Liquid Crystal Polymer-Based Flexible Trap Ultra-Wideband Antenna Design

Te wearable ultra-wideband antenna that is appropriate for body area networks is designed and implemented in this article. Stepped circular radiation patches, microstrip feeders, and trapezoidal foors make up the majority of the antenna. Good bending qualities have been attained by using liquid crystal polymer, a fexible material, as a dielectric substrate. An antenna measuring 30 mm × 35mm × 0.1mm was designed with two C-shaped slots etched on it to achieve notch function in the 3.20–4.10GHz and 5.19–5.98GHz frequency bands. Its working frequency range was 1.96–11.61GHz, and its specifc absorption rate for the human body was less than 2W/kg. Te test fndings demonstrate that ultra-wideband body area network systems can be implemented using the antenna impedance and radiation characteristics.


Introduction
Since 2002, when the Federal Communications Commission (FCC) allocated the 3.1-10.6GHz band to the civil ultrawideband (UWB) technology, it has received a lot of attention in the feld of wireless communication because of its advantages of low power consumption, high accuracy, and low complexity [1,2].Te performance of antennas will have the most direct impact on the overall performance of wireless communication.Ultra-wideband antennas have the advantages of high transmission efciency, high processing gain, good concealment, high multipath resolution, low cost, and low power consumption [3].It is with these advantages that ultra-wideband technology is widely used in highprecision positioning, high-speed multimedia wireless communication, radar, control, medicine, and other felds [4].
Te research direction of ultra-wideband antennas is mainly focused on the traditional bandwidth as well as on the antenna [5,6] size, where microstrip antennas have signifcant advantages over other forms of antennas [7].In the literature [8], a tapered slot UWB antenna was designed to increase the bandwidth and reduce the area of the antenna by etching a series of rectangular slot lines, and the bandwidth was increased by 4.72 GHz compared to that without the etched slot lines.Te UWB antenna designed in the literature [9] was fed by microstrip lines and operated from 2.8 to 12.5 GHz, and the bandwidth was extended by increasing the resonant point by etching the slot lines on the ground plate.
In addition to this, another key issue of ultra-wideband antennas is the interference of narrowband signals in the ultra-wideband band [10].To solve the interference of narrowband signals, the trapping function is designed.Currently, the main methods to achieve trapping are etching geometric slits of diferent shapes [11], placing parasitic cells, and adding resonant branches [12].In the literature [13], an ultra-wideband antenna with trapping characteristics was proposed to achieve the trapping function in the 5.3-5.8GHz and 7.85-8.55GHz bands by adding two different sizes of open resonant ring SRRs next to the microstrip feed line.However, when the antenna operates at a low-frequency band, the placed parasitic units and the added resonant branches are too long, which increases the antenna size and is not conducive to antenna miniaturization.In the paper [14], a "swan-" shaped ultra-wideband antenna is designed to avoid interference from WLAN systems by etching a "U-" shaped gap in the microstrip feed line.Te ultra-wideband antenna proposed by the authors in reference [15] has an operating bandwidth of 2.5-12 GHz, and the trap function in the operating band is achieved by etching slot lines on the radiating patch and loading the electromagnetic band gap structure (EBG) on the side of the microstrip feed line.A circular nested UWB antenna was designed in the literature [16].Te introduction of the electromagnetic band gap structure (EBG) at the center of the front of the dielectric substrate makes the antenna produce the trap feature at 4.0-5.2GHz, and the antenna size is 60 mm × 60 mm, which is large and can suppress the interference in only one frequency band.Te dielectric substrate of these ultra-wideband antennas [17] is generally made of rigid material, which is less fexible and cannot meet the application of the wearable system and body area network WBAN.
With the development of miniaturization and convenience of electronic communication devices and wearable terminal devices, many antennas using fexible materials as dielectric substrates have emerged.In the literature [18], a fexible ultra-wideband antenna is proposed, which is fabricated by miniaturized full inkjet printing and has an operating bandwidth of 3.43-10.1GHz and no trap function.In reference [19], a wearable antenna is designed for wireless body area networks with an operating bandwidth of 4.55-13 GHz, using a fexible textile substrate with a thickness of 4.68 mm and no trap band.In the literature [20], an ultra-wideband antenna for a body area network is designed with a three-petal fower-shaped patch printed on a substrate of jeans fabric with an antenna size of 60 mm × 60 mm × 2.16 mm and with no trapping functions.
In this paper, a fexible band trap ultra-wideband antenna based on LCP (liquid crystal polymer) is designed.A novel polymer material called liquid crystal polymer (LCP) has good bending properties, which can be used to create antennas that are appropriate for wearable technology and body area networks.In this work, we designed an ultrawideband antenna with a liquid crystal polymer dielectric substrate.Te antenna size is 30 mm × 35 mm × 0.1 mm, and its operating band range is 1.96-11.61GHz.It is fed by a coplanar waveguide, which makes it easy to integrate into the circuit.Te antenna is made up of a modifed radiation patch, a 50 Ω microstrip feed line, and a trapezoidal ground plate.Etching slits on the radiation patch can suppress 3.20-4.4GHz.It is able to suppress the interference in the bands of 3.20-4.10GHz and 5.19-5.98GHz.Te efect of fexible bending of the antenna on the S-parameters is also analyzed, and the efect on the human-specifc absorption rate is experimented.

