All the simulations in this work are conducted using Ansys HFSS, full wave simulator. The simulated S-parameter comparison of the monopole antenna, monopole antenna with AMC, monopole antenna with AMC and superstrate is shown in Fig. 6.
The comparison of the realized gain for monopole antenna with AMC and superstrate is shown in Fig. 7. It can be observed that there is 3.84 dBi increment in gain after applying AMC on the monopole antenna. Furthermore, when the superstrate is applied, the gain of the antenna is 5.5 dBi at 3.5 GHz.
A. Parametric analysis of the antenna
The proposed antenna is fed by CPW as shown in Fig. 1. The parametric studies of the dimensions of the monopole antenna, gap between antenna and superstrate has been carried out to achieve good impedance matching and desired frequency band.
i) Length of monopole antenna, l
The length of the monopole antenna, l, is varied from 20 mm to 30 mm with the step size of 5 mm. As shown in Fig. 8 (a) the reflection coefficient \({S}_{11}\)is lower than − 10 dB for desired frequency bands. It can also be observed that the S11 resonance moves to lower frequency as the length of the monopole increases.
ii) Gap between the antenna and superstrate
The gap between antenna and superstrate is varied from 4.2 mm to 10.2 mm with the step size of 3 mm. It can be seen from Fig. 8. (b) that there is hardly any effect of varying the gap on the antenna performance. In this way, the antenna will give almost the same results even if the gap changes due to slight mishandling of the antenna.
B. Near field analysis
In order to get insights of working of the superstrate, near field behavior is very helpful [25]. Far field pattern being the Fourier transform of the near-field pattern, can give better understanding of how the superstrate is changing the near field that will eventually change the far field characteristics. To accomplish it, a vertical plane in the center of the antenna is placed on which the dominant component of the near field is studied. In our case, Ex is the dominant electric field as the monopole is aligned along the x-axis. In Fig. 9 and Fig. 10, Ex magnitude and phase behavior in absence and presence of the superstrate are shown. It is evident from Fig. 9 (a) and Fig. 10 (a) that the placement of the superstrate confines the fields and hence shows slightly higher field magnitude in case of the superstrate. It is also evident from Fig. 9 (b) and Fig. 10 (b) that due to placement of the superstrate, the direction of equi-phase front changes. The phase front is changed towards left, signifying that the outward radiation is expected towards left side when the superstrate is placed. From these figures, it is easy to understand that the absence or presence of the superstrate decides the beam steering direction.
After getting the desired radiation properties, the prototype of the antenna is fabricated as shown in Fig. 5. The proposed antenna is tested using R & S ZNB40 vector network analyzer and anechoic chamber for its properties.
C. S-parameter measurement
The simulated and measured S-parameters of the antenna are shown in Fig. 11. For acceptable performance, the proposed antenna needs to provide good reflection coefficients, |S11|, in the desired frequency band. From Fig. 11, it is evident that the fabricated antenna has |S11| < -10 dB over the frequency band of 3.1–3.7 GHz. The simulated and measured S-parameters are in good agreement. The fabricated antenna has wide impedance bandwidth for 5G/WiMAX/WLAN applications.
D. Radiation pattern of the proposed antenna
Figure 12 shows the radiation pattern of the proposed antenna for φ = 0° cut. The non-uniform superstrate was designed to steer the beam in θ=±18°. The superstrate needs to be mechanically rotated or removed depending upon the desired steering beam direction. While, currently, we manually did the operation of mechanically placing and removing the superstrate but the superstrate can be moved by mechanical motor to automate this operation. This type of mechanical movement of a superstrate is simple and can be easily accomplished by mechanical motors.
It can be seen from Fig. 12 that the beam scans in ± 18° and 0° in elevation directions in presence of superstrate and in absence respectively. It is also interesting to note that the antenna does not show sidelobes in this plane. The realized gains at these steered angles are 5.5 dB signifying that the antenna shows almost zero scan loss for ± 18° scanning angles.
The proposed antenna radiation patterns (co- and cross-polarization) are measured in an anechoic chamber at 3.5 GHz as shown in Figs. 13 and 14. In these figures, co-polarization is shown by red lines and cross-polarization by black lines. It is evident from Figs. 13 and 14, that the antenna has very low cross-polarization components. The simulated and measured results are also in good agreement.
E. Realized gain
Figure 15 shows the simulated and measured gain of the antenna with and without superstrate. It is observed that the gain of the antenna in both cases is 5.5 dBi at 3.5 GHz. The simulated and measured values are also in good agreement.
Comparison table
In Table 2, all existing designs are compared with the current work. It is evident from Table 2 that the proposed antenna gives a good beam steering with compact design.
Table 2
Comparison of present work with the previously published antennas
Ref
|
Antenna type
|
Size (\({{\lambda }^{3}}_{0}\))
|
Frequency band (GHz)
|
Realized gain (dBi)
|
Beam scanning
(Degrees)
|
[11]
|
Phased array and reconfigurable PRS structure
|
\(3.1\times 3.1\times 0.55\)
|
5.5 to 5.7
|
12
|
\(\pm {15}^{o}\)
|
[14]
|
Parasite patches and lumped capacitances
|
\(1.56\times 0.63\times 0.013\)
|
2.45
|
5–8
|
\(\pm {15}^{o}\)
|
[15]
|
Parasitic phased array antennas
|
\(0.30\times 0.78\times 0.005\)
|
1
|
7.4
|
\(\pm {15}^{o}\)
|
[16]
|
Tunable parasitic
|
\(0.49\times 0.49\times 0.0013\)
|
2.43–2.47
|
3.36
|
\(\pm {40}^{o}\)
|
This Work
|
Metasurface monopole Antenna
|
\(0.58\times 0.58\times 0.16\)
|
3.1–3.7
|
5.53
|
\(\pm {18}^{o}\)
|