I. Introduction
Printed electronics is a rapidly emerging technology that is gaining increasing popularity due to vast market demands in the electrical and electronic industries, particularly in biosensors, medical electronics, and health-monitoring devices. This technology enables the production of low-cost and large-area electronic components using printing techniques such as screen printing, flexography, inkjet, or gravure [1, 2]. Accordingly, it has attracted much interest in the development of conformal and flexible devices [3–9]. Moreover, this technology can be used to fabricate antennas and radio frequency components using papers, plastics, textiles, and a variety of potential flexible substrates [10]. Since printing can deposit functional materials directly onto a substrate to generate patterns, this technology offers several merits, such as a new way to produce low-cost antennas, quick fabrication, good production versatility, and roll-to-roll compatibility [11, 12]. Moreover, as it can efficiently fabricate antennas for wearable applications, this technology offers a cost-effective method for producing light and flexible antennas as well. The etching technique, though a reliable fabrication method, involves environmentally unfriendly chemicals. In contrast, printing techniques, owing to their additive nature, generate minimal material waste during fabrication [13]. One principal benefit of screen printing antennas is its potential for cost-effective and high-volume manufacturing. Screen printing technology also enables cost-optimized inline reel-to-reel manufacturing, which means that antennas can be thinner, lighter, more flexible, and cheaper than those produced by conventional etching [14].
Presently, 3D printing has emerged as the latest additive manufacturing technique. It can print complex 3D models using the layer-by-layer deposition of versatile materials [15–17], enabling customized substrate structures (thickness, filling, and shape), electrical properties (permittivity), and mechanical properties (weight and flexibility) [7]. To print electronic circuits and antennas, several types of conductive inks are used, as they can produce fine pattern structures. Owing to their high conductivity, the metal particles commonly used in inks are silver, gold, nickel, and platinum [18, 19]. Silver nanoparticles, however, are the most preferable, as they possess low bulk resistivity and, as silver oxide is also conductive, their conductivity does not get severely degraded by oxidation [20, 21]. Although printing techniques may significantly lower antenna fabrication costs [22], the printed antennas may suffer performance degradation when compared with solid metal antennas [23]. Since the conductive inks produced from the mixture of metal particles and adhesive fluids generally have lower conductivities than the inks produced from conventional metal traces [24–27], printed antennas' performance, in terms of their gain and radiation efficiency, is noticeably degraded [28–31]. It has been found that adding two split ring resonators (SRRs) to the antennas' biplanar dipole can generate additional resonance modes at low and high frequencies [32, 33] and that the position of the SRRs can adjust the antennas' input impedance [34].
Inkjet-printed antennas that use conductive paint are simpler to manufacture than conventional printed circuit board antennas, which use etching and are ideal for mass production. However, as highly conductive paint is quite expensive, using it significantly increases the manufacturing cost [24–27]. In light of this, discovering the minimum electrical conductivity needed to maintain an antenna's characteristics can help reduce its manufacturing cost. We investigated how the conductivity of silver nanoparticles affect the performance characteristics of a flexible compact wideband antenna comprised of a printed dipole and two rectangular SRRs. The antenna can generate a wide impedance bandwidth with different conductivities; however, its gain and efficiency change significantly at different conductivity values within the impedance bandwidth. To validate the antenna design, we analyzed three prototypes with different conductivities and, using the ANSYS HFSS software, compared their performance characteristics.
II. Antenna Geometry and Performance
1. Antenna Geometry
The proposed configuration of the antenna is shown in Fig. 1. The dipole is printed at the top and bottom of a thin Rogers RO5880 dielectric substrate (ɛr = 2.2, tanδ = 0.0009, and h = 0.254 mm), and two SRRs are placed on both sides of the substrate to encompass each arm of the dipole. The slits on each SRR are located at the opposing sides with respect to the feed position of the antenna, and a coaxial feed with a characteristic impedance of 50 Ω was used for antenna excitation. The outer conductor of the coaxial line is connected to the bottom arm of the dipole, and the inner conductor of the coaxial line extends through the substrate and connects to the top arm of the dipole [32]. We simulated the antenna using a sheet structure with no thickness. The optimized design parameters of the antenna that resulted in a wide impedance bandwidth and omnidirectional radiation patterns are as follows: e = 17.3 mm, W1 = 10 mm, gap = 1 mm, Ld = 34.7 mm, Wd = 4.3 mm, h = 0.254 mm, Ws = 2.2 mm, Gp = 12.2 mm, Ls = 74.8 mm, and Ll = 54.95 mm.
