The dimensions of the inverted U-shaped slot for the desired operating frequencies 2.5/3.5/5.5 GHz predicted by the ANN structure are depicted in Table 5:
Table 5
Dimensions | ANN values |
X1 | 0.60 mm |
Y1 | 13.19 mm |
X2 | 11.69 mm |
Y2 | 0.70 mm |
The geometric evolution of the presented antenna is illustrated in Fig. 4. From Fig. 5 (curve ANT-1) we can observe that, for the first antenna geometry Fig. 4(a), which starts with a conventional rectangular patch and a coplanar ground plane, two resonant frequencies are obtained at approximately 3.46 and 5.49 GHz, with a reflection coefficient of -24 dB and − 27 dB respectively. By including an inverted U-shaped slot on the radiating patch, as illustrated in antenna2 Fig. 4(b), and by predicting its optimized dimensions using the ANN technique (Table 5), we can observe three resonant modes and two distinct wide bands centered at approximately 2.51, 3.51, and 5.50 GHz, with a good reflection coefficient almost equal to -38 dB, -32 dB, and − 32.5 dB, respectively, as seen in Fig. 5 (curve ANT-2).
Therefore, the proposed antenna covers all the 2.4/5.2/5.8 GHz WLAN and 2.5/3.5/5.5 GHz WIMAX bands.
For better comprehension of antenna performance, various antenna parameters needed to be determined. For this reason, we studied and discussed the reflection coefficient, the radiation pattern, the VSWR, the surface current distribution, the gain, and the radiation efficiency of the proposed antenna, which was simulated based on the initial antenna dimensions presented in Table 1 and the predicted ANN values of the dimensions [X1, Y1, X2, Y2] given in Table 5 using both CST and HFSS electromagnetic simulators.
From the − 10 dB bandwidth simulated results illustrated in Fig. 6, it is clear that the antenna presents three resonant frequencies and two wide operating bands. The lower band has an impedance bandwidth of 140 MHz (2.40–2.58 GHz) centered at 2.51 GHz, while the impedance bandwidth for the upper band is 3260 MHz (3.00-6.41 GHz), which presents two resonant frequencies of 3.51 GHz and 5. 50 GHz. Using a Rohde & Schwarz ZVB20 vector network analyzer, we measured the reflection coefficient of the fabricated prototype antenna (Fig. 7), and the measured results showed that the antenna has |S11| ≤ − 10 dB in the frequency bands of 2.31–2.43 GHz, 3.10–3.44 GHz, and 3.94–5.88 GHz (Fig. 6). These results prove that the proposed antenna can operate over 2.4/5.2/5.8 GHz WLAN and 2.5/3.5/5.5 GHz WIMAX bands. The measured and simulated S11 are not exactly the same, this can be due to the fabrication process, manual soldering of SMA connector, and measurement circumstances.
To verify the radiation pattern characteristics, we measured the radiation pattern using the Geozondas principle as shown in Fig .8. In Fig. 9(a), Fig. 9(b), and Fig. 9(c) the measured and simulated far-field radiation patterns of the proposed antenna in the elevation xoz plane (E-plane) and the azimuthal xoy plane (H-plane) for the desired operating frequencies of 2.50 GHz, 3.50 GHz, and 5.50 GHz, are given respectively. From these figures, we note that over the three operating frequencies, the antenna presents an omnidirectional radiation pattern in the xoy plane, while it has a bidirectional pattern in the xoz plane. An excellent agreement between measured and simulated results is also noted.
To evaluate the impedance matching between the transmission line and the antenna, we used the VSWR parameter, which is a function of the reflection coefficient. A good antenna is defined by an acceptable value of VSWR (1 ≤ VSWR ≤ 2). The VSWR of the designed CPW antenna was simulated and plotted in Fig. 10. The VSWR of the proposed antenna is approximately 1.20, 1.04, and 1.05 at the resonant frequencies 2.50 GHz, 3.50 GHz, and 5.50 GHz, respectively, which indicates good matching.