Antenna Design and Analysis
2.1.Antenna Structure Design.Figure 1(a) illustrates the antenna structure which was designed in this paper.Te processed antenna is physically depicted in Figure 1(b).Te size of the antenna is (L × W) 35 mm × 30 mm.Liquid crystal polymer is used as the dielectric substrate, the relative dielectric constant is 2.9, the loss angle tangent is 0.002, and the thickness of the dielectric substrate is 0.1 mm.
A modifed rectangular plus semicircular patch, a 50 Ω microstrip feed line, and a trapezoidal grounding plate make up the antenna unit.By cutting the corners of the foor and rounding the corners of the patch, the antenna's bandwidth can be expanded.Te antenna is fed by a coplanar waveguide.Te dual trap characteristics of the WLAN band and WiMAX band are obtained by etching two C-shaped slits in the radiating patch to prevent interference from narrowband signals.Te length of the etched C-shaped slit is determined by the following equation, where f notch is the trap center frequency, ε e is the efective permittivity, ε r is the relative permittivity, c is the speed of light in free space, and L is the slit length. ( Table 1 gives the optimized antenna parameters, according to the data processed and produced by the antenna physically as shown in Figure 1.

Antenna Design Process. Te antenna's design process is
shown in Figure 2. Two rectangular grounding surfaces, called Ant-a, and a rectangle radiating patch make up the antenna in Figure 2(a).Ten, in Figure 2(b), the top of the rectangular radiation patch is improved to be semicircular, and the rounded corners are cut on both sides of the patch, named Ant-b.In Figure 2(c), the bottom of the radiation patch is improved by cutting the rounded corners, named Ant-c.In Figure 2(d), the rectangular ground plane is improved to a trapezoidal ground plane, named Ant-d.Te comparison of the four antennas' return loss simulation results is displayed in Figure 3.
Te design process of the antenna is shown in Figure 2, and the results of the S-parameters in the design process are shown in Figure 3. From the loss curve in the fgure, it can be seen that the antenna in step 1 is obtained by estimating the patch size through the equivalent cylindrical method, and the antenna bandwidth is 2.13-4.55GHz at this time, which is insufcient for ultrawideband communication.Steps 2 and 3 improve the resonant frequency and further expand the bandwidth of the antenna by improving the top of the patch with a semicircular shape and cutting the bottom with rounded corners.Finally, step 4 improves the resonant frequency and S 11 parameters of the antenna by changing the rectangular foor to a trapezoidal foor, and after the abovementioned steps, the bandwidth of the antenna is 2.17-11.74GHz with the basic performance of the ultrawideband antenna.

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Impedance Characteristics Analysis of the Antenna.
In the design of the ultra-wideband antenna, the height of the ground plate Lg and the gap between the ground plate and the feed line g are two important parameters to simulate and analyze the impact on the impedance characteristics of the antenna, and the simulation curve is shown in Figure 4. From Figure 4(a), we can see that Lg takes the value of the change, and the antenna performance at high frequency has a certain impact and almost no impact on the low frequency part.As the value of Lg increases, the bandwidth at high frequency gradually decreases, so we take Lg as 9.2 mm. Figure 4(b) shows that, with the gap between the ground plate and the feed line g value becoming smaller, S 11 parameters in the ultra-broadband band to −10 dB, so take g as 0.13 mm.

Analysis of the Trap Characteristics of the Antenna.
Figure 1(a) depicts the antenna structure.Te trap function is introduced by etching slits on the radiating patch.Figure 5 shows the simulation curve.Te frequency of the blocking band can be infuenced by the width of the slot line and the radius of the two etched C-shaped slits.Te efect on the trapping characteristics is simulated and analyzed by using the radius R 3 of the relatively large Cshaped slit and the width S 2 of the relatively small C-shaped slit.As seen in Figure 5(a), the trap center frequency shifts to the low frequency with an increase in the radius R 3 of the Cshaped slit.Te relatively wide C-shaped slit causes the antenna to trap in the WiMAX band, while the comparatively small C-shaped slit causes the antenna to trap in the WLAN band.As the width of the C-shaped slit S 2 grows in Figure 5(b), the WLAN trap band center frequency changes to higher frequencies.

Results and Discussion
Tis section discusses the various performances of the antenna based on Ansoft HFSS 19 and Keysight N5234A PNA-L vector network analyzer.Figure 6(b) gives the antenna simulation and the measured antenna gain; by the test environment, the measured antenna gain at low frequency is slightly lower than the simulated gain, but at high frequency, it is better than the simulated gain; the gain can reach 7.78 dB, and in the trap band, the gain has a signifcant reduction, indicating that the antenna cannot work properly in the trap band and can shield the interference of the two trap bands.

