Single antenna element

The single antenna element is shown in Fig. 1. The overall dimensions are 30 × 25 × 1.57 mm³. It is composed of a 50 Ω microstrip feedline etched on the top layer of the substrate. The bottom layer contains the ground plane as shown in Fig. 1. In order to achieve wide frequency band operation, different modifications are made on the ground plane of the initial structure, as shown in Fig. 2. Initially, the ground plane consisted of partial rectangular ground plane with dimensions 25 × 10 mm² as shown in Fig. 2. The simulated reflection coefficient in the case of a partial ground plane (antenna 1) in Fig. 3 shows that the antenna operates at 4.2 GHz, with an impedance bandwidth (IBW) of 1.7 GHz (relative bandwidth of 40.5%). Then, the ground plane is modified by extending its surface (antenna 2) as shown in Fig. 2. This new configuration allows a frequency operation from 2.6 to 6 GHz as shown in Fig. 3. Finally, further bandwidth improvement is achieved by introducing a vertical open slot with dimensions 5 × 0.5 mm² in the ground plane (antenna 3) as shown in Fig. 2 with an operating range extending from 2.4 to 6 GHz as shown in Fig. 3.

Figure 1.

Layout of the single antenna element: (a) front view and (b) back view.

Figure 2.

Geometric evolution of the single antenna element (ground plane).

Figure 3.

The evolution of the simulated reflection coefficient curve according to the modifications.

Two different filters are integrated into the antenna element. The first is used to create a notched band at the WiMAX band (3.3–3.8 GHz) by introducing a half-wavelength (λ/2) RSRR etched in the top layer of the substrate with dimensions L_r × W_r, as shown in Fig. 4(a). The second is used to filter the higher frequencies of the band (5.15–5.925 GHz) by introducing a horizontal straight slot (LPF) on the back side of the antenna in the ground plane with a length of 21 mm and a wide of 0.5 mm as shown in Fig. 4(b).

Figure 4.

Layout of the reconfigurable antenna element: (a) top view and (b) back view.

The dimensions of RSRR and LPF that correspond to the filtering at 3.5 and 5.15 GHz are calculated using equations (1) and (2), respectively:

(1)\begin{equation}{f_{notche}} = {c \over {2 \times {L_{RSRR}} \times \sqrt {{\varepsilon _{eff}}} }},\end{equation}

(2)\begin{equation}{f_{filter}} = {c \over {2 \times {L_{LPF}} \times \sqrt {{\varepsilon _{eff}}} }},\end{equation}

where ${L_{RSRR}} = 2 \times {l_r} + 2 \times {w_r} - {g_r}$ is the length of the RSRR at the desired notch frequency, ${L_{LPF}} = {w_l} + {g_l}$ is the length of the straight slot (LPF) at the desired filter frequency, and ε_eff is the effective dielectric constant of the substrate. The photograph of the realized prototype is shown in Fig. 5. Two switches S₁ and S₂ are used for frequency reconfiguration. The role of each switch is to enable/disable the filtering caused by the two mechanisms (RSRR and straight slot in the ground plane). The antenna has four different operating modes depending on the state of the two switches as shown in Table 1.

Figure 5.

Realized antenna element prototype with ideal switches (case of notched band mode).

Table 1.

Switches configuration for each mode

Operating modes of the proposed switchable single antenna element

In a first simulation phase, we modeled the switch with a conductive strip (ideal case) before proceeding with the implementation of the final prototype with PIN diodes. Both ON and OFF states are modeled in the simulation by the presence/absence of a conductive strip. The notch filter (RSRR) is activated when the switch S₁ is in the ON state. The LPF is activated when switch S₂ is in the OFF state. With this configuration, the antenna offers four different frequency operating modes summarized in Table 1. First, the initial wideband operating mode is obtained when S₁ is in the OFF state and S₂ is in the ON state (RSRR and LPF are disabled). The reflection coefficient curve shows that the working band at −10 dB is from 2.4 to 6 GHz for simulation and 2.55 to 6 GHz for measurement as shown in Fig. 6(a). Second, the notched band mode at 3.5 GHz (WiMAX band) is obtained when all switches are turned to ON state (RSRR is enabled, and LPF is disabled). From the S-parameters, which are shown in Fig. 6(b), we note that the simulated notched band is from 2.92 GHz to 4.06 GHz, whereas the measured band is from 3.1 to 3.86 GHz. The filtering property, with a good insertion loss, can be noticed at 3.49 GHz (greater than −1 dB), in simulation as well as in measurement. This notched band can be shifted by changing the electrical length of the RSRR as shown in Fig. 7(a). Third, the LPF mode is obtained when the switches (S₁ and S₂) are in the OFF state. The LPF mode gives a simulated reflection coefficient with bandwidth at −10 dB from 2.43 GHz to 4.69 GHz (relative bandwidth of 63.4%), where the measured bandwidth is from 2.68 GHz to 4.82 GHz (relative bandwidth of 58.07%) as shown in Fig. 6(c). The bandwidth can be reduced/increased by changing the electrical length of the horizontal slot as shown in Fig. 7(b). Finally, dual-band operation is obtained when S₁ is in the ON state and S₂ is in the OFF state. As shown in Fig. 6(d), the antenna has two frequency operating bands, the first from 2.4 to 2.85 GHz with a relative bandwidth of 17.7%, and the second from 3.9 to 4.7 GHz with a relative bandwidth of 17.9%. For measured results, the antenna operates in two bands of operation, 2.58–3.27 GHz (23.2%), and 3.86–4.58 GHz (17.06%). This last mode is used when WLAN and WiMAX bands are filtered simultaneously. We can note a good agreement between simulation and measurements. We only notice a small shift in the bands, which is essentially due to the quality of the manufacturing process.

