Wide-Band Antenna on Flexible and Thin Substrate for Wireless Systems

This paper presents the design of a flexible monopole antenna for wideband applications. The proposed antenna is printed on flexible substrate material having a dielectric constant of 3 for the design and fabrication of the proposed antenna. The antenna dimensions are 3.8 cm × 4.1 cm × 0.025 cm. The antenna characteristics are investigated not only in flat conditions but also under bending configuration. Both flat and bending configurations are operated at frequency bands from 2.4 GHz up to 10 GHz with S- parameters lower than − 10 dB. The antenna parameters such as gain, efficiency, and radiation patterns are studied for both configurations. A deep study for time-domain response is discussed to validate the performance of the proposed antenna to operate at a wide band with a highly correlated response.


Introduction
In recent years, the repaid development of wearable applications and biomedical systems requires bendable and flexible devices. These devices can be used in smartphones, laptops, RFID tags, sensors, sports, fire fighters, IoT applications, and biomedical devices [1,2]. Flexible and curved antennas are considered the main part of wireless wearable systems [3]. Flexible antennas can take any shape curved, or folded without affecting the antenna parameters such as matching and radiation characteristics.
Wideband and ultra-wideband (UWB) systems can support applications with low power, fading reduction, security, lower penetration loss, and high data rates [4]. Federal Communications Commission (FCC) states that the spectrum of the UWB systems is from 3.1 GHz up to 10.6 GHz [5]. Thanks to UWB system features, researchers developed new 2 Antenna Design

Flat monopole Antenna Structure
First of all, the flexible substrate material RO3003 with good performance is chosen in the design. The substrate has a thickness, dielectric constant, and loss tangent equal to 0.254 mm, 3, 0.0013, respectively. The antenna configuration and the fabricated photo of the antenna are illustrated in Fig. 1. The antenna consists of a monopole antenna with a rectangular patch attached to a microstrip line with a width of 0.6 mm to obtain 50 Ω impedance. The rectangular patch has tapered edges and the ground plane has partial ground to achieve the wideband operation we need. The tapered edges angle and the partial ground length are optimized to obtain the desired frequency band. The proposed antenna has an overall size of 4.1 × 3.8 cm 2 . The substrate has the advantages of flexibility and robustness. The antenna is etched using chemical etching and the SMA connector is soldered using welding tin. The antenna is simulated using CST microwave studio and measured using VNA Agilent N9918A. Figure 2 shows the simulated and measured S 11 which confirms that first; the antenna is operated from 2.4 GHz up to 10 GHz with S 11 lower than − 10 dB. Secondly, the simulated results have the same trend as the measured results. However, a very small shift is seen between the two results this is due to the fabrication and the measured process. Figure 3 illustrates the flat antenna radiation patterns measurement inside an anechoic chamber. The measurements are obtained by using the antenna measurement system Starlab 18 [27] with a dynamic range of 60 dB in the frequency range from 1 to 18 GHz.The normalized simulated and measured radiation patterns at 3 GHz, 5 GHz, and 8 GHz are illustrated in Fig. 4. The antenna is measured at three planes x-z plane, y-z plane, and x-y planes. The patterns have low distortion at higher frequency bands

Effect of Bending on the Antenna Performance
In this section, we compare the results from the bent antenna with the previous flat antenna to show the effect of bending on the antenna performance and validate the flexibility of the  proposed antenna. The effect of the bending on the antenna performance at different antenna radii of curvature R is illustrated in Fig. 6a. It is clear that the antenna still achieved wideband operation from 2 GHz up to 10 GHz with S 11 lower than − 10 dB which confirms the antenna stability with bending. The previous antenna is curved using a cylindrical structure with a radius R of 62 mm and a dielectric constant of 1.03 as shown in Fig. 6b. The S 11 of the bending antenna is shown in Fig. 7. It is noticed that the antenna still has a wide band feature with bandwidth extended from 2.4 GHz up to 10 GHz with S 11 lower than -10 dB. Also, the bent antenna shows the same S 11 performance as the previous flat antenna which confirms the stability of the proposed antenna.
The bent antenna radiation patterns are measured inside an anechoic chamber as shown in Fig. 6c. The radiation patterns are compared with the radiation patterns of the flat antenna at 3 GHz, 5 GHz, and 8 GHz and at x-z, y-z and x-y planes as shown in Fig. 8. It is illustrated that the antenna has the same results as the previous flat antenna with slight deformation at the