Current Distribution.
In order to analyze the trap characteristics of the antenna, two trap frequencies of 3.6 GHz and 5.4 GHz are selected, and the current distribution on the antenna's surface is then simulated and examined, as illustrated in Figure 7.When the antenna operates at 3.6 GHz, the feed line and the relatively large Cshaped slit receive the majority of the antenna surface current.Since the current fowing in both directions from either side of the slit is opposite at this point, the generated felds cancel each other out and are unable to radiate out efectively, which results in the trap characteristics.When the antenna works at 5.4 GHz, the surface current is mainly concentrated at the relatively small C-shaped slit and the feed line.

Radiation Patterns.
Figure 8 shows the anechoic chamber environment at the time of the antenna's actual radiation direction diagram.Figures 9(a)-9(d) show the simulated radiation patterns of the antenna at 3 GHz, 5 GHz, 7 GHz, and 9 GHz, respectively.As can be seen from this fgure, the H surface of the antenna is approximately circular, the radiation pattern of the antenna is approximately omnidirectional and does not change much within the operating bandwidth, and the E surface of the antenna is "8" shaped.Work in the high-frequency band will produce more electromagnetic waves, and the direction of maximum radiation will change.Overall, the radiation properties of the antenna in the working band range are good.Te measured direction of the antenna is shown in Figures 10(a)-10(d).Te test environment is to blame for the modest distortion of the observed radiation direction map.Te omnidirectional antenna's radiation properties are afected by the bracket that was installed behind it during testing, making it impossible to measure the radiation direction map of the antenna correctly in an anechoic chamber.

Flexible Properties.
With a relative permittivity of 2.9 and a loss angle tangent of 0.002, liquid crystal polymer (LCP) material is used as the dielectric substrate in this paper's ultra-wideband antenna design.LCP is a fexible material that exhibits good fexibility, allowing the antenna to be bent and conformal to the carrier surface with good molding performance.In addition, LCP ofers a broad research prospect in the feld of wearable antennas.
Figure 11(a) depicts the antenna's bending simulation design.Te antenna is bent horizontally, and the infuence on the antenna's S 11 characteristics under various bending radii is simulated.R i represents the bending radius.Ten, as indicated in Figure 11(b), R j is chosen as the bending radius, and the impact of bending in a vertical direction on the antenna's S 11 characteristics is examined and simulated.Te simulated S 11 parameters for the antenna bending in both the horizontal and vertical directions are shown in Figures 12(a) and 12(b), respectively.When the antenna is bent along the horizontal and vertical directions, the operating bandwidth at high frequency becomes smaller compared

Te Efect of the Human Body on the Antenna.
In order to verify the wearability of the antenna, the designed antenna is placed on the human chest position, arms, and legs to measure the return loss S 11 , so as to verify the impact of the human body on the antenna performance, and the placement position is shown in Figure 13.Te antenna is placed on the clothing and the human body conformal will have a slight bend; the measurement results are given, the lowfrequency part of the operating band and the center frequency of the trapped wave and the horizontal placement of the measurement results change slightly, but basically can meet the design requirements.
3.6.Human-Specifc Absorption Rate Simulation.Te human simulation model, as illustrated in Figure 14, is established in accordance with the literature [21] to verify the scenario of the fexible antenna applied to the body domain network.Te analysis of the SAR value of the human body-specifc absorption rate is used to measure the efect of the radiated energy on the human tissue during the operation of the fexible antenna.Te dimensions of the human body model are 45 × 50 × 40 mm 3 .It is composed of four layers: a skin layer that is 2 mm thick, with a conductivity of σ � 1.49 (S/m) and where E is the root mean square value of the electric feld strength, ρ is the density of human tissue, and σ is the electrical conductivity.Table 2 displays the highest SAR values that were obtained from the simulation at various operating frequencies when the input power was set to 0.2 W and the distance between the antenna and the human tissue model was h 1 � 5 mm.Te human body simulation model is established for SAR value simulation [22].Te highest SAR value of 10 g of human tissue is less than 2 W/kg, which is lower than the EU standard for all simulated maximum SAR values.Assuming a distance of h 1 � 5 mm between the antenna and the human tissue model, Figure 15 displays the SAR distribution on the 10 g model at various operating frequencies.Te comprehensive comparison between this work and some other wideband antennae is conducted in Table 3.International Journal of Antennas and Propagation

Figure 4 :
Figure 4: Efect of diferent Lg and g taking values on S 11 .

Figure 5 :Figure 6 :
Figure 5: Efect of diferent values of R 3 and S 2 on S 11 .

Figure 7 :
Figure 7: Current distribution at the trap frequency of the antenna.(a) 3.6 GHz.(b) 5.4 GHz.

Figure 11 :
Figure 11: Schematic diagram of the antenna under diferent directions of bending.(a) Bending along the horizontal direction.(b) Bending along the vertical direction.

Figure 12 :FigureFigure 14 :
Figure 12: Efect of bending in diferent directions on S 11 .(a) Bending along the horizontal direction.(b) Bending along the vertical direction.

Figure 15 :
Figure 15: SAR distribution of 10 g of tissue simulated on the human tissue model at diferent operating frequencies.(a) 3.5 GHz.(b) 6 GHz.(c) 9.5 GHz.
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Table 2 :
Maximum SAR values at diferent frequencies.

Table 3 :
Comparison of several types of antennas.