Time domain analysis for foldable thin UWB monopole antenna

Accepted Manuscript Regular paper Time Domain Analysis for Foldable Thin UWB Monopole Antenna Sherif R. Zahran, Mahmoud A. Abdalla, Abdelhamid Gaafar PII: DOI: Reference:

S1434-8411(17)30418-1 http://dx.doi.org/10.1016/j.aeue.2017.09.006 AEUE 52059

To appear in:

International Journal of Electronics and Communications

Received Date: Accepted Date:

20 February 2017 9 September 2017

Please cite this article as: S.R. Zahran, M.A. Abdalla, A. Gaafar, Time Domain Analysis for Foldable Thin UWB Monopole Antenna, International Journal of Electronics and Communications (2017), doi: http://dx.doi.org/ 10.1016/j.aeue.2017.09.006

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Time Domain Analysis for Foldable Thin UWB Monopole Antenna Sherif R. Zahran, M.Sc, Arab Academy for Science and Technology, Cairo, Egypt ([email protected]) Mahmoud A. Abdalla Ph.D MTC College, Cairo Egypt ([email protected]) Abdelhamid Gaafar Ph.D, Arab Academy for Science and Technology, Cairo, Egypt ([email protected])

Time Domain Analysis for Foldable Thin UWB Monopole Antenna Sherif R. Zahran Mahmoud A. Abdalla Abdelhamid Gaafar

Abstract In this paper, the time domain analysis of an Ultra Wide Band antenna flexible circular monopole antenna is presented. The antenna is fabricated on liquid crystalline polymer flexible substrate with a compact geometry that makes it suitable for wearable applications under different bending conditions. The antenna is fed by coplanar waveguide transmission line and has a compact total size of 40 × 22 mm2. Moreover, the antenna has good radiation efficiency (97 %) over the bandwidth. The presented antenna has a good performance over the operating spectrum for straight and bending configurations. The design principals along with simulation and experimental results are presented in this contribution. Index Terms—Antenna, CPW, flexible, ultra wide band, wearable.

I.

Introduction

The need for mechanically flexible wireless devices is growing exponentially due to their wide area of applications such as wearable and implantable devices for health monitoring systems in addition to the daily life wireless devices such as cell phones, tablets, laptops and others. For this purpose, the need for printed flexible antennas has increased in the recent years to be applied in biomedical applications [1], [2], wearable applications and body mounted applications [3], [4]. One technology that is in vital need for flexible devices is the ultra wide band (UWB) technology [5]. From spectrum point of view, ultra wide band technology has occupied the spectrum (1) from 3.1 GHz to 10.6 GHz as stated by Federal Communications Commission (FCC) [6] (2) two bands MB-OFDM (3.1 GHz-4.8 GHz) and DS-UWB (6 GHz-8.5 GHz) as stated by CEPT’s ECC [7]. Accordingly, the enormous bandwidth available, the capacity for high data rates and the potential for small size and low processing power along with low implementation cost present an appealing opportunity for UWB to become a widely used radio solution for future wireless indoor home-networking technology. Many novel tends in designing UWB antennas have been reported in recent years [8]. However, the need for designing flexible antennas for wearable applications require continuous innovation [9]. Different planar monopole antenna structures have been widely used in designing UWB antennas due to its capability to demonstrate constant omnidirectional pattern and input impedance parameters over the UWB frequency band. Moreover, it has a simple structure, small size, and can be printed on the very same PCB circuitry [10]-[13].

From another aspect, designing flexible antennas would require thin substrates, as a consequent the antenna’s radiation pattern properties tends to be degraded. Examples for flexible antennas in planar configurations are compared in Table I [14]-[27]. Time domain investigation for UWB antennas became recently a necessary study as the UWB pulse endures very short time given that huge frequency range, it is more likely to be distorted through the Tx Rx system this is one reason to study UWB antennas in time domain in order to predict the produced signal as in [27], [32]-[35]. In this paper, we use an ultra thin substrate to demonstrate a typical UWB flexible antenna with compact size and high bending functionality. The studied antenna is fed with CPW transmission line, the antenna design has been addressed and its performance has been checked using electromagnetic full wave simulations and experimental measurements. Time domain analysis of the antenna in both straight and bent cases are introduced.

