude ratio (jE/ j=jEh j), (b) Phase difference (\E/  \Eh ). [Proposed offset conformal-strip fed DRA element].

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 6. Influence of varying notch width (b) on: (a) Magnitude ratio (jE/ j=jEh j), (b) Phase difference (\E/  \Eh ). [Proposed offset conformal-strip fed DRA element].

Fig. 7. Simulated outcomes during the evolution stages of proposed offset conformal-strip fed DRA element. (a) Reflection coefficient, (b) Axial ratio [Towards bore-sight
direction (h = 0°)].

Fig. 8. Electric field distributions inside the rectangular DRA at 4.4 GHz when center-fed by a conformal-strip. (a) Top view, (b) Side view.

(c)). A parametric investigation is performed to illustrate the CP be noticed from this figure, the value of ‘a’ affects the phase differ-
generation for the proposed offset conformal-strip fed DRA ele- ence considerably, and the magnitude ratio and the phase differ-
ment. Fig. 5 depicts the influence of notch length ‘a’ on magnitude ence can be tuned by adjusting the value of ‘a’. The influence of
ratio (jE/ j=jEh j) and phase difference (\E/  \Eh ) outcomes. As can notch width ‘b’ on magnitude ratio and phase difference outcomes

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 9. (a) Electric field distributions on the top surface of the proposed offset conformal-strip fed DRA element at different frequencies, (b) Electric field distributions on the
top surface of the proposed offset conformal-strip fed DRA configurations at different time instances [At 5.68 GHz].

Fig. 10. Proposed offset microstrip-slot fed DRA element. (a) Top view, (b) 3-D view. [LS = 40 mm, WS = 40 mm, W = 20 mm, L = 17 mm, H = 7 mm, HS = 1.6 mm, r = 2 mm,
a = 18 mm, b = 7 mm, d2 = 4 mm, d3 = 7.75 mm, d4 = 4.7 mm, WF = 2.6 mm, s1 = 12 mm, s2 = 1.5 mm].

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 11. Evolution of the proposed offset microstrip-slot fed DRA element.

Fig. 12. Simulated orthogonal electric far-field components showing the conditions for the generation of CP. (a) Conf-A, (b) Conf-B, (c) Conf-C. [Towards bore-sight direction
(h = 0°)].

also investigated and is displayed in Fig. 6. From this figure, similar of Conf-I is due to the excitation of TEy 111 mode in the rectangular
observations are obtained. Thus, it is evident from Fig. 5 and Fig. 6 DR. To verify the presence of TEy 111 mode, the resonance frequency
that the desired CP conditions are fulfilled at the operating fre- of this mode is calculated using the relations (1)-(3) [26], and it is
quency by considering the value of ‘a’ and ‘b’ to 18 mm and found to be 4.5 GHz. In relations (1)-(3), c signifies the speed of
7 mm, respectively. light in vacuum, k0 signifies the free-space wave number, and kx,
The reflection coefficient and axial ratio (AR) results of the pro- ky and kz signify the wave numbers inside the DR along the three
posed DRA element during its evolutionary stages are shown in directions. In addition, a study of the electric field distributions
Fig. 7. The minima at 4.4 GHz in the reflection coefficient curve inside the DR is done and the result of this study is displayed in
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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