2. Antenna Performance
We analyzed the changes in the antenna's performance with different conductivities of silver nanoparticles. We first noted the changes in the reflection coefficient. As illustrated in Fig. 2, the antenna generated three resonances with high conductivity [33, 34]. The first resonance resulted from the entire lengths of both SRRs oscillating in unison as a dipole, and it occurred at the lowest frequency. Meanwhile, the intermediate resonance was generated by the dipole, while the third resonance was generated at a high frequency because of the SRRs being partially excited. In the third mode, only the overlapping portion of both SRRs resonated at a half wavelength. The interactions between the resonances produced a wide impedance bandwidth, and it has been shown that the current distribution, thus, offers substantial evidence of the mechanism that generates the three resonances [33].
According to Fig. 2, which illustrates the antenna's reflection coefficient when conductivity varies, the impedance bandwidth remains almost unchanged with high conductivity values. When the conductivity is less than 5.8 × 104 S/m, the reflection coefficient curve moves toward lower frequencies, producing only two resonances—that is, when conductivity is less than approximately 5.8 × 104 S/m, the high-frequency resonance disappears (Fig. 3). However, the low-frequency resonance does not disappear, as the entire SRR retains strong electromagnetic interactions with the excited dipole, and this produces sufficient coupling to excite the SRRs to resonate. The resonance shifting is a result of a slight increase in the capacitance of the antenna with decreased conductivity. This increased capacitance produces slight changes in the input impedance, which generates an impedance mismatch at low frequencies. The reflection coefficient curves depict the gradual degradation of the high-frequency resonances as the conductivity changes from 1.6 × 104 S/m to 1.2 × 105 S/m. The third resonance immediately ceases to exist when the conductivity changes from 6.0 × 104 S/m to 3.0 × 104 S/m.
The disappearance of this resonance can be explained via the surface current distribution, with a conductivity of σ = 3.0 × 104 S/m, as shown in Fig. 4(a). By comparing this to the surface current distribution with a conductivity of σ = 5.8 × 107 S/m, as shown in Fig. 4(b), it is evident that the first two resonances have similar current distributions on both the dipoles and SRRs. However, dissimilarities exist in the current distributions at the high-frequency resonance. A strong current circulating within the overlapping region of both SRRs generates high-frequency resonance for the high-conductivity copper case. When the conductivity falls below 6.0 × 104 S/m, the coupling between the two SRRs becomes weak; consequently, the current circulating within the overlapping region of the SRRs is degraded and becomes insufficient to excite the overlapped area of the SRRs at a high frequency. The high-frequency resonance thus disappears, and the impedance bandwidth narrows. Accordingly, with the conductivity range between 1.2 × 105 S/m and 5.8 × 107 S/m approximately, the antenna behavior with respect to the reflection coefficient remains almost the same, and the same bandwidth is obtainable within the same conductivity range. The antenna bandwidth characteristic is thus similar when the conductivity values are larger than approximately 1.2 × 105 S/m. In addition, for conductivity values below 3.0 × 104 S/m, the antenna produces only two resonances, with a shift in the reflection coefficient profile toward lower frequencies, resulting from a slight increase in the antenna's capacitance.
In contrast to the impedance bandwidth, the antenna's gain performance is appreciable when conductivity changes. The gain attains stable values within the antenna bandwidth, while it immediately decreases outside the bandwidth. The gain performance is confirmed with Fig. 5, which illustrates the antenna gain response with different conductivities. When conductivity decreases, the gain also decreases. At low conductivity, the ohmic resistance increases, and more electromagnetic energy is lost thermally, which leads to poor gain performance. At high conductivity values, there is high gain, as ohmic resistance falls and less electromagnetic energy is lost thermally. When conductivity decreases and becomes 5.8 × 104 S/m, the gain decreases and approaches a 3-dB loss. Below this conductivity value, the gain degrades significantly. With high conductivities around the boundaries of copper, the gain decreases slightly. However, for conductivities far below that of copper, the drop in gain is significant. Fig. 6 shows the variation in gain with different conductivities at 1.6 GHz. A thorough examination of the curve revealed a rapid rise in gain to −2 dBi at low conductivities (up to 104 S/m). Above 104 S/m, the gain rises slowly with a gentle slope and attains stability at about 2 dBi, with little or no increase in gain at conductivities of >106 S/m. High conductivity thus results in the best gain performance; however, at certain limits, conductivity increments of >106 S/m do not result in any gain improvements. From the above analysis, it is evident that various gain values are attainable depending on the conductive ink used in antenna fabrication. The radiation efficiency curve follows a trend similar to that of the gain curve, and Fig. 7 shows the radiation efficiency as a function of conductivity. The efficiency is almost constant within the impedance bandwidth, with unnoticeable fluctuations, and the same general trend between gain and conductivity can be extended to the radiation efficiency and conductivity due to the direct relationship between gain and radiation efficiency. With conductivities of >5.8 × 107 S/m, a radiation efficiency of >93% is attainable within the impedance bandwidth. However, the reverse occurs with a conductivity of <5.8 × 104 S/m, as the radiation efficiency drops to <70%. As shown in Figs. 5 and 7, the antenna efficiency and gain are directly related to conductivity: when conductivity increases, gain and efficiency increase. This relationship can be expressed mathematically by considering the general antenna model that shows the equivalent model of a transmitting antenna [35].
