Design of multi-band flexible microstrip antenna based on micro drop jetting

Due to the high compatibility of micro-droplet jetting 3D printing technology within the realm of printed electronics, a flexible and miniaturized multi-band microstrip antenna was designed. The purpose of this is to extend the wireless signal response range of wearable devices and to investigate the feasibility of producing wearable devices with high efficiency. The antenna uses polydimethylsiloxane (PDMS) as the dielectric substrate and nanosilver as the conductive material for the radiating patch, demonstrating remarkable flexibility. The antenna’s structure underwent simulation and analysis through frequency sweeping using ANSYS HFSS simulation software. The outcomes illustrate the antenna operating within three frequency bands at 2.5GHz, 3.5GHz, and 5.8GHz, and the return loss is kept below -18dB for each central frequency. Simultaneously, it displays favorable flexibility. The radiation pattern of the antenna indicates that it has good directivity and no extra side lobes are generated. Ultimately, Antennas were fabricated using microdroplet spraying technology, and the final product’s characteristics and morphology were analyzed. The aforementioned findings demonstrate that micro-droplet jetting technology’s remarkable precision and efficiency render it a viable approach for the processing and production of flexible microstrip antennas.


Introduction
Wireless communication is widespread in modern society and relies heavily on antennas to transmit and receive electromagnetic signals.The adoption of technologies such as the Internet of Things and Body Area Networks has led to the integration of wireless communication in wearable devices.The trend towards lightweight, highly flexible antennas with desirable characteristics has resulted in an increased demand for such devices.A popular area of research in the field of antenna engineering relates to the development of flexible antennas, which have a primary focus on wearable devices.
In recent years, the transmission frequency range for short-range wireless communication has expanded, causing household wireless communication frequency bands to extend beyond the conventional 2.5 GHz range.Additional frequency bands, such as 5.8 GHz and 3.5 GHz, have been introduced [1].As a result, conventional single-frequency antennas are inadequate for everyday use, and flexible wearable antenna demands have grown.
Among various types of multi-frequency antennas, microstrip antennas offer advantages such as ease of fabrication, simple structure, and the ability to achieve circular polarization.Additionally, the radiating patch of a microstrip antenna has high flexibility, making it easier to achieve multi-frequency characteristics.Microstrip antennas also exhibit excellent compatibility with Polydimethylsiloxane (PDMS), utilizing PDMS's excellent heat resistance, corrosion resistance, non-toxic properties [2], and good electrical characteristics, which enable PDMS-based microstrip antennas to have good radiation characteristics and flexibility.
Previous research on multi-frequency antennas, conducted by Sushanta Sarkar et al. [3], presented a design featuring three resonant frequencies.The tuning of one frequency has been demonstrated, highlighting that the ratio of the tuneable frequency f2 to the fixed fundamental frequency f1 can range from 1.6 to 1.The tuning of one frequency has been demonstrated, highlighting that the ratio of the tuneable frequency f2 to the fixed fundamental frequency f1 can range from 1.6 to 1.The tuning of one frequency has been demonstrated, highlighting that the ratio of the tuneable frequency f2 to the fixed fundamental frequency f1 can range from 1.6 to 1.The study also discusses the potential for optimum tuning to result in a wide-band antenna, which would have a significantly higher gain in comparison to a monopole antenna.K. Mahendran et al. [4] propose a quad-band Triangular Microstrip antenna that is designed for use in IEEE 802.16eWi-MAX, IEEE 802.11aWLAN, C band downlink communication, and X-band radar applications.The antenna features a triangular patch with a triangular split ring resonator and is preferred for its conservativeness and data transfer capacity.The proposed antenna demonstrates improved return and gain by resonating at 3.5 GHz, 4.1 GHz, 5.6 GHz, and 9.7 GHz frequencies.
Micro drop jetting, as a processing technique in inkjet printing, has broad application prospects in electronic printing, flexible electronics, and panel displays.It can also be effectively deployed in the preparation and assembly of nanoparticles within the domain of nanotechnology.In addition, micro drop jetting has a range of other applications, such as inkjet printing, coating, microfluidics, and micro/nano processing.The benefits of micro drop jetting technology comprise exceptional precision, rapid speed, effortless integration, and scalability.This technology enables meticulous command over droplet dimensions, velocity, and orientation, resulting in an enhancement in the exactness and consistency of experimental outcomes [5].Patrick Cooley et al [6] fabricated a single and multi-fluid piezoelectric micro-jet device which has been used for targeted synthesis of peptides, micro-printing of biodegradable polymers for cell proliferation, etc.The basic principle of the device is the use of piezoelectric ceramics resulting in volume contraction and expansion to create a pressure pulse inside the nozzle, which ultimately ejects the droplets.Chandra S et al [7].proposed a direct pneumatic pressure-driven microdroplet injection technique, the main principle of which is to control the compressed gas through a solenoid valve to generate pneumatic pressure pulses in the fluid chamber and use this to achieve the purpose of injection.
In order to enhance the wireless signal response range of wearable devices and address their usage environment more effectively, a flexible microstrip antenna with a slotted structure loaded on a PDMS substrate was designed to operate in three distinct frequencies.The efficient two-dimensional structure of the antenna's radiating surface facilitates an uncomplicated fabrication process using a method known as micro drop jetting printing within the field of printed electronics.
After optimization using simulation, the antenna's center operating frequency covers the present mainstream frequency bands of 2.5 GHz, 3.5 GHz, and 5.8 GHz.The antenna's multi-frequency attributes and bending performance were scrutinized.The flexible microstrip antenna's fabrication conditions were assessed using micro drop jetting, and the PDMS surface underwent cleaning using oxygen plasma treatment.Lastly, the antenna was manufactured to create the final product.