reflection coefficient curve of Conf-III is attributed to the existence

of TEx 111 and TEy 111 modes. From Fig. 7(b), it can be noted that Conf-
III is able to provide the desired CP radiation (AR  3 dB) from 5.31
to 6.60 GHz. The minimum AR value of 0.16 dB is observed at
5.68 GHz, which is around the center of two resonance frequencies
(5.15 GHz and 6.05 GHz). This is obvious because, at the center fre-
quency, the lower and upper resonances provide the net 90° phase
difference between the two orthogonal modes [28], which also
clearly noticed from Fig. 3(c). To illustrate this, the electric field
distributions at 5.68 GHz (with phase angles 45° and 45°),
5.15 GHz, and 6.05 GHz are depicted in Fig. 9(a). Observe that
the field distribution outcome at 5.15 GHz and 6.05 GHz is similar
to that of at 5.68 GHz \45° and 5.68 GHz \45°, respectively,
which validates a quadrature phase difference between the orthog-
onal modes. From Fig. 3(c), it can also be noticed that the phase of
E/ component is lagging behind the Eh component (as ‘‘\E/  \Eh
(deg)” is providing negative value) by 90° at the minimum AR fre-
quency point. Hence, a right hand circular polarization (RHCP)
radiation can be estimated from the proposed DRA configuration.
To clarify this, a qualitative analysis based on the electric field dis-
tributions on the top surface of the DR (seen from the + z-axis) at
different time instances (t = 0, T/4, T/2, and 3 T/4) is depicted in
Fig. 9(b), where T signifies the period. Observe that as the time
increases, the direction of dominant electric field vectors rotates
counterclockwise to ensure RHCP radiation. Furthermore, by sim-
ply considering the mirror image of the proposed DRA configura-
tion, it is possible to secure a left hand circular polarization
Fig. 13. (a) Rotated electric field vectors on the top surface of the offset microstrip- (LHCP) radiation, which can be seen from Fig. 9(b).
slot fed rectangular DRA, (b) Different amount of paths covered by the different qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
c 2 2 2
electric field components inside the proposed offset microstrip-slot fed DRA fr ¼ pffiffiffiffiffiffi kx þ ky þ kz ð1Þ
element. 2p erd
where
Fig. 8. From Fig. 8, the existence of TEy 111 mode at 4.4 GHz is con- p p
firmed since the electric fields are oriented along the x-axis. In kx ¼ ; kZ ¼ ð2Þ
L 2H
Conf-II, due to the introduction of offset feed, two mutually orthog-
  qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
onal degenerate modes (TEx 111 and TEy 111 ) are excited in the rectan- ky W
¼ ðerd  1Þk0  ky
2 2
gular DR. As displayed in Fig. 7(a), the reflection coefficient ky tan ð3Þ
2
response of Conf-II is nearly the same with that of Conf-I, except
a better impedance matching is observed in the case of Conf-II.
In Conf-III, by carving out a notch from the rectangular DR, the 2.2. Offset microstrip-slot fed DRA element
effective permittivity of the DR is decreased. Consequently, the res-
onance point is shifted to a higher frequency (shown in Fig. 7(a)). Fig. 10 depicts the schematic picture of the proposed offset
An approximate value of the effective permittivity can be calcu- microstrip-slot fed DRA element. It is composed of same DR as that
lated as the volume fraction weighted average of the permittivities of conformal-strip fed DRA element. The DR is employed on the top
of DR and air [27]. The two minima at 5.15 GHz and 6.05 GHz in the of the ground plane and fed by a 50-O microstrip line through a

Fig. 14. Influence of varying notch length (a) on: (a) Magnitude ratio (jE/ j=jEh j), (b) Phase difference (\E/  \Eh ). [Proposed offset microstrip-slot fed DRA element].

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 15. Influence of varying notch width (b) on: (a) Magnitude ratio (jE/ j=jEh j), (b) Phase difference (\E/  \Eh ). [Proposed offset microstrip-slot fed DRA element].

Fig. 16. Simulated outcomes during the evolution stages of proposed offset microstrip-slot fed DRA element. (a) Reflection coefficient, (b) Axial ratio [Towards bore-sight
direction (h = 0°)].

Fig. 17. Electric field distributions inside the rectangular DRA at 4.8 GHz when center-fed by a microstrip-line. (a) Top view, (b) Side view.

slot which is positioned at an offset distance ‘d2’ (along Y-direction) To clarify the operational principle of the proposed offset
and ‘d3’ (along X-direction) from the center of the DR. Note that the microstrip-slot fed DRA element, three antenna prototypes (de-
microstrip line is etched on the bottom of the FR4 substrate and is noted as Conf-A, Conf-B, and Conf-C) are designed and are dis-
used to feed at an offset distance ‘d40 (along Y-direction) from the played in Fig. 11. Conf-A is a basic design, where a rectangular
center of the slot. DR is fed by a central microstrip line through a slot which is posi-

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 18. (a) Electric field distributions on the top surface of the proposed offset microstrip-slot fed DRA element at different frequencies, (b) Electric field distributions on the
top surface of the proposed offset microstrip-slot fed DRA configurations at different time instances [At 5.5 GHz].