In order to demonstrate the contribution of the embedded slots in the radiating element patch to the resonant frequencies, we present in Fig. 11 the proposed antennas simulated surface current distribution at 2.50, 3.50, and 5.50 GHz. From Fig. 11(a), it is clear that most of the surface currents are concentrated around the inverted U-shaped slot. This result indicates that the embedded slot is the essential contributor to the occurrence of the lower resonant mode at 2.50 GHz. It is observed from Fig. 11(b) and Fig. 11(c), that the surface current distribution is slightly around the inner and the outside of the slot, which leads with the help of the ANN technique to match perfectly the second and third resonant mode. Thus, from the surface current distribution, we can conclude that the embedded slot is important in creating the proposed antenna resonance and generating two wide bands covering the 2.4/5.2/5.8 GHz WLAN and 2.5/3.5/5.5 GHz WiMAX bands.
The antenna gain and the radiation efficiency across the operating frequency bands were also verified and plotted. As shown in Fig. 12, for the first operating band (2.4–2.69 GHz), the antenna has a peak value of 1.35 dBi at 2.47 GHz, and a maximum gain of 2.15 dBi in the middle band (3.403.69 GHz) at 3.69 GHz, whereas a gain of 3.17 dBi at 5.85 GHz is obtained in the upper operating band (5.15–5.85 GHz). The radiation efficiency varies approximately from 70–87%.
All the simulated results of the presented CPW fed microstrip patch antenna are summarized in Table 6:
It can be seen from Table 6 that for the presented antenna, fundamental parameters like the reflection coefficient, VSWR, gain, and radiation efficiency show reasonable characteristics over the 2.4/5.2/5.8 GHz WLAN bands and 2.5/3.5/5.5 GHz WIMAX bands. The antenna is perfectly matched with the transmission line, because the VSWR is between 1 and 2. These results also show that the proposed CPW antenna exhibits an acceptable gain value and radiation efficiency, which indicates a minimum loss within the structure of the antenna and at the input terminals. Hence, this antenna is recommended for the WLAN/WIMAX communication systems.
Table 6
Simulated results of the designed antenna.
Antenna performance | | WLAN | | | WIMAX |
2.4 | 5.2 | 5.8 | 2.5 | 3.5 | 5.5 |
Reflection coefficient (dB) | -10 | -22.65 | -20.21 | -21.87 | -31.98 | -32.43 |
VSWR | 1.93 | 1.15 | 1.22 | 1.20 | 1.04 | 1.05 |
Gain (dBi) | 1.23 | 2.78 | 3.13 | 1.01 | 1.85 | 3.00 |
Radiation efficiency (%) | 74.21 | 86.99 | 85.88 | 69.07 | 82.78 | 86.79 |
To support the simulated results, we present in Table 7a comparison between the obtained results and the results found in the literature.
Table 7
Comparison of alternative WLAN/WIMAX antennas.
References | Operating bands [GHz] | Gain [dBi] | Size [mm × mm ] |
[1] | 2.402.52, 3.403.60, 5.06.0 | 1.62, 1.47, 2.96 | 10 × 26 |
[2] | 2.392.69, 3.43.85, 4.557.85 | 2, 3, 2.5 | 15 × 15 |
[5] | 2.0-2.76, 3.04-4.0, 5.2-6.0 | 1.5, 1.7, 3.05 | 19 × 25 |
[12] | 2.25-4.00 | 1.5, 2 | 14.8 × 29.6 |
[33] | 2.40 -2.4835, 3.40–3.61 | 7, 7.4 | 60 × 45 |
[34] | 2.36–2.70, 3.35–3.74, 5.01–6.12 | 0.94, 1.45, 2.61 | 32 × 12 |
[35] | 2.20–2.52, 3.30–4.20 | 1.4, 1.6 | 14.75 × 26 |
Proposed | 2.40–2.58, 3.00-6.41 | 1.01, 1.85, 3.00 | 24 × 33 |
Considering the covered bandwidths, gain, and antenna size, we conclude that compared with the references listed in Table 7, the designed CPW antenna has a simpler structure that can be fabricated easily, and can cover all the frequency bands of WLAN/WIMAX applications.