a compact 8-element mimo antenna system for 802.11ac wlan applications.pdf

8/18/2019 A Compact 8-Element MIMO Antenna System for 802.11ac WLAN Applications.pdf

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MIMO ANTENNA DESIGN

Initially a patch antenna was designed using HFSSTM

. The substrate used in the design was FR4 with a dielectric

constant of 4.4 and thickness of 0.8 mm. The dimensions chosen for the patch were 11 x 8 mm2. These dimensions

were chosen keeping in view that when eight such elements were to be used in the MIMO antenna, they fit well in anarea of 100 x 50 mm2 (standard mobile device board size). The patch antenna was excited by a microstrip feed line with

inset feed to match the antenna with 50Ω. From the simulations, the antenna was found to be resonating at 6.52 GHz.

To lower the resonant frequency of the patch antenna and tune it to 5 GHz, a CSRR was etched out underneath the

center of the patch. The CSRR is a metamaterial structure which is a negative image of split-ring resonator (SRR). ASRR is also a metamaterial element which is made up of two concentric conducting rings. The inner ring resides inside

the outer ring with spacing and both rings have slits that are oriented opposite to one another. The CSRR is made by

cutting from a conducting sheet (usually ground plane) the shape of an SRR. The CSRR is a resonant structure and behaves as an LC tank circuit [7]. It interacts with the axial electric field and exhibits a negative permittivity around its

frequency of resonance. Antenna miniaturization of a patch antenna in single antenna application has been

demonstrated by loading a patch antenna with the CSRR [8],[9].

The resonant frequency of the patch antenna loaded with the CSRR was related to the dimensions of the CSRR. These

dimensions included the radius of the outer ring of the CSRR ‘r ’, the width of each ring ‘w’, the spacing between therings ‘ s’ and the width of the slit in the rings ‘d ’. Due to the anisotropic nature of the CSRR, its orientation underneath

the patch also affects the resonant frequency of the patch antenna. In the design, the CSRR was oriented in such a way

that the slits of rings were facing the radiating edges of the patch antenna. The slit of the outer ring was facing the edgeof the patch that was excited by the microstrip line. The dimensions of the CSRR were varied to tune the patch antenna

at 5 GHz. The patch antenna resonated at 5 GHz with the CSRR dimensions shown in Table I. The microstrip feed line

was shifted 1.5mm along the width of the patch antenna from its central axis for proper mode excitation.

Once the design of single patch was finalized, eight similar patches were placed together on the same FR-4 substrate to

make an eight element MIMO antenna system. The geometry of the MIMO antenna system is shown in Fig. 1. The topside of the MIMO antenna is shown in Fig. 1(a) where eight elements are placed together with a spacing of 5 mm.

Fig. 1(b) shows the bottom side of the MIMO antenna where the CSRR’s are etched out underneath each element.

Table I: CSRR DIMENSIONS FOR THE PROPOSED ANTENNA

Radius of the outer ring r 2.5 mm

Width of the rings w 0.25 mm

Spacing Between the Rings s 0.5 mmWidth of the slit d 0.5 mm

Fig. 1 Geometry of the 8-element MIMO antenna

system, (a) Top side, (b) Bottom side

Fig. 2 Fabricated 8-element MIMO antenna system, (a)

Top side, (b) Bottom side

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RESULTS AND DISCUSSION

The MIMO antenna was designed and its operation verified through simulations in HFSSTM

. It was then fabricated.

Fig. 2 shows the top and bottom side of the fabricated antenna. The scattering parameters of the fabricated MIMO

antenna were measured using an Agilent HP8510 network analyzer. Fig. 3(a) shows the simulated reflection coefficientcurves. The simulated reflection coefficient of the MIMO antenna showed a minimum -6 dB (VSWR < 3) bandwidth of

170 MHz which covered 4.94 – 5.11 GHz.

Isolation between the antenna elements of a MIMO antenna system is an important parameter. The diversity performance of a MIMO antenna system is directly related to the number of uncorrelated antenna elements. The

isolation curves between the antenna elements of the proposed MIMO antenna system found from simulation are shownin Fig. 3(b). Only the isolation between antenna element 1 with the rest of the antenna elements is shown. Due to

symmetry, the isolation for all other combinations of antenna elements follows one of the curves shown in Fig. 3(b). A

minimum isolation of 10.95 dB was achieved between antenna elements whose radiating edges were facing each other.

For all other combinations, the isolation was more than 20 dBs.

Once the antenna was fabricated, its measured scattering parameters were compared with the simulation results.Fig. 4(a) shows the measured reflection coefficient curves of the MIMO antenna elements. Compared with the

simulation results, a shift in the resonant frequency of the antenna elements was observed accompanied by a lower

bandwidth. The antenna elements resonated between 5 – 5.15 GHz with a minimum -6 dB (VSWR < 3) bandwidth of

80 MHz. This deviation of measurement results from simulation results were attributed to the difference in the material properties of the substrate defined in the simulation from the one used for fabrication.

4.85 4.9 4.95 5 5.05 5.1 5.15 5.2 5.25 5.3 5.35-30

-25

-20

-15

-10

-5

0

Frequency (GHz)

R e f l e c t i o n

C o e f f i c i e n t ( d B )

S11

S22

S33

S44

S55

S66

S77

S88

(a)

4.85 4.9 4.95 5 5.05 5.1 5.15 5.2 5.25 5.3 5.35-70

-60

-50

-40

-30

-20

-10

0

Frequency (GHz)

I s o l a t i o n ( d B )

S12

S13

S14

S15

S16

S17

S18

(b)

Fig.3 S-parameters of the MIMO antenna system obtained by simulation, (a) Reflection coefficient, (b) Isolation

4.85 4.9 4.95 5 5.05 5.1 5.15 5.2 5.25 5.3 5.35-30

-25

-20

-15

-10

-5

0

Frequency (GHz)

R e f l e c t i o n

C o e f f i c i e n t ( d B )

S11

S22

S33

S44

S55

S66

S77

S88

(a)

4.85 4.9 4.95 5 5.05 5.1 5.15 5.2 5.25 5.3 5.35-70

-60

-50

-40

-30

-20

-10

0

Frequency (GHz)

I s o l a t i o n ( d B )

S12

S13

S14

S15

S16

S17

S18

(b)

Fig.4 Measured S-parameters of the MIMO antenna system (a) Reflection coefficient, (b) Isolation

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a compact 8-element mimo antenna system for 802.11ac wlan applications.pdf

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