Figure 6.

Measured and simulated reflection coefficient of the proposed reconfigurable single antenna element: (a) wideband mode, (b) notched band mode, (c) LPF mode, and (d) dual band mode.

Figure 7.

Parametric studies on the reflection coefficient |S₁₁|: (a) variation of W_r and (b) variation of W_l.

Two-element MIMO Antenna design

The reconfigurable MIMO antenna consists of two reconfigurable single antenna elements, arranged symmetrically relative to the y-axis and shares a common ground plane, with an edge-to-edge distance of 10 mm, as shown in Fig. 8. The MIMO antenna is fed by two 50 Ω SMA connectors. To improve the isolation between the two ports of the reconfigurable MIMO antenna, while keeping a compact structure, some modifications are introduced to the common ground plane as shown in Fig. 9, to obtain the final result. Due to the symmetry of the element S₁₁ = S₂₂ and S₁₂ = S₂₁. For the study, the mode without filtering with the largest bandwidth at −10 dB was chosen (mode wideband). We can note a clear improvement in the isolation inside the operating band of the antenna thanks to the modifications made to the ground plane. The superposed simulated scattering parameters of the MIMO antenna with and without isolation (configurations A and B) for the four operating states are presented in Fig. 10.

Figure 8.

Layout of the reconfigurable two-element MIMO antenna: (a) front view and (b) back view.

Figure 9.

|S|-parameters of the antenna for each configuration (wideband case).

Figure 10.

Simulated |S|-parameters for the four operating modes with and without ground plane modifications: (a) wideband mode, (b) notched band mode, (c) LPF mode, and (d) dual band mode.

These simulation results demonstrate that with the modifications made to the common ground plane, the minimum isolation in the operating bands has been improved by at least 6 dB for both wideband and notched band modes.

For the other two modes, the minimum isolation has increased from 11 to 19 dB. We can also see that the lowest operating frequency returns to its initial value of 2.43 GHz (single antenna element) for all operating modes, as shown in Fig. 10. The switch states for each operating mode are listed in Table 2.

Table 2.

Switches configuration for each mode of the proposed MIMO antenna

To better understand the physical operation of the antenna, a study of the surface current distributions has been carried out. Surface current distributions of the proposed reconfigurable MIMO antenna are observed in the four operating modes at 3.5 GHz (WiMAX) and 5.2 GHz (WLAN), to understand the operation of the two mechanisms of filtering (RSRR and LPF). Figure 11 shows the surface current densities for wideband mode, notched band mode, LPF mode, and dual-band mode, respectively. The surface current distributions for wideband mode (RSRR and LPF are disabled) are shown in Fig. 11(a) and (b) at 3.5 and 5.2 GHz, respectively.

For both frequencies, there is a current concentration on the feedline and the ground plane, which proves that the antenna works at these frequencies. In notched band mode at 3.5 GHz in Fig. 11(c), it can be noticed that a strong current distribution exists on the RSRR, which proves that the filtering mechanism (RSRR) at 3.5 GHz works.

We can also see that the current distribution is normally distributed between the feedline and the ground plane at 5.2 GHz, as shown in Fig. 11(d). For the LPF mode, there is a strong current concentration along the right slot in the ground plane at 5.2 GHz, as shown in Fig. 11(f). This concentration disappears at 3.5 GHz, as shown in Fig. 11(e), which proves that the horizontal slot in the ground plane filters the high frequencies of the working band (the LPF filtering mechanism is activated). Finally, in dual-band mode (RSRR and LPF are enabled), we can see a high current distribution on the RSRR in Fig. 11(g), and along the horizontal slot in Fig. 11(h), which confirms that the two filters operate simultaneously, respectively at 3.5 and 5.2 GHz. We also notice very good isolation between the two ports for all modes of operation, with very low surface current distribution on the second antenna element as shown in Fig. 11.

