3.1 Synthesis of Au NPs

Au NPs were synthesized following a method proposed by Turkevich et al. (1951). Chloroauric acid (HAuCl₄) of 5 mM is prepared in deionized (DI) water and used as stock solution. 2 mL of this solution is added to 50 mL of DI water and then heated under stirring until it reaches its boiling point. 2.8 mL of 25 mM Trisodium Citrate Dehydrate (Na₃C₆H₅O₇) in DI water is then added into the above mixture and boiled further. When the color of the solution gradually changes from yellowish to reddish, it is kept in an ice bath and allowed to stabilize to get Au NPs.

Au ions through the dissociation of HAuCl₄ are reduced using trisodium citrate as the reducing agent. The thermal agitation through heating results in the nucleation of Au NPs which increase in size through Oswald ripening. When the desired size is reached the reaction is quenched using an ice bath.

3.2 Synthesis of Ag NPs

For synthesis of Ag nanospheres, a similar method as described in subsection 3.1 for synthesis of Au NPs is adopted where the salt is Silver Nitrate (AgNO₃) and the reducing agent is Na₃C₆H₅O₇. After the synthesis of both the nanomaterials, the particles are deposited over the slot using drop and dry or drop casting method (Kaliyaraj et al., 2020). For the fabrication of the Au NPs based antenna firstly, 200 µL of the synthesized material is dropped over the slot area and dried at 100 °C in a hot plate. The parameters of the designed antenna are then measured and analyzed. The same procedure is repeated again by applying three droppings of the same material. For the fabrication of the Ag-Au-Ag NPs based antenna, alternate droppings of Ag NPs, AuNPs and then again Ag NPs of 200 µL each is first dropped over the slot and dried. After analysis of the antenna performance parameters, the process is repeated with three droppings of alternate Ag-Au-Ag NPs. The structure of the fabricated antennas with Au NPs and Ag-Au-Ag NPs are shown in Fig. 2(a) and (b), respectively.

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Fig. 2

Structure of fabricated (a) Au NPs deposited antenna and (b) Ag-Au-Ag NPs deposited antenna.

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5.1 Return loss & bandwidth analysis

Fig. 5(a) shows S₁₁ parameters of the fabricated conventional slotted MSA and Au NPs deposited antenna for nanofilm thickness of 88 nm and 264 nm. The S₁₁ measurements are done using using ZNB20 Vector Network Analyzer.

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Fig. 5

(a) Comparison of S11 parameters of conventional MSA & Au NPs based MSA for nanofilm thickness of 88 nm & 264 nm (b) Comparison of S11 parameters of conventional MSA & Ag-Au-Ag NPs based MSA for nanofilm thickness of 110 nm & 330 nm.

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From the figure it can be seen that the conventional slotted MSA resonates at 5.8 GHz and so does the nanomaterial deposited antenna for both 88 nm and 264 nm film thickness. During the theoretical analysis the antenna was designed to operate at a frequency of 4 GHz. The observed shift in operating frequency in the fabricated antenna is due to the introduction of the slot in the original rectangular patch antenna which actually decreases the electrical length of all the three antennas. The return loss and the bandwidth for the Au NPs deposited antenna with 88 nm film thickness is found to be −15 dB and 0.10 GHz respectively. However, with a film thickness of 264 nm the same antenna shows a return loss of −31 dB which is quite low compared to the conventional slotted MSA which shows a return loss of only −20.5 dB. Minimized return loss signifies improvement in impedance matching in the antenna design. This antenna also shows a bandwidth improvement of 12.5 %. The bandwidth is found to be 0.18 GHz which is better than 0.16 GHz as achieved by the conventional slotted MSA. Fig. 5(b) shows S₁₁ parameters of the fabricated conventional slotted MSA and alternately layered Ag-Au-Ag NPs antenna with the two different nanofilm thickness. It can be seen from the figure that both the antennas with 110 nm film thickness and 330 nm film thickness resonates at 5.81 GHz and shows a return loss of −25.5 dB and −30 dB, respectively. Further, both antennas show an improvement of 18.75 % in the bandwidth compared to the conventional slotted MSA. It can be found in all the cases that compared to the conventional slotted MSA the nanomaterial-based antennas show a considerable improvement in return loss and bandwidth specifically with three droppings of deposition (∼264 nm for Au NPs and ∼330 nm for Ag-Au-Ag NPs). Also, all the antennas provide a decrease in the electrical length thereby contributing towards physical size reduction of 32.09 % (Au NPs based MSA) and 31.15 % (Ag-Au-Ag based MSA). Since all the fabricated antennas resonates at the 5.8 GHz to 5.9 GHz range they are well suited for WLAN applications. Table 3 highlights the measurement parameters of this section.