Table I A comparison between recent published flexible antennas Ref.

Substrate

Size (mm3)

-10 dB S11 BW (GHz)

Avg gain (dB)

Antenna Type

[14]

Polymide

40×30×0.15

3.1-4.8

2

Planar dipole

[15]

Liquid Crystalline Polymer (LCP)

44×40×0.1

3.1-10.6

2

Monopole

[16]

Kapton Polymide

47×33×0.05

2-14

4

Monopole

[17]

Flexible Copper Clad Laminate (FCCL)

35×23.6×0. 05

3.2, 4.5 & 6-10.6

4

Monopole

[18]

ShieldIt

97×88×3

3.5-11

6

Surface radiators

[19]

Liquid Crystalline Polymer (LCP)

26×16×0.05

3.1-4.5&6-11

2.5

Monopole

[20]

Kapton Polymide

33×30×0.05

2 - 14

N/A

Monopole / slot

[21]

Polymide

32×18×0.12 7

3.5-10.6

3.5

Monopole

[22]

Cloth

40×30×0.75

3-10.6

2.5

Monopole

[23]

Kapton polymide

35×35×0.01 25

2.4

N/A

Inverted-F antenna (IFA) Monopole

[24]

Ceramic-filled Polytetrafluoroethylene (PTFE)

62×42×4

2.4

2

Monopole

[25]

Polymide

26×26×2.5

2.4

N/A

Slot dipole

[26]

Paper

92×25×0.00 6

1

-4

Meandered line dipole

[27]

Liquid Crystalline Polymer (LCP)

130×66×0.2

3.5-11

8

Planar Dipole

Presented

Liquid Crystalline Polymer (LCP)

40×22×0.1

2.5-11

2

Monopole

Fig. 1 The proposed antenna (a) geometry layout (b) photograph of the bent antenna Table II Antenna dimensions

II.

Par.

Dim. (mm)

Par.

Dim. (mm)

Par.

Dim. (mm)

L W

40 22

Lg Wg

18.9 9.7

Rp Wf

10.6 2.2

Par.

Dim. (mm)

D

0.45

Structure Geometry and Design In this section, the antenna structure is discussed with simulated and measured matching results.

A. Antenna Structure Geometry The design of the antenna is started by selecting a material that satisfies robustness, thermal endurance and flexibility conditions at which the antenna can perform effectively under extreme bending. Out of many materials filtered, Ultralam 3850 fulfilled the above mentioned mechanical requirements. The selected material consists of 100 m liquid crystalline polymer substrate with dielectric constant of 2.9 sandwiched by two 18 m thick copper cladding. The antenna was fabricated using careful photolithography process followed by chemical etching process. The antenna is existed by 50  CPW transmission line with width of 2.2 mm on its edge a SMA connector was soldered using special type of liquid solder. The antenna has total size of 40 22 mm2. As shown in Fig. 1 (a), the patch antenna is designed to be typical circular monopole the labeled parameters are indicated in Table II. Fig. 1 (b), represents a photograph of the fabricated antenna bent easily with fingers stress.

B. Antenna Matching Results The electromagnetic full wave simulation of the proposed flexible antenna was achieved using the commercial software (ANSYS-HFSS). The practical matching was emphasized by measuring the reflection coefficient using the Agilent FieldFox N9918A Vector Network Analyzer. Fig. 2, represents the simulated and measured reflection coefficient (S 11) of the proposed antenna, the simulated reflection coefficient result indicating that the associated -10 dB bandwidth of 2.6 GHz up to more than 10.6 GHz with high resonance at almost 7.4 and 10.6 GHz satisfying the total FCC and ECC UWB allocated bandwidth regulation while on the other hand the measured reflection coefficient indicates three resonances occur at 5.3 GHz, 6.4 GHz and 9.4 GHz there is a good agreement between the two results in the main shape. However, there is a shift in frequency between them more than 1 GHz at high frequencies while a total mismatch at lower frequencies, this is mainly due to the small electrical size of the antenna. Moreover the very thin substrate of the antenna along with the available SMA connectors made it difficult to measure using the accessible measuring tools.

Fig. 2 Simulated and measured reflection coefficient in dB against frequency in GHz

Fig. 3 Measurement setups for bent configuration (a) simulation (b) practical

III.