Figure 11.

Surface current distribution: (a) wideband mode at 3.5 GHz, (b) wideband mode at 5.2 GHz, (c) notched band mode at 3.5 GHz, (d) notched band mode at 5.2 GHz, (e) LPF mode at 3.5 GHz, (f) LPF mode at 5.2 GHz, (g) dual band mode at 3.5 GHz, and (h) dual band mode at 5.2 GHz.

Implementation with Pin diodes

Figure 12 shows the layout of the reconfigurable MIMO antenna with the integration of real switches and the bias circuit necessary for the operation of the PIN diodes. The type of diode that is used is BAR50-02 V from the manufacturer’s Infineon. To take their effect into account in the simulations its S-parameters (s2p file) downloadable from the manufacturer’s website have been inserted in the antenna with CST Microwave Studio software. For the biasing of the diodes, decoupling capacitors of 100 pF and inductors (RF chokes) of 100 nH were used. Photographs of the realized prototype with PIN diodes are shown in Fig. 13.

Figure 12.

Layout of the two-element reconfigurable MIMO antenna with PIN diodes and biasing circuit: (a) front view and (b) back view.

Figure 13.

Photographs of the realized prototype with PIN diodes and bias circuit: (a) front view and (b) back view.

Envelope Correlation Coefficient (ECC)

The ECC is used to determine the correlation between the various antenna elements in a MIMO antenna system. In practice, ECC values should be less than 0.5. It is possible to calculate it from the S-parameters [Reference Sharawi27], using equation (3) or from the far-field radiation using equation (4). The simulated and measured ECC using S-parameters of the proposed antenna is illustrated in Fig. 17. A good agreement between simulations and measurements is obtained. It is less than 0.008 in the whole operating band for the various modes which means a very low correlation between the two antenna elements. The ECC value increases at filtered frequencies (WiMAX/WLAN) while maintaining a good value as shown in Fig. 17(b–d). This indicates a good decoupling between the two ports even inside the filtered bands:

(3)\begin{equation}{\rm{ECC}} = {{{{\left| {{\rm{S}}_{11}^{\rm{*}}{{\rm{S}}_{12}} + {\rm{S}}_{21}^{\rm{*}}{{\rm{S}}_{22}}} \right|}^2}} \over {\left( {1 - \left( {{{\left| {{{\rm{S}}_{11}}} \right|}^2} + {{\left| {{{\rm{S}}_{21}}} \right|}^2}} \right)} \right)\!\left( {1 - \left( {{{\left| {{{\rm{S}}_{22}}} \right|}^2} + {{\left| {{{\rm{S}}_{12}}} \right|}^2}} \right)} \right)}},\end{equation}

(4)\begin{equation}ECC = {{{{\left| {\smallint {\smallint _{4{{\pi }}}}\left[ {\overrightarrow {{F_1}} \left( {\theta ,\varphi } \right).\overrightarrow {{F_2}} \left( {\theta ,\varphi } \right)} \right]{\rm{d}}\Omega {\ }} \right|}^2}} \over {\smallint {\smallint _{4{{\pi }}}}{{\left| {\overrightarrow {{F_1}} \left( {\theta ,\varphi } \right)} \right|}^2}{\rm{d}}\Omega \;\smallint {\smallint _{4{{\pi }}}}{{\left| {\overrightarrow {{F_2}} \left( {\theta ,\varphi } \right)} \right|}^2}{\rm{d}}\Omega {\ }}}.\end{equation}

Diversity Gain (DG)

Figure 17 shows simulated and measured DG for the four operating modes. It can be calculated using the following equation [Reference Kumar, Ansari, Kanaujia, Kishor and Kumar17]:

(5)\begin{equation}{\textrm{DG}} = 10\sqrt {1 - {{\left( {0.99{\textrm{ECC}}} \right)}^2}} .\end{equation}

It can be observed that the DG of the MIMO antenna is greater than 9.9 for the four modes of operation. This value changes only in the filtering bands related to the WiMAX and WLAN bands as shown in Fig. 17(b–d). These results validate the good diversity performances of the MIMO structure. Table 4 gives the simulated and measured ECC and DG for the different operating modes of the antenna.

Table 4.

Simulated and measured values of ECC and DG for each mode