Optical and electrical analysis of saltwater. Figure 3 depicts the measured conductivity and optical transparency of saltwater at various salinity levels ranging from 35 to 200 parts per thousand (ppt). The measured conductivity is carried out using a portable electrical conductivity meter, while a UV/VIS spectrophotometer connected to a computer is used to measure the optical transparency. It should be noted that optical transparency in Fig. 3 is the average value in the visible band.
As illustrated in Fig. 3, when the salinity rises, the conductivity rises rapidly while the optical transparency falls slightly. At room temperature, the maximum salinity level is 263 ppt. With salinity of 200 ppt, saltwater has efficient conductivity of 20 S/m and very high optical transparency of 91.5%. Therefore, to reduce the ohmic loss of the antenna, we use saltwater with salinity of 200 ppt in the antenna design.
Single feeding port transparent liquid antenna. A transparent liquid antenna usually has low radiation efficiency compared to a typical metal antenna due to its higher ohmic loss. Therefore, it is important to understand the mechanism and optimize the radiation efficiency of the liquid antenna. The radiation of an antenna can be defined as Eq. (1).
\(\eta =\frac{{R}_{rad}}{{R}_{rad}+{R}_{loss}}\),
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(1)
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where \({R}_{rad}\) and \({R}_{loss}\) are correspondingly the radiation resistance and loss resistance of the antenna. Because the antenna is a monopole type, \({R}_{rad}=73 \varOmega\) [10]. The loss resistance can be determined from the conductivity of saltwater (\(\sigma\)) and from the dimension of the antenna, as expressed by Eq. (2).
\({R}_{loss}=\frac{L}{\sigma Wt}\),
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(2)
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where \(L, W, \text{a}\text{n}\text{d} t\) are the length, width, and thickness of the antenna, respectively.
Eqs. (1) and (2) show that the radiation efficiency of the saltwater antenna can be enhanced when the conductivity of the saltwater is increased. As reported in a previous study by the authors [6], under certain temperature and pressure conditions, the conductivity of saltwater increases with an increase in the salinity level. However, the salinity of saltwater at a certain point reaches its saturation limit, meaning that the conductivity of saltwater is limited. Under ambient conditions, the maximum salinity of saltwater at 263 parts per thousand (ppt) corresponds to conductivity of 25 S/m. To prevent the saltwater from becoming saturated, we use saltwater at 200 ppt for our antenna design. This salinity level ensures that conductivity of the saltwater remains as high as 20 S/m. The efficiency of the antenna can also be improved by optimizing the antenna’s dimensions, such as the length of the metal strip (c) and the width of the antenna (W), as presented in the next sections.
In order to improve the radiation efficiency of the liquid antenna, a metal strip is loaded on the top of the feeding probe (see Fig. 2) [6]. In this case, the metal strip is very thin (thickness 0.05 mm) and therefore does not affect the transparency of the antenna. Figure 4 shows the reflection coefficient and radiation efficiency of the single-port transparent liquid antenna when the length of the metal strip varies from 5 to 15 cm. As shown in Fig. 4(a), when c increases, the resonant characteristics of the antenna do not have much of an effect and the impedance matching level is slightly poorer. However, the radiation efficiency of the antenna is significantly improved when c increases from 5 to 15 cm, as shown in Fig. 4(b). This occurs because the metal strip acts as a radiator, which contributes to an increase in the total radiation efficiency of the proposed liquid antenna. Therefore, the length of the metal strip is kept as long as the width (W) of the saltwater antenna to maximize its efficiency.