Far Field Radiation Results

In this section, two antenna setup configurations are presented; one is the straight antenna and the other one is the setup of bent antenna configuration over an imaginary cylinder with an adjustable radius (Rx) as shown in Fig. 3 (a). Rx is selected to be 25 mm. Fig. 3 (b), represents a photograph of the bent antenna configuration setup mounted over Styrofoam semi-cylinder. Distributed frequencies over the UWB spectrum (4 GHz, 6 GHz, 8 GHz and 10 GHz) were selected in order to investigate far field parameters for the antenna.

A. Gain Investigation Fig. 4 (a), and Fig. 4 (b), represent the simulated polar three-dimensional total gain in dBi for the straight and bent antenna configurations respectively, the figure indicates omnidirectional radiation for lower frequencies and the pattern tends to deform as frequency increases for both bent and straight configurations. As shown in the subfigure of Fig. 2, that illustrates the antenna’s far field measurement setup inside the anechoic chamber for both straight and bent scenarios. A comparison between measurement and simulation results in the sense of total gain radiation pattern in dBi for the three principal planes, at the investigated frequencies, is introduced in Fig. 5, for straight antenna configuration setup and Fig. 6, for bent configuration. Fig, 5, indicates that the antenna maintains its omnidirectional pattern represented in figure of eight for X-Y and X-Z planes for lower part of the spectrum and tends to deform as frequency increases aligning with Fig. 4 (a). Fig. 6, indicates some agreement between simulation and measured gain for the bent configuration in sense of gain magnitude.

Fig. 4 3D polar total gain in dBi for (a) straight configuration (b) bent configuration

Fig. 5 Measured and simulated 2D gain radiation pattern in dBi for the straight configuration

Fig. 6 Measured and simulated 2D gain radiation pattern in dBi for the bent configuration

From the previous investigated far field radiation patterns, the boresight angle is chosen to be at Φ=θ=0°. Fig. 7, represents the simulation of the total gain in dBi along with normalized radiation efficiency at boresight angle showing that the antenna gain tends to increase till it maximizes with value 4.2 dBi at 7.5 GHz and continues to decrease in value afterword’s, the radiation efficiency on the other hand tends to linearly increase as frequency increases.

Fig. 7 Simulated total gain and radiation efficiency at borsight angle

B. Polarization Investigation The linear polarization of an emitted radiation is examined by studying the ratio between the two projection components coexisting in a plane perpendicular to the direction of wave propagation (gain phi and gain theta). Fig. 8, represents a comparison between measured and simulated gain theta and gain phi at the formerly selected frequencies for straight antenna setup scenario, it is indicated that the difference between simulated and measured magnitude of gain theta and gain phi reaches 10 dBi to 17 dBi at the investigated planes. As a conclusion we can say that the polarization associated with the generated radiation within UWB spectrum is linear polarization.

Fig. 8 Measured and simulated 2D radiation pattern of gain theta and gain phi in dBi for X-Y plane

IV. Time Domain Analysis In the previous sections frequency domain characteristics had been studied and verified however, a good UWB antenna should show consistent time domain characteristics. As the UWB pulse endures very short time (<2 ns) utilizing that huge frequency range it is more likely to be distorted through the Tx Rx system, this is one reason to study UWB antennas in time domain in order to predict the produced signal. The transmission coefficient S21 is measured between two separated identical straight antennas connected to VNA ports in three different configuration setups: face to face, face to side and side to side, the three S21 measurement setups are photographed in Fig. 9. The distance between the antennas is set to 1 meter which is larger than the far field distance calculated for 10.6 GHz (shortest wavelength in UWB spectrum). This setup was intentionally constructed in such multi-reflections environment in order to mimic the real operational environment in which an indoor wearable UWB antenna operates in.

