Design A Flexible Antenna Integrated With Artifical Magnetic Conductor

A flexible printed monopole antenna integrated with artificial magnetic conductor (AMC) working at 2.45 GHz is designed and analyzed. The proposed antenna is suitable for the industrial, scientific, and medical (ISM) band, WIFI and Bluetooth applications. The artificial magnetic conductor (AMC) is used to enhance the antenna characteristics and to isolate the antenna from human body effects that produces by proximity coupling. The antenna has ultra–low profile which is about 0.028 λ0. The simulated results show a good improvement in gain from 0.82 dB to 2.61 dB, efficiency from 78.56 % to 88.72% and the radiation pattern change from omni-direction to uni-direction which it suitable for wearable applications.


Introduction
Recently, flexible antennas have spawned a considerable research interest due to their applicability to a large range of applications such as military, medical, scientific, firefighter, personal communication and radio frequency identification ( RFID) [1].Flexible wireless systems require specific antenna properties such as lightweight, low profile, compact and flexible.
Production and hosting by ISPACS GmbH.http://www.ispacs.com/journals/acte/2017/acte-00215/International Scientific Publications and Consulting Services These bring some challenges in communications field.There are different techniques are used to face these challenges and the important one is using metamaterial.The term "metamaterial" had been originally employed to describe any artificial (engineered) structure possessing effective electromagnetic properties not encountered among natural materials [2].On the other hand, metamaterial has spawned a considerable research interest due to their unique properties that is not found in the nature [2].One of the metamaterial classes which is adopted in this work is the artificial magnetic conductor (AMC).The AMC is an artificial material designed in a special manner to exhibit the perfect magnetic conductor behavior which is not existent in the nature.The Perfect Electric Conductor (PEC) exhibits 180º phase shift while Perfect Magnetic Conductor (PMC) exhibits a reflection phase of 0º.The reflection phase of AMC varies from -180º to +180º with frequency.When it is between -90 to +90 the image currents are more in phase than out of phase.It means that in a certain frequency band, AMC behaves as PMC.This in phase reflection behavior enables low profile antenna design using AMC as a ground plane [3].Therefore, the integration of an antenna with an AMC leads to exciting applications in flexible wireless system.In [4], design ultra-thin metamaterial absorber which can be helpful to achieve low profile antenna but, still not very low-profile.In [5], reduction in the flexible textile antenna back radiation achieved by using two mushroom-like structure metamaterial but the structure get more complicated by using vias.Moreover, textile based antennas are prone to discontinuities in substrate materials in addition to the textile nature of fluid absorption.In [6], a 2 x 2 array of mushroom-like transmission line metamaterial utilized to achieve compact, low-profile antenna but the antenna has monopole-like pattern which it is undesirable for on body communication.In [7], four cell U-shape metamaterial used to achieve low-profile antenna but, the final overall size still large.In this paper, a flexible antenna based on AMC with slotted Jerusalem Cross shape (SJC) is introduced.The antenna operates at 2.45 GHz.The AMC structure is utilized to provide in phase reflection which significantly enhances the front to back ratio and reduces the specific absorption rate (SAR).Furthermore, the proposed antenna is flexible and compact which makes it suitable for aforementioned application.The paper is organized as follows.In Section II, the proposed antenna and its properties before using AMC is described.In Section III, the AMC unit cell structure and the test results at 2.45 GHz are reported.Section IV discusses the final structure design after being combined with the AMC structure.Section V display the results of the final design and compared to the antenna properties without using the AMC.Section VI concludes the paper.The Computer Simulation Technology (CST) software 2015 based on finite integral technique (FIT) [8], has been used to simulate the antenna in the designed frequency and band.