Time Domain Study
Studying time-domain response is considered a vital part of wideband antenna design. The time-domain response claims that the designed antenna can transmit a narrow pulse in the time domain which translated to a broad band in the frequency domain to support wideband applications. This study can be done by fabricating two identical antennas (transmitting and receiving). Instead of putting the two antennas inside an anechoic chamber one is fixed and the other is rotated with different φ angles as [28,29], three different layouts face to face, face to the side, and side to side are used. Studying time-domain response is done for both flat and bent antennas. Firstly, the two identical flat antennas are separated by 110 cm as shown in Fig. 10 and two ports VNA are used to measure the insertion loss S 21 between them. The S 21 magnitudes of the three antennas orientations are shown in Fig. 11. It is seen that the S 21 magnitudes from 2.4 GHz up to 5.5 GHz have the same levels of around − 35 dB and the levels are reduced from − 35 dB to around − 55 dB at the upperfrequency band from 6 to 10 GHz. This is due to the radiation pattern features at the lower and upper-frequency bands. The measured S 21 phases in rad for the three configurations are illustrated in Fig. 12. By studying the phase between the two antennas is important to show the linearity behavior of the system. The nonlinear features can be distorted in the transmitted signal. The unwrapped rad phase as shown in Fig. 12b is calculated to enhance the visualization and to detect easily the nonlinearity regions of the phase. From Fig. 12, we can notice that the phase responses for all orientations are the same with a linear feature. However, the face-to-side orientation has deviated slightly from 8.5 GHz to the end of the band. So, after this study, we can claim that the proposed antenna behaves well in time domain response. At this point, to show the behavior of the received signal, the input signal is converted to the frequency domain by applying fast Fourier transfer (FFT) in Matlab and multiplied with the transfer function of the system S 21 as shown in Eq. (1). The Gaussian Pulse is chosen as the input pulse to apply to the system as shown in Fig. 13a, also the pulse parameters are chosen to cover all operated frequency bands as shown in Fig. 13b. The response of the system is calculated by converting the frequency domain to the time domain by inverse fast Fourier transform (IFFT) as illustrated in Eq. (2). The normalized received pulse at different three orientations is illustrated in Fig. 14. First, the three pulses have the same shape as the input pulse with the pulses centered at 2.2 ns with 1.7 ns delayed from the input signal. Second, the three pulses have small fluctuations around the pulse this is due to the nonlinear behavior of the phase response. Finally, we can conclude that the received signals are correlated with the input signal which means a good time-domain response.   Another important factor that should be studied to evaluate the time response and the quality of the received signal of the wideband systems is the system fidelity factor (SFF) [28]. The SFF is a factor ranging from 0 to 1 also this factor should be more than 0.5 to achieve an undistorted signal. The SFF can be calculated using Eq. (3).
(2) R s (t) = IFFT R s ( ) The input signal in the time domain is convoluted with the received signal in the time domain with variable shift τ then the output is divided on both input and received signals, then we took the maximum of the output and normalize it to achieve the calculated SSF. All of this process can be calculated using Matlab. The measured SSF equals 0.842, 0.853, and 0.913in the side-to-side, face-side, and face-to-face orientations. From SFF results, we can claim that the transmitted pulse can be received without distortion which means the proposed antenna can be used in wideband systems.
Secondly, The measured setup of the bending antenna is achieved with the same steps as the previous flat antenna as shown in Fig. 15. There are three different layouts of the antenna to measure the insertion loss S 21 between them. The S 21 magnitudes and phase are measured and plotted in Figs. 16 and 17, respectively. The S 21 magnitudes have the same levels of -30 dB up to 6 GHz and are reduced to around − 55 dB at the end of the band. The phase shows linear operation up to 8.5 GHz for the three configurations but the face-to-face configuration and side-by-side configuration are deviated from 8.5 GHz up to 10 dB.   The normalized Gaussian pulse is used as the input pulse and the normalized received pulses calculated from Eq. (2) are shown in Fig. 18. The received pulses show a good correlation compared to the input signal instead of little fluctuations around the pulses. The SFF is calculated for three configurations using Eq. (3). The measured SFF for the curved antenna equals 0.853, 0.912, 0.901 for side-to-side, face-to-side, and face-to-face orientations. From the previous results, we can claim that the proposed antenna gives stable performance even if the antenna is bent. Finally Table 1 shows our work compared with other research. From Table 1, we can conclude that our work has good performance which makes it a good choice for wideband and wireless systems.

Conclusion
A wide-band flexible monopole antenna printed on a thin substrate has been designed and fabricated. The proposed antenna has been worked at 2.4 GHz up to 10 GHz with well-matched behavior. Two flat and bending configurations are studied to show the flexibility behavior of the proposed antenna. The antenna average gain equals 3.8 dB with omnidirectional and bidirectional radiation patterns. The time-domain performance has been carried out with a cross-correlation higher than 90% between the Gaussian input pulse and the received pulse for both configurations. The measured and simulated results conclude that the proposed antenna can be used in wireless systems.