5.2 Radiation pattern & gain analysis

As discussed in subsection 5.1, the return loss for both the NPs deposited antennas show better results for three droppings (∼264 nm for Au NPs and ∼330 nm for Ag-Au-Ag NPs) compared to single dropping of material, and hence in this section the radiation pattern and gain is analyzed only for antennas having thicker nanofilms. The radiation pattern plots for the proposed antennas are illustrated in Fig. 6. These measurements are done using Antenna Radiation Measurement System from Diamond Engineer, US. Fig. 6 (a) and 6 (b) shows the comparison of 2D radiation pattern in the elevation plane and azimuth plane for the conventional slotted MSA and the Au NPs deposited antenna for nanofilm thickness of 264 nm, respectively.

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Fig. 6

(a) Comparison of 2D radiation pattern of conventional slotted MSA & Au NPs in both elevation and azimuth planes for 264 nm film thickness (b) Comparison of 2D radiation pattern of conventional slotted MSA & Ag-Au-Ag NPs in both elevation and azimuth planes for 330 nm film thickness.

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It can be seen from the pattern that a slight improvement in gain (about 0.5 dB) is observed in both elevation and azimuth plane for the Au NPs deposited antenna. Fig. 6(c) and (d) shows the comparison of 2D radiation pattern in the elevation plane and azimuth plane respectively, for the conventional slotted MSA and the alternate Ag-Au-Ag NPs deposited antenna. From both the figures, the nanomaterial-based antenna is found to have a considerable improvement in the pattern with a 2 dB improvement in gain for both elevation and azimuth planes. The cross-polarized components for both the nanomaterial-based antennas is also found to be less compared to the co-polarized components which signifies improvement in the pattern. The radiation pattern exhibits a wide beam in the azimuth plane with minimized back lobe.

The current distribution in the MSA with a rectangular slot is along the edges of the antenna boundary and the slot area. This introduces an inductive effect in the copper patch area and a capacitive effect in the slot region. This capacitance in the slot region shifts the resonating frequency of the original MSA without slot from 4 GHz to 5.8 GHz in the slotted MSA. Thus, it can be said that the shift in frequency decreases the electrical length thereby causing size reduction of the original patch. Now, when Au NPs are dropped into the slot, it forms thin layers of NPs over the slot. These NPs have extremely huge relative surface area which provides high contact area for reactivity with surrounding materials. As such, the presence of the conductive Au NPs layer provided the necessary contact between the slot and the copper patch with increased surface area for better impedance matching and reduced S₁₁ value compared to the conventional slotted MSA. However, the gain in this antenna is almost similar to the conventional slotted MSA with only 0.5 dB enhancement. In case of the Ag-Au-Ag NPs based MSA, considerable gain enhancement can be seen along with return loss and bandwidth enhancement. This is because of the introduction of two different types of nanomaterials in the slot area. The particle size of Au NPs is 22 nm which is larger compared to Ag NPs whose particle size is only 11 nm. Smaller nanoparticles tend to have higher reactivity due to a high surface area to volume ratio, whereas larger particles may act more as reinforcement. Surface contact between neighboring Au NPs will create pathways through which thermal radiation and electrical current gets enhanced causing increased radiation. From circuits point of view also, the presence of alternate layers of Ag and Au in the later antenna introduces a capacitance in between the two different materials at high frequency which also had a positive effect on the performance of the antenna. However, for both the proposed antennas, an interesting observation has been made that the performance saturates at three droppings of nanomaterial (∼264 nm for Au NPs and ∼330 nm for Ag-Au-Ag NPs) and no further improvement was observed in the impedance matching beyond those thicknesses. This is because the thin film achieves a level of smoothness and beyond that the surface morphology simply repeats. The performance with nanofilm thickness of ∼88 nm for Au NPs based MSA and a ∼110 nm for Ag-Au-Ag based antenna is satisfactory without much improvement because of the fact that necessary electrical contact between the copper patch and the nanofilm is not obtained in the slot depth of 0.05 mm with those thicknesses. Thus, it can therefore be concluded that return loss, bandwidth and gain of a slotted MSA can be enhanced through deposition of metallic nanoparticulate films of significant size and thickness to create necessary electrical conductivity in the antenna. Moreover, the proposed designs are also cost effective in the sense that the nanoparticles synthesized were of very low concentration of reactants, and hence, the expenditure involved for the nano-coating is very nominal.