Figure 5 shows the resonant characteristics and radiation efficiency of the liquid antenna when its width varies from 5 to 15 cm. We can observe from Fig. 5(a) that the resonant frequency of the antenna does not change much, whereas the impedance matching level is significantly improved when W increases. On the other hand, when W increases from 5 to 15 cm, the radiation efficiency of the antenna also increases significantly (Fig. 5(b)) despite the fact that the impedance matching becomes poorer with an increase in W. This likely stems from the ohmic loss of the antenna, which decreases when W increases. However, we can also observe from Fig. 4(b) that the total radiation efficiency of the antenna increases rapidly with W below 10 cm and that it increases slowly when W exceeds 10 cm. Therefore, in this study, we chose \(W=c=15 cm\) as the optimum dimensions of the antenna. The optimum dimensions of the antenna are listed in Table 1. It should be noted that the thicknesses of the saltwater and glass layers (not shown in Fig. 2) are 10 mm and 2 mm, respectively.
Dual-port feeding transparent liquid antenna. Figure 6 shows the reflection coefficient of the antenna in the advanced configurations. Figures 6(a) and (b) correspond to the configuration with two ports arranged opposite to each other along the z-axis (see Fig. 2(b)) and arranged orthogonally along the z-axis and x-axis (see Fig. 2(c)), respectively. For each configuration, two ports are fed in a co-phase and reserve-phase configuration relative to each other in an effort to investigate their resonant characteristics.
As shown in Fig. 6(a), when we change from the co-phase feeding mode to the reserve-phase feeding mode, the resonant frequency of the antenna moves from the higher band to the lower band. However, in the configurations where the two ports are arranged orthogonally, we can observe a reserve trend because the resonant frequency of the antenna moves from a lower to a higher band when we change from reserve-phase feeding to co-phase feeding. Therefore, both configurations demonstrate that a tunable frequency can be realized by the interchange between co-phase feeding to reserve-phase feeding. However, the configuration with opposite ports shows a much wider frequency-tunable range compared to the configuration with orthogonal ports, making it a more suitable configuration for frequency-tunable purposes.
In order to understand the mechanism associated with the resonant characteristics of the two advanced configurations, the current distributions at the resonant frequencies of the configurations are assessed at the resonant frequencies, as shown in Fig. 6. In each case, two ports are also fed in the co-phase and reserve-phase configurations relative to each other. As shown in Fig. 7(a), the current is distributed into different short segments with different directions, demonstrating that the antenna is working at a higher resonance level. When the two ports are fed in the reserve-phase configuration, as shown in Fig. 7(b), the current is uniformly distributed along with the antenna from port 1 forward to port 2. This type of current distribution indicates that the antenna is working in the fundamental resonance condition. Therefore, the resonant frequency in this case is much lower than that of co-phase feeding in Fig. 7(a).
Figures 7(c) and (d) show the current distribution of the configuration with orthogonal feeding ports when ports 1 and 2 are fed in the co-phase and reserve-phase conditions, respectively. We find that the current distribution in these cases is quite similar, indicating that both cases are operating at the fundamental resonance. The length of the current in Fig. 7(c) is slightly longer than that in Fig. 7(d), resulting in the resonant frequency of co-phase feeding being slightly lower than in reserve-phase feeding, as shown in Fig. 7(b).
Figure 8 shows the simulated 3D radiation pattern of the transparent liquid antenna with different advanced configurations and feeding phases. By using opposite feeding ports, the antenna acts as an omnidirectional antenna (Figs. 8(a) and (b), whereas it shows directional radiation characteristics when two feeding ports are arranged orthogonally to each other, as shown in Figs. 8(c) and (d). This result shows that the configuration with opposite feeding ports can be used for broadcasting applications such as television while the configuration with orthogonal feeding ports is more suitable for applications that require a high-directivity antenna, such as satellite communications.
Measurement results and discussion. Figure 9 shows the measurement setup of the proposed antenna in an anechoic chamber. The inset image in Fig. 9 depicts the fabricated transparent liquid antenna (advanced configuration), where the two feeding ports are orthogonally arranged along the x- and z-axis. We find that the fabricated antenna retains its very good transparency.
Figs. 10 shows the simulated and measured reflection coefficients ( ) of the transparent liquid antenna with different feeding configurations. The measured S11 is assessed using a network analyzer E5071B, with the findings in good agreement with the simulation results. As shown in Fig. 10(a), the transparent liquid antenna with single-port feeding (see Fig. 2(a)) shows a very wide -6 dB bandwidth ranging from 350 to 680 MHz (of 330 MHz; 64%). This frequency nearly covers the band for ultra-high-definition TV (UHD TV) applications. The reflection coefficient of the antenna with feeding using two opposite ports (see Fig. 2(b)) is shown in Fig. 10(b). As expected, the resonant frequency of the antenna moves significantly to a higher band when we change from the reserve-phase feeding to the co-phase feeding configuration. It was observed that the antenna in the reserve-phase configuration shows a wide -6 dB bandwidth ranging from 260 to 750 MHz (490 MHz; 97%), while the resonant band of the co-phase configuration is much higher, from 1.03 to 1.32 GHz (290 MHz; 24.6%). This result demonstrates that frequency-tunable characteristics of the transparent liquid antenna can be achieved by feeding a different phase between two opposite ports. Fig. 10(c) presents the simulated and measured reflection coefficients of an antenna with a feeding configuration of two orthogonal ports (see Fig. 2(c)). For the configuration with orthogonal ports, the antenna shows a small left-shift of the resonant frequency from 600 to 550 MHz when we change from reserve-phase to co-phase feeding. The -6dB bandwidths of the antenna in the reserve-phase and co-phase configurations are 330 MHz (500 to 830 MHz; 49.6%) and 240 MHz (460 to 700 MHz; 41.3%), respectively.
Fig. 11 depicts the simulated and measured gain and the radiation efficiency of the transparent liquid antenna. The antenna in the basic configuration with a single port shows typical performance outcomes with the average gain and radiation efficiency on the UHF band being 1.73 dBi and 69%, respectively (Fig. 11(a)). As shown in Figs. 11 (b), the antenna in the advanced configuration with two opposite co-phase ports feeding shows an efficiency rate as high as 63% with an average value of 58.8% and gain up to 5.4 dBi on the resonant band from 1.03 to 1.32 GHz. Meanwhile, as shown in Fig. 11(c), the feeding configuration with the two opposite reserve-phase ports exhibits higher efficiency up to 71% and lower gain levels up to 4.5 dBi compared to the co-phase feeding configuration. This can be explained by the radiation characteristic of the antenna in the co-phase feeding configuration, which is more directive than that of the reserve-phase configuration, as shown in Figs. 8(a) and (b). Figs. 11(d) and (e) show the gain and efficiency of the antenna when two ports are arranged orthogonally. We find that the co-phase and reserve-phase feeding configurations exhibit similar average gain and efficiency outcomes over the resonant bands. The average gains of the co-phase and reserve-phase configurations are 5.8 and 6.8 dBi, respectively, whereas the corresponding average efficiency rates are 56% and 60%.
The simulated and measured radiation patterns of the proposed antenna with different feeding configurations are depicted in Fig. 12. We observed good agreement between the simulation and measurement outcomes. As shown in Fig. 12(a), the antenna shows an omnidirectional radiation pattern on the xoy-plane, while it acts as a directional antenna on the xoz-plane. The radiation pattern of the proposed antenna is similar to that of a conventional metal monopole. Figures 12 (b) and (c) show the radiation pattern of the antenna with two opposite co-phase and reserve-phase feeding ports at their resonant frequencies, respectively. Both configurations exhibit omnidirectional radiation characteristics. However, we find that the co-phase configuration is operating at a higher resonance level while the reserve-phase configuration acts as a typical dipole at the fundamental resonance level. On the other hand, the antenna using two orthogonal feeding ports shows directional radiation patterns, as presented in Figs. 12 (d) and (e). This result demonstrates that a tunable radiation pattern can be achieved by changing from the configurations with single and two opposite feeding ports to that with orthogonal feeding ports.