tioned at the center of the DR. In Conf-B, the position of the slot is tions for CP generation. So, in the next evolution step (Conf-C), the
shifted at an offset distance ‘d20 and ‘d30 . In addition to this, in Conf- phase difference E/ and Eh is adjusted by removing a notch from
B, the position of the microstrip line is displaced by a distance ‘d40 the rectangular DR, which can be interpreted from Fig. 13(b). Then,
from the center of the slot. Afterward, a proper asymmetrical notch by properly optimizing the position and dimensions of the rectan-
is removed from the rectangular DR to configure the proposed DRA gular notch, the desired conditions are nearly yielded over a band
element (denoted as Conf-C). of frequencies (shown by the shaded region in Fig. 12(c)). A para-
Fig. 12 illustrates the magnitude and phase response of orthog- metric investigation is performed to illustrate the CP generation
onal electric field components (E/ and Eh) present in each evolu- for the proposed offset microstrip-slot fed DRA element. Fig. 14
tionary configuration. As displayed in Fig. 12(a), Conf-A is and Fig. 15 depicts the influences of the notch length ‘a’ and notch
predominantly delivering only one electric field component (Eh) width ‘b’, respectively. From these figures, it can be noted that the
and thus can be used for better LP wave radiation. In order to phase difference between the orthogonal electric field components
acquire CP wave radiation, the position of the slot is displaced in affected considerably as the value of ‘a’ and ‘b’ varies. Furthermore,
the next evolution step (Conf-B). As a result of this, in Conf-B, both the magnitude ratio and the phase difference can be tuned by
E/ and Eh are induced inside the rectangular DR, which can be changing the value of ‘a’ and ‘b’. Thus, for a = 18 mm and
clearly noticed from Fig. 12(b). The reason is attributed to the b = 7 mm, the desired CP conditions are achieved at the operating
decomposition of the electric field vectors into two orthogonal frequency.
components. Consequently, the resultant electric field vectors are The reflection coefficient and AR results for each of the evolu-
aligned at an angle about 45° with respect to x-axis, shown in tionary configurations are displayed in Fig. 16. The reflection coef-
Fig. 13(a). In general, by moving the slot from center position, it ficient response of Conf-A has one minima at 4.8 GHz, owing to the
is possible to excite mutually orthogonal nearly degenerate modes excitation of TEy 111 mode in the rectangular DR. The resonance fre-
inside the DR. But, Conf-B is unable to completely fulfill the condi- quency of the proposed isolated rectangular DR operating in TEy 111

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 19. Proposed two-port MIMO antenna configuration (Dimensions are in millimeters).

Fig. 20. (a) S-Parameters, (b) Axial ratio [Port 1 (h = 0°), Port 2 (h = 180°)] of the proposed two-port MIMO antenna configuration.

Table 3
Performance of proposed diversity antenna. (Frequency Unit: GHz).

Results Reflection Coefficient Axial Ratio

Port 1 (S11) Port 2 (S22) Port 1 (h = 0°) Port 2 (h = 180°)
Sim. 4.60 – 6.90 5.12 – 6.12 5.19 – 6.51 5.30 – 5.89
Meas. 4.88 – 6.53 5.15 – 6.12 5.30 – 6.39 5.23 – 5.87

mode can be calculated using the relations (1)-(3). Based on these tence of TEy 111 mode is clearly confirmed from the electric field dis-
relations, the resonance frequency is found to be 4.5 GHz, which is tribution outcomes displayed in Fig. 17. Moving the slot from the
close to the observed minima at 4.8 GHz. Furthermore, the exis- center to a particular offset distance causes mode degeneracy.