Microstrip Antenna Design Principle
As depicted in Figure 1, the rectangular microstrip antenna boasts a length of L equivalent to half of the wavelength, a width of W, and a substrate thickness of h, whereby h is considerably smaller than the antenna wavelength λg.Per transmission line theory, the antenna may be deemed as an open-ended microstrip transmission line.The electric field in the W direction of the transmission line remains stable and unchanging.However, the electric field produced by the length L, equivalent to the wavelength, can be determined by From Equation (1), it is evident that the two open ends, a and b, always lie at the electric field nodes, and the gap between these ends and the ground plane creates far-field radiation.
The electric field produced by these gaps, a and b, can be separated into tangential and normal components.The tangential components of the electric field abolish each other.The vertical components of the electric field combine to produce the strongest radiation in the direction perpendicular to the surface.Consequently, the antenna emits electromagnetic waves in the upper half-space above the ground plane.
where c is the speed of light, 0 is the target operating frequency, is the effective permittivity, ∆ is the effective gap width, and its value is given by Similarly, the width W can be obtained from the formula In the design of microstrip antennas, the substrate's length and width are typically selected to be about twice that of the patch's length and width, respectively.

Performance Simulation and Analysis of Flexible Tri-Band Microstrip Antenna
Design a microstrip antenna according to the schematic in Figure 2. The antenna comprises three components: a radiating patch, a dielectric substrate, and a ground plane.PDMS is the selected dielectric substrate material, with dimensions of 60mm × 60mm × 2.5mm.The radiating patch employs nano silver as the conductive material, which has been demonstrated to exhibit excellent conductivity and flexibility, rendering it ideal for achieving antenna flexibility when combined with the PDMS dielectric substrate [8].
The antenna employs microstrip line edge feeding, offering the benefits of easy processing and impedance matching.Concurrently, the microstrip transmission line used for feeding is inserted inside the radiating patch, mitigating input impedance and lessening the impact of the transmission line on the overall radiation performance of the antenna.
The radiating patch section of the antenna undergoes a structured design.The central section of the patch incorporates two inverted "U" shaped slots and one strip slot to alter the distribution of surface current and attain multi-band features.Additionally, rectangular blocks are cut at the edges of the microstrip patch to precisely adjust the resonant frequency of each of the three frequency bands.Please refer to Table 1 for precise parameters of the radiating patch section.3 illustrates the return loss (S11) of the antenna.In practical engineering, an S11 value below -10dB is commonly regarded as the effective resonant frequency range and represents the frequency range of the antenna's operation.The antenna displays three bands of resonant frequency with centre frequencies of 2.51GHz, 3.48GHz, and 5.80GHz, respectively.The antenna's minimum loss reaches approximately -26dB.The formula for relative bandwidth can be utilized.
= 2 By calculation, it can be determined that the three frequency bands from left to right have relative bandwidths of around 3.9%, 3.5%, and 7.2%.Applying the narrowband (0%~1%), broadband (1%~25%), and ultra-wideband (25%~) standards, it can be inferred that the antenna's three resonant frequency bands satisfy the bandwidth requirements for microstrip antennas.

