Antenna array design

The proposed geometry of 1 × 2 B-shaped antenna array is shown in Fig. 1. The design consists of a modified rectangular patch antenna on one side of the substrate and finite ground plane on the other side. The antenna is fabricated on FR4 substrate (ε_r = 4.4, loss tangent 0.02, thickness 0.02λ_o, where λ_o denotes the free space wavelength at 4 GHz). The antenna array is fed by the microstrip line with 50 Ω characteristics impedance. The microstrip line terminates at T-shaped impedance-matching microstrip line [Reference Pozar34] whose values have been optimized in the simulator. The inset feed line and the round edges have been used for wide band impedance matching. Two annular-shaped strip lines have been attached on the rectangular patch to obtain dual-band and wide band characteristics. Simulations have been carried out in CST Microwave Studio 16 [35]. The dimensions of the proposed antenna array are illustrated in Table 1.

Fig. 1.

Geometry of the proposed B-shaped antenna array.

Table 1.

Dimensions of the proposed B-shaped antenna array

Parametric study

The impact of the B-shaped patch antenna has been further analyzed in this section. The length of parameter W ₄ has been varied and the reflection coefficient is shown in Fig. 2. Reflection coefficient plotted on Smith chart for W ₄ = 1.5, 2.5, and 3.5 mm is illustrated in Fig. 2(a) and its corresponding value in dB is represented in Fig. 2(b). Best results have been achieved at an optimum length W ₄ = 2.5 mm. Further increase or decrease in parameter length results in an impedance mismatch.

Fig. 2.

Effect of change in the parameter W ₄ on the reflection coefficient (a) plotted on Smith chart (b) plotted in dB.

Theory of characteristics mode (TCM) describes the current patterns and existence of the resonant modes in the B-shaped antenna. TCM can be obtained by solving the eigenvalue equation [Reference Bouezzeddine and Schroeder36, Reference Chen and Wang37]

(1)$${\boldsymbol X}{\boldsymbol J}_n = \lambda _n{\boldsymbol R}{\boldsymbol J}_n, \;$$

where J_n represents characteristic currents, λ _n is the eigenvalue, X and R represent the imaginary and real part of the generalized impedance.

A large quantity of modes can be calculated at any operating frequency. However, modes close to the operating frequency represent significant importance and calculated by the n modal significance (MS) equation:

(2)$$MS = \displaystyle{1 \over {\vert {1 + j\lambda_n} \vert }}.$$

A mode resonates when its eigenvalue (λ _n) is zero and MS equals to 1. TCM analysis of B-shaped unit cell has been performed in FEKO [38] and illustrated in Fig. 3. Surface current flows along the edges of the B-shaped unit cell in case of mode 1, mode 2, mode 3, and mode 4, respectively, as shown in Figs 3(a)–3(d). However, a strong surface current has been observed along the two annular-strip lines in case of mode 5 as illustrated in Fig. 3(e). The eigenvalue of the first five resonant modes of the B-shaped antenna unit cell is presented in Fig. 4. A mode resonates when its eigenvalue is zero [Reference Hassan, Zahid, Khan and Maqsood39]. For the proposed B-shaped structure, mode 1 and mode 2 resonate at 3.9 and 4.1 GHz, respectively. Mode 3, mode 4, and mode 5 resonate at 7.7, 7.8, and 8 GHz.

Fig. 3.

Surface current density of B-shaped antenna unit-cell resonance at (a) mode 1, (b) mode 2, (c) mode 3, (d) mode 4, and (e) mode 5.

Fig. 4.

Eigenvalue of the proposed B-shaped antenna unit cell.

The proposed antenna array has been fabricated and the simulated results have been compared with the measured results as shown in Fig. 5. The measured results are in good agreement with the simulated results. The measured impedance bandwidth reference −10 dB is observed at 3.84–4.16 GHz (320 MHz) and 7.78–8.38 GHz (600 MHz), respectively.

Fig. 5.

Reflection coefficient of the proposed B-shaped antenna array.

Surface current distribution of the B-shaped antenna array has been performed as shown in Fig. 6. At 4 GHz, maximum surface current flows along some portion of annular-shaped strip lines and T-shaped matching network as shown in Fig. 6(a). Strong surface current flows in the B-shaped antenna array at 8 GHz as depicted in Fig. 6(b).

Fig. 6.

Surface current distribution at (a) 4 GHz, (b) 8 GHz.

The simulated three-dimensional radiation patterns of the proposed B-shaped antenna array have been displayed in Fig. 7. The peak gain at 4 GHz is 9.17 dB as depicted in Fig. 7(a), whereas the peak gain at 8 GHz is 9.73 dB as shown in Fig. 7(b). The fabricated prototype of 1 × 2 B-shaped antenna array is shown in Fig. 8.

Fig. 7.

3D radiation pattern of the proposed B-shaped antenna array at (a) 4 GHz, (b) 8 GHz.

Fig. 8.

Prototype of fabricated 1 × 2 B-shaped antenna array.

The radiation pattern of the proposed antenna array has been measured in the anechoic chamber with dimensions 9 × 4.5 × 4.9 m³. The measured radiation patterns and the simulated radiation patterns of B-shaped antenna array have been compared in Fig. 9. E-plane radiation pattern is directional whereas the H-plane radiation pattern is omni-directional. At 4 GHz, peak gain 8.3 dB occurs at 0°, whereas minimum gain −29.5 dB occurs at 240° as shown in Fig. 9(a). At 8 GHz, peak gain 9.4 dB exists at 0°, whereas minimum gain −27.1 dB exists at 160° as depicted in Fig. 9(c).

Fig. 9.

Radiation pattern of B-shape antenna array: (a) E-plane at 4 GHz, (b) H-plane at 4 GHz, (c) E-plane at 8 GHz, and (d) H-plane at 8 GHz ( simulated, ––– measured).

Peak gain and radiation efficiency of the proposed antenna array are shown in Fig. 10. Measured peak gain ranges 7.5–8.3 and 7.8–9.4 dB at 3.7–4.2 GHz and 7.7–8.4 GHz, respectively. The difference between the measured and simulated results is due to the fabrication tolerance. Measured radiation efficiency ranges 77.3–82.5% in the operating bands.

Fig. 10.

Gain and efficiency of the proposed antenna array.

The comparison of the proposed B-shaped antenna array with the existed literature has been reported in Table 2, where λ_o denotes free space wavelength at the lowest frequency. The table shows that the proposed antenna array is compact and possesses better gain than [Reference Yu and Lim24, Reference Prakash and Srinivasan25, Reference Dewan, Rahim, Malek, Ausordin, Yusuf and Azemi32]. Peak efficiency of the proposed antenna is comparable with [Reference Yang, Qin, Liu, Yin and Guo27]. Moreover, the impedance bandwidth (reference −10 dB) is better than [Reference Yu and Lim24, Reference Prakash and Srinivasan25, Reference Wang, Zhang, Yin and Wu30, Reference Dewan, Rahim, Malek, Ausordin, Yusuf and Azemi32], respectively.

Table 2.

Comparison with recently published dual-band antenna array