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

of effective permittivity which significantly influences the reso-

nance frequency. Observe in Fig. 16(b) that the minimum AR value
of 0.23 dB is attained at 5.5 GHz, which is around the center of two
minima points noticed in the reflection coefficient curve of Conf-C.
These two minima points (5.45 GHz and 5.6 GHz) are due to the
existence of TEx 111 and TEy 111 modes. A quadrature phase difference
between E/ and Eh is acquired at 5.5 GHz due to the presence of
these two nearly degenerate orthogonal modes, which can be con-
firmed from Fig. 18(a). Fig. 18(a) shows the electric field distribu-
tions at 5.5 GHz (with phase angles 45° and 45°), 5.45 GHz, and
5.6 GHz. Observe that the field distribution outcome at 5.45 GHz
and 5.6 GHz is similar to that of at 5.5 GHz \45°and 5.5 GHz
\45°, respectively. From Fig. 16(b), it can also be observed that
Fig. 21. 3-D far-field radiation patterns of the proposed two-port MIMO antenna
configuration. [At 5.5 GHz]. Conf-C is able to deliver the desired CP performance (AR  3 dB)
from 5.35 to 5.73 GHz. From Fig. 12(c), it can also be observed that
the phase of Eh component is lagging behind the E/ component (as
Hence, in Conf-B, two mutually orthogonal degenerate modes ‘‘\E/  \Eh (deg)” is providing positive value) over the shaded
(TEx 111 and TEy 111 ) are excited in the rectangular DR. In Conf-B, region. Hence, a LHCP radiation can be estimated from the pro-
the microstrip line is placed at a suitable offset distance from the posed DRA configuration. This can be further clarified from a study
center of the slot to acquire the optimum impedance matching. of time-varying electric field distributions, as illustrated in Fig. 18
Referring to Fig. 16(a), it can be seen that the reflection coefficient (b). Fig. 18(b) shows the electric field distributions on the top sur-
response of Conf-B is almost similar to that of Conf-A. But, in Conf- face of the DR (seen from the +z-axis). Observe that as the time
C, when a rectangular notch is removed from the DR, the resonance increases, the direction of dominant electric field vectors rotates
point is shifted to the higher frequency. This is due to the reduction clockwise to ensure LHCP radiation. In the same way, a RHCP radi-

Fig. 22. Normalized 2-D far-field radiation patterns of the proposed two-port MIMO antenna configuration. (a) Port 1, (b) Port 2. [At 5.5 GHz]

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 23. (a) Gain and Radiation efficiency [Port 1 (h = 0°), Port 2 (h = 180°)], (b) Envelope correlation coefficient of the proposed two-port MIMO antenna configuration.

Fig. 24. Some possible two-port MIMO antenna systems.

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Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

Fig. 25. (a) Mutual coupling, (b) Envelope correlation coefficient, (c) Diversity gain, (d) Multiplexing efficiency for three systems.

ation can also be realized by simply taking the mirror image of the ments, as indicated in Fig. 21. Fig. 21 shows the simulated 3-D far-
proposed DRA configuration, which can be clearly noticed from field radiation patterns of the proposed diversity antenna. In the
Fig. 18(b). proposed MIMO antenna configuration, two different feeding tech-
niques are employed to facilitate different radiation directions
(pattern diversity), which in turn provides high isolation between
3. MIMO/Diversity antenna development
the closely placed antenna elements. The simulated and measured
AR variations are plotted in Fig. 20(b) and the 3-dB AR frequency
This section examines the diversity performance of a simple
bands are documented in Table 3.
two-port compact MIMO antenna developed using the above pro-
Fig. 22 shows the simulated and measured normalized 2-D CP
posed DRA elements. Fig. 19 depicts the schematic diagram and
radiation patterns of the proposed diversity antenna at the mini-
fabricated prototype of the proposed MIMO antenna configuration.
mum AR frequency point. Note that, for port 1, the RHCP waves
The DRA elements are arranged side-by-side (parallel configura-
are being radiated from the antenna towards +Z-direction (as the
tion) on a 40
60 mm2 substrate. Note that the DR associated with
RHCP field component is stronger than the LHCP component in
port 1 (conformal-strip fed DRA element) is placed on the top side,
h = 0° direction). Similarly, for port 2, the RHCP waves are being
whereas the DR associated with port 2 (microstrip-slot fed DRA
radiated from the antenna towards –Z-direction (as the RHCP field
element) is placed on the bottom side of the substrate.
component is stronger than the LHCP component in h = 180° direc-
Numerical simulations of the antenna are carried out using the
tion). The gain and radiation efficiency values are depicted in
ANSYS HFSS software package. The simulated and measured
Fig. 23(a). Observe that the measured gain value is about 4.7 dB
S-parameters of the proposed diversity antenna are shown in
for port 1 and 4.4 dB for port 2 at 5.5 GHz. Similarly, the simulated
Fig. 20(a). The simulated and measured outcomes are closely
radiation efficiency is about 96.7% for port 1 and 94% for port 2 at
matched with each other, however, some slight disagreements
5.5 GHz. The diversity characteristics can be examined using the
between them may be due to measurement and fabrication errors.
envelope correlation coefficient (ECC). For uniform multipath envi-
The 10-dB impedance bandwidths (Sii   10 dB) are documented
ronment, the ECC between port 1 and port 2 is computed using the
in Table 3. Fig. 20(a) indicates that the proposed antenna has a very
far-field pattern based relation (as discussed in [29]) and is illus-
low inter-port mutual coupling (Sij <  17 dB) throughout the
trated in Fig. 23(b). The fundamental necessity of a diversity
operating frequency band. This can be attributed to the low inter-
antenna is that the radiation pattern of the antenna elements
secting radiated spatial field components between the antenna ele-
14
Nikesh Kumar Sahu, G. Das and Ravi Kumar Gangwar Engineering Science and Technology, an International Journal 28 (2022) 101015