Figure 3. Antenna Return Loss
Impedance matching is a critical factor when assessing an antenna's energy loss.The antenna's impedance matching may be measured using the Voltage Standing Wave Ratio (VSWR).This ratio represents the amplitude ratio of the maximum voltage (current) to the minimum voltage (current) on the transmission line.The reflection coefficient may be used to express this ratio.
Where Γ is the reflection coefficient, which can be obtained from the characteristic impedance.Equation ( 6) always yields a VSWR greater than 1, with the best impedance matching of the antenna occurring when the VSWR is closer to 1. Said antenna is typically matched with a 50Ω transmission line.
The VSWR, as obtained through frequency sweep simulation for this design, is displayed in Figure 4. Figure 4 illustrates the VSWR of the antenna between 2GHz and 7GHz.The simulation outcomes indicate that the VSWR exhibits a good match between the antenna and the 50Ω transmission line at each center frequency, with values of 1.11, 1.40, and 1.32 observed at the three respective center frequencies.The circular radiation pattern indicates that the antenna evenly emits energy in all directions, without any unwanted secondary lobes or concentrated radiation in particular directions.This feature is advantageous for applications that require a balanced and consistent coverage of the electromagnetic field.

Analysis of Antenna Flexibility Performance
To examine the antenna's flexibility performance, we utilized the HFSS simulation software to subject the antenna model to bending.The antenna was bent to conform to cylindrical objects with differing radii to analyze its performance under various bending conditions.Figure 7 presents the return loss (S11) of the antenna for diverse bending radii.The bending radii of 120mm, 100mm, and 80mm were tested.The obtained test results show that the antenna's resonance frequency bands remain stable across various bending radii with no significant variations.Furthermore, with an increased bending radius, the minimum return loss drops from -21.8dB (no bending) to -31dB.This signifies that as the bending angle rises, the loss reduces, indicating an improvement in the antenna's impedance matching.
The simulation test results demonstrate that the antenna exhibits good flexibility performance, rendering it appropriate for supple designs in wearable gadgets, thereby fulfilling the prerequisites of flexibility for wearable appliances.

Physical Processing
Based on the above design, the fabrication of the tri-band microstrip antenna was accomplished using a droplet jetting 3D printing process.The device utilizes a piezoelectric printhead, which operates based on the principle that when an electric field is generated between the internal and external electrodes, the piezoelectric element inside the printhead undergoes axial expansion or compression.This rapid deformation is transmitted to the glass tube through epoxy adhesive, causing the inner surface of the glass to move outward, creating a negative pressure (relative to the equilibrium state).The negative pressure travels at the speed of sound through the liquid along the glass tube, propagating as an extended acoustic wave to the orifice and the liquid supply end.The expansion wave (pressure higher than the equilibrium pressure in the glass tube) reflects at the liquid supply end to form a compression wave, which returns toward the orifice.This compression wave causes the solution at the nozzle to break the surface tension and generate small, fast-moving droplets [9].
To enable material deposition on the highly hydrophobic PDMS surface, it requires undergoing a process of hydrophilic modification treatment.As reported in reference [10], the PDMS surface is exposed to vacuum oxygen plasma treatment (oxygen flow rate of 200SCCM, treatment time of 30s, power of 120W).Afterwards, a solution containing sodium dodecyl sulfate (SDS) with a concentration of 0.5% is applied for 30s.After observing the PDMS surface for any cracks, we transferred 2.5μl of deionized water using a pipette to evaluate the surface modification degree.Figure 8 illustrates the hydrophilicity of the PDMS surface after the above-mentioned treatment method.During testing, it was discovered that the contact angle of water droplets on the treated PDMS surface is less than 10°, signifying a super hydrophilic surface.By comparison, water droplets exhibit a contact angle of approximately 90°-100°on the untreated surface of PDMS.Additionally, PDMS that has undergone SDS treatment is capable of sustaining long-term hydrophilicity, thereby meeting the prerequisites of 3D printing processing.

Figure 8．(a) Hydrophobicity of Untreated Surface (b) Hydrophilicity after Modification
Treatment A 25mm-long nano silver wire was printed onto the modified PDMS surface using a droplet jetting device, and the line morphology was analyzed using a surface profiler to explore the optimal printing parameters.Figure 9 illustrates the line morphology at various printing speeds(Substrate temperature 65 ℃ , solution nanosilver concentration 25wt%, piezoelectric ceramic nozzle vibration frequency 80Hz, back pressure -10kPa).As can be seen from the figure in the speed of 20mm/s or less, the width of the printed line is basically positively correlated with the speed, more than 20mm/s, the droplets due to the relative distance of the elongation of the line can not be connected to a complete line, in the speed of 25mm/s when the droplets are basically independent of each other.In terms of line width, microdroplet injection can form the minimum line width and nozzle diameter has a great correlation, such as 60 μm nozzle diameter, the print line width is usually in the range of 60 80 μm or so, this topic based on improving the printing efficiency and printing accuracy and other considerations, the selection of the nozzle diameter of 120 μm of the piezoelectric ceramic nozzle as a processing device.After testing, when the printing speed reaches 15mm/s, the line width can reach the optimal 150μm.