Fig. 9 Time response measurements setup for different configurations (a) face to face (b) side to face (c) side to side

A typical first order Rayleigh pulse with characteristic time (a=50 ps) is convoluted with system’s transfer function. The time domain representation of the S 21 (the inverse Fourier transform of (magnitude and phase)) previously extracted from VNA, considered as the system’s transfer function, using a simple MATLAB code the impulse response (IR) can be calculated as in equation (1). IR = Input signal * IFFT (S21)

(1)

It is worth to comment that any consistency in S21‘s magnitude and linearity in its phase within operation spectrum is considered as a good impulse response indicator. It can be noticed from Fig. 10, that the magnitude of S21 for different configurations is fluctuating between -40 and -60 dB. It is worth to mention that a curve smoothing function was applied to the S21 values, for clearer view of curves. Fig. 11 (a) and (b), represent linearity in phase that can be highly noticed in the unwrapped phase. As observed in Fig. 11(b), that face to face and face to side scenarios coincide in phase while side to side setup’s phase shows slight increase in its slope after 6 GHz. This can be explained as the transfer function (S 21) resembles the antenna radiation pattern, to have a linear system (with linear phase) the radiation pattern should be identical at all frequencies. Any degradation in the radiation pattern at a given direction would distort the received signal from the same direction. However, we can observe at Fig. 4(a), that there is some deformation in the pattern at higher frequencies compared to lower one. This problem becomes more critical if the magnitude of radiation is small in the case of direct transmission as the case of side to side.

Fig. 10 Measured S21 magnitude (transfer function) in dB for different straight antenna setups

Fig. 11 Measured S21 phase in rad for three straight antenna setups (a) normal (wrapped) phase (b) unwrapped phase

The interpretation of the time domain representation of the transfer function S21 (by applying the Inverse Fourier transform) is shown in Fig. 12. For better understanding, the simulation of the measurement setting in Fig. 9 was carryout out. As shown in Fig. 12(a), the simulated S21 values is not larger than 0 dB. However, the curve obtained based on the measured values which is shown in Fig. 12(b), has some ripples that may increase a

little bit above 0 dB. This can be claimed due to some possible constructive interference due to the multiple reflection from surrounding walls.

Fig. 12 The S21 in time domain for three straight antenna configurations (a) Simulation (b) Measured

To conclude, for emphasizing the functionality of the designed ultra-wide band antenna, Fig. 13, represents a comparison between the input signal and calculated impulse response pulses for three setups, noting that the received pulses are delayed by 1 ns in order to illustrate any distortions, it is observed that received signals are highly correlated to the input signal except for minor distorted repels.

Fig. 13 Input and received calculated normalized pulses for straight antennas setups

On the other hand, the same measurement procedures were conducted while both antennas were bent upon semi cylinder Styrofoam in order to study the effect of radiation pattern deformation due to bending on the impulse response of the antenna. Fig. 14, represents considerable linearity of unwrapped phase in radians for face to face and side to side scenarios in which they almost coincide in phase while face to side setup’s phase shows higher slop. This deviation is considered due to the deformation of face to side radiation pattern generated from bent antenna as shown in Fig. 4 (b). Similar to the studied straight antennas, the time domain representation for the measured transmission coefficient (S 21) between two bent antennas is extracted and plotted in Fig. 15 for the three setups. It is obvious that the response has slight higher variations in the magnitude compared to the straight one in Fig. 12 (b). Finally, for completeness of results, Fig. 16, represents a comparison between the input signal and calculated received pulses (delayed by 1 ns) for the three bent antennas setups, it is observed that received signals are highly correlated to the input signal except for minor distortion in side ringing patterns.

Fig. 14 Measured S21 phase in rad for three bent antenna setups (a) normal (wrapped) phase (b) unwrapped phase

Fig. 15 Measured S21 in time domain for three bent antenna configuration setups

Fig. 16 Input and received calculated normalized pulse for bent antennas setups

V.

Conclusion

This work presents a typical circular monopole CPW-fed antenna fabricated using flexible LCP substrate. Simulation and measurement were conducted for both straight planar and bent scenarios. The measured 10 dB reflection coefficient bandwidth is 81% of the FCC’s UWB regulation. Anechoic chamber measurement of the far field characteristics showed omni-directive radiation pattern at lower frequencies, boresight gain reaches 4.2 dBi at 7.5 GHz while maintaining high radiation efficiency. Linearity in radiation polarization is concluded in agreement with simulation results. Time domain characteristics represented in impulse response was estimated, the received pulses from different setup scenarios showed high correlation when compared to the input pulse with slight degradation in received pulses in case of bent antenna scenario. The antenna’s compliance with UWB requirements even under bending conditions considering its compact size and basic design makes it suitable for commercial UWB wearable applications.

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