ought to be uncorrelated. As noticed in Fig. 23(b), the ECC is less III. Despite of very closely placed two antenna elements, the
than 0.12 throughout the operating frequency range. Such low proposed two-port MIMO antenna delivers high inter-port
ECC can be attributed to the spatially uncorrelated radiation pat- isolation (>17 dB) and low ECC (<0.12) without using any
tern of the antenna elements, as demonstrated in Fig. 21. special decoupling mechanism.
The role of pattern diversity is examined by employing three IV. For both MIMO modes of operation (diversity and multiplex-
different MIMO antenna systems, as shown in Fig. 24. System-1 ing), the system performance has been investigated by eval-
consists of two offset conformal-strip fed DRA elements which uating specific matrices and the results convey that the
are arranged side-by-side (parallel configuration) and are intended suggested antenna model can be utilized as a proficient con-
to radiate towards + Z-direction. Similarly, System-2 consists of tender for MIMO applications.
two offset microstrip-slot fed DRA elements which are intended V. The diversity/MIMO system of having two CP radiators has
to radiate towards –Z-direction. However, System-3 (proposed been attempted to overcome the appearances of channel
diversity antenna) consists of both offset conformal-strip and off- fading in rich multipath environments.
set microstrip-slot fed DRA elements. In System-3, each element
radiates towards different direction to avail pattern diversity 4. Conclusion
decoupling scheme. The inter-port isolations and ECCs for three
systems are studied and are illustrated in Fig. 25(a) and (b), respec- Two offset-fed circularly polarized DRA elements are designed
tively. Fig. 25(a) reveals that for both System-1 and System-2, the and studied. A proper offset position is chosen to induce orthogo-
inter-port isolation values vary between 8.57 and 15.9 dB, whereas nal electric field components inside the DR. A simple two-port
for System-3, the values vary between 17.1 and 27.6 dB across the compact MIMO antenna is proposed, and the prototype antenna
operating frequency range. Thus, the inter-port isolation is remark- is successfully realized and measured. The diversity performance
ably increased when the System-3 is used. Similarly, as illustrated is examined to validate the suitability of the proposed MIMO
in Fig. 25(b), the ECC is remarkably decreased when the System-3 antenna and all the features demonstrate that the proposed
is used. Both high isolation and low ECC are the principal require- antenna can be utilized as a proficient contender for MIMO
ments for a MIMO antenna system with good diversity perfor- applications.
mance. Based on the signal-to-noise ratios (SNRs) of the received
signals, different MIMO modes are selected to enhance the system
performance. For low-SNR scenarios, diversity techniques are uti- Declaration of Competing Interest
lized to diminish the fading, and the MIMO antenna system perfor-
mance can be estimated by investigating diversity gain (DG). On The authors declare that they have no known competing finan-
the other hand, for high-SNR scenarios, parallel data streams are cial interests or personal relationships that could have appeared
transmitted simultaneously to provide additional data capacity, to influence the work reported in this paper.
and the MIMO antenna system performance can be measured by
investigating multiplexing efficiency (MUX). For two-port MIMO References
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