Figure 9．Line
Morphology at Different Printing Speeds (a) 2.5mm/s (b) 5mm/s (c) 10mm/s (d) 15mm/s (e) 20mm/s (f) 25mm/s After establishing the most suitable line parameters, the lines were arranged in a "Z" pattern path to form a surface, which can be observed in Figure 10(a).To guarantee both conductivity and flexibility of the surface structure, nanosilver was picked as the conductive material for the radiating patch.Furthermore, a copper foil with a thickness of 20μm was preferred as the ground plane material to cut costs and enhance module interchangeability.Finally, in order to verify that the finished antenna meets the requirements of use, the use of vector network analyzer on the antenna was to do an S11 return loss performance test, the test results are shown in Figure 11, it can be seen that the measured data of the antenna compared to the simulation design has a certain offset, but still in the three central frequency points still maintain a high degree of matching, to ensure the stability of the three-frequency performance.

Figure 1 .
Figure 1.Schematic Diagram of Microstrip Antenna Principle The length L of the antenna patch is affected by the edge-shortening effect, and its length formula is given by

Figure 4 .
Figure 4. Antenna Voltage Standing Wave Ratio (VSWR) Figures 5(b) and 5(c) depict the two-dimensional radiation pattern of the antenna in the E-plane and H-plane, respectively.Through thorough analysis of the two-dimensional patterns, the antenna's maximum gain of around 3.92dB becomes apparent.Furthermore, the two patterns demonstrate circular characteristics, signifying the antenna's excellent omnidirectional radiation abilities, which lack significant side lobes.The circular radiation pattern indicates that the antenna evenly emits energy in all directions, without any unwanted secondary lobes or concentrated radiation in particular directions.This feature is advantageous for applications that require a balanced and consistent coverage of the electromagnetic field.

Figure 5 .
Figure 5. (a) Three-dimensional Gain Pattern of the Antenna (b) E-plane Gain Pattern of the Antenna (c) H-plane Gain Pattern of the Antenna

Figure 6 .
Figure 6.represents a schematic diagram of the antenna bending simulation.

Figure 7 .
Figure 7. displays the return loss of the antenna for different bending radii.The bending radii of 120mm, 100mm, and 80mm were tested.The obtained test results show that the antenna's resonance frequency bands remain stable across various bending radii with no significant variations.Furthermore, with an increased bending radius, the minimum return loss drops from -21.8dB (no bending) to -31dB.This signifies that as the bending angle rises, the loss reduces, indicating an improvement in the antenna's impedance matching.The simulation test results demonstrate that the antenna exhibits good flexibility performance, rendering it appropriate for supple designs in wearable gadgets, thereby fulfilling the prerequisites of flexibility for wearable appliances.
Figure 9．LineMorphology at Different Printing Speeds (a) 2.5mm/s (b) 5mm/s (c) 10mm/s (d) 15mm/s (e) 20mm/s (f) 25mm/s After establishing the most suitable line parameters, the lines were arranged in a "Z" pattern path to form a surface, which can be observed in Figure10(a).To guarantee both conductivity and flexibility of the surface structure, nanosilver was picked as the conductive material for the radiating patch.Furthermore, a copper foil with a thickness of 20μm was preferred as the ground plane material to cut costs and enhance module interchangeability.Figure 10(b) illustrates the bending ability of the antenna model.The radiating components on the antenna surface preserved excellent conductivity despite multiple tests with large-angle bending.

Figure
Figure 10．(a) Antenna Physical Prototype (b) Antenna Flexibility TestFinally, in order to verify that the finished antenna meets the requirements of use, the use of vector network analyzer on the antenna was to do an S11 return loss performance test, the test results are shown in Figure11, it can be seen that the measured data of the antenna compared to the simulation design has a certain offset, but still in the three central frequency points still maintain a high degree of matching, to ensure the stability of the three-frequency performance.

Table 1 .
Table of Structural Parameters for the Microstrip Antenna