Fig. 8 shows the radiation efficiency at 1.6 GHz for different conductivity values, indicating that the radiation efficiency rises as a function of conductivity. The radiation efficiency increases linearly with a steep sloping increase in conductivity. At a conductivity of >106 S/m, the slope becomes gentle and gradual, as it approaches the maximum attainable radiation efficiency. Consequently, the curves indicate that conductivity is a crucial parameter for achieving optimum antenna performance. By considering the different variations in conductivity, different antenna performance limits can be observed. Evidently, for cost-effective antenna fabrication, a low-cost material with a conductivity value within the possible range for satisfactory performance can be utilized.
From the aforementioned analysis and explanations, it is evident that changes in conductivity values generate corresponding changes in antenna performance. At a certain conductivity value or within a range of conductivity values, the antenna showed major changes in its performance. These changes were noticeable at the boundary of a conductivity value of 5.8 × 104 S/m, particularly in the antenna's reflection coefficient. As a result, three different antennas were examined with conductivity values higher and lower than the previously mentioned conductivity value to validate and further demonstrate the major changes. The antenna samples were of the same design parameters and conditions but constructed of copper (5.8 × 107 S/m), high-conductivity silver nanoparticle (8.0 × 105 S/m), and low-conductivity silver nanoparticle (1.7 × 104 S/m) [24, 25]. Fig. 9 shows the impedance bandwidth generated by the three sample antennas. The copper cladding and high-conductivity silver nanoparticle antennas produced the same number of resonances (three) with an almost identical bandwidth. However, the low-conductivity silver nanoparticle antenna generated only two resonances, with a slight shift toward low frequencies, because the conductivity (1.7 × 104 S/m) of this antenna was not high enough for the overlapping area of the SRRs to be sufficiently well-excited for generating a third resonance.
The copper cladding antenna generated the highest gain because of its high conductivity and, hence, fewer ohmic losses and more radiated power (Fig. 10). With lower conductivity values than that of copper, the gain degraded gradually. Among the three antennas under discussion, copper demonstrated the highest gain values, while the gain for the high-conductivity silver nanoparticles slightly decreased compared to that of the copper cladding antenna. The low-conductivity silver nanoparticle antenna produced a significant decrease in gain (about 3 dB) compared to that of copper, and we recorded a similar trend for radiation efficiency (Fig. 11). We found that the low-conductivity silver nanoparticle produced the lowest efficiency, while the efficiencies of the high-conductivity silver nanoparticle and copper were significantly improved due to their high conductivity values. As shown in Fig. 12, all the antennas generated symmetric dipole radiation patterns with few fluctuations in gain. The gain values in the radiation patterns of the high- and low-conductivity silver nanoparticles were lower when compared to that of copper, which was due to the drop in antenna gain performance with a corresponding decrease in conductivity. The copper cladding antenna demonstrated a slightly better performance than the high-conductivity silver nanoparticle in terms of gain and radiation efficiencies, but both antennas offered similar impedance bandwidths. Nonetheless, the high-conductivity silver nanoparticle antenna approached the performance of copper at a reduced fabrication cost and with improved flexibility.
III. Experimental Results
To validate the effect of conductivity on the antenna under investigation, we fabricated and measured three prototypes. We fabricated two antennas printed with conductive ink using a Brother inkjet printer, which is easy to charge and whose ink is easy to replace, and a substrate with a thickness of 0.254 mm (the maximum thickness available for the printer). The conductive paste used for printing was Kratoz Xlink (σ = 8.0 × 105 S/m) and CANS A-201 (σ = 1.7 × 104 S/m). For inkjet printing, the conductive paste was diluted in acetone. The electrical conductivity of the fabricated actual antenna was thus lower than that of the paste used in the fabrication. We air-dried the antenna was at room temperature for more than 12 hours. All the three antennas were identical in size—10 mm × 74.8 mm × 0.254 mm (0.053λo × 0.399λo × 0.0014λo at 1.6 GHz)—but differed in their conductivities.
Fig. 13 shows the fabricated antennas. To prevent changes in the radiation patterns due to leakage current, we used a sleeve balun on the coaxial cable. The first antenna was fabricated with copper, with a conductivity of 5.8 × 107 S/m, while the other two were made of silver nanoparticles with high (8.0 × 105 S/m) and low (1.7 × 104 S/m) conductivities. The prototype's reflection coefficient was measured using an Agilent E8362B Network Analyzer and a 3.5 mm Coaxial Calibration Standard (GCS35M). Fig. 14 shows the measured impedance bandwidths of the fabricated antennas. These antennas demonstrated bandwidths almost identical to those of the simulated results. Moreover, the antenna made of copper and the antenna made of high-conductivity silver nanoparticle generated three resonances in their |S11| profile. The measured impedance bandwidth for a |S11| < −10 dB ranged between 1.4 GHz and 2.4 GHz (52.63%) and 1.38 GHz and 2.4 GHz (53.96%) for copper and the high-conductivity silver nanoparticle, respectively. Meanwhile, the antenna made of the low-conductivity silver nanoparticle portrayed two resonant points in the reflection coefficient curve, with dual bands ranging between 1.18 GHz and 1.3 GHz (9.6%) and 1.45 GHz and 1.95 GHz (29.41%), respectively, for |S11| < −10 dB. Minute disparities were noted between the measurements and the HFSS simulation of the low-conductivity silver nanoparticle antenna, which could be associated with misalignment during printing and fabrication.
The far-field measurements were made in a full anechoic chamber with dimensions of 5.5 m (W) × 5.5 m (L) × 5.0 m (H) at the Electromagnetic Wave Technology Institute, Yongsan, Korea. During the radiation pattern measurement procedure, a standard wideband horn antenna was used for transmitting, while the fabricated antennas were used for receiving. The transmitter and receiver were separated at a distance of 2.9 m, the horn antenna was fixed, and the fabricated antennas were rotated from −180° to 180° at a scan angle of 1° and a speed of 3°/s. The broadside gain and radiation patterns of the fabricated prototypes at different frequencies were measured (shown in Figs. 15 and 16, respectively). The broadside gains for all three antennas followed the simulated results, with the gain rising and attaining stable values within the impedance bandwidth before decreasing rapidly. The following measured peak gains of 1.8 dBi, 0.9 dBi, and −0.6 dBi were recorded for the copper, high-conductivity silver nanoparticle, and low-conductivity silver nanoparticle antennas, respectively, and the radiation patterns for all three antennas at the resonant frequency points are shown in Fig. 16. The fabricated antennas generated good broadside dipole-like radiation patterns, with a symmetrical profile within the entire impedance bandwidth. At 1.8 GHz, the measured patterns were fairly symmetrical in both x–z and y–z planes. The radiation patterns were slightly tilted, indicating tiny ripples, which were due to the foams and racks employed in the radiation pattern measurement setup. These foams and racks were used to make the antenna rigid and prevent it from sagging at the edges during measurements. This degraded the backside of the radiation patterns.
IV. Conclusion
We investigated a low-cost flexible composite antenna made of a dipole and two SRRs with various conductivities and observed the effects of different conductivities on antenna performance in terms of antenna gain, radiation efficiency, and reflection coefficient. The number of resonances changed from three to two when there was a large decrease in conductivity from that of copper. When conductivity was less than approximately 5.8 × 104 S/m, the high-frequency resonance vanished, with a consequent constriction in the impedance bandwidth. It was thus found to be important to use silver nanoparticle with a conductivity higher than 5.8 × 104 S/m to maintain the resonance characteristics of the coupled antenna structure. With a drop in conductivity, the antenna gain diminished, and at 5.8 × 104 S/m, a loss of approximately 3 dB occurred. Below this aforementioned conductivity value, more significant drops in gain were noticeable, and similar trends in gain were recorded for the radiation efficiency, with changes in conductivity. Finally, we printed three prototype antennas with copper, high conductivity silver nanoparticle, and low-conductivity silver nanoparticle to validate the antenna's performance with different conductivities. The simulated and measured results aligned, demonstrating that an antenna's conductivity has significant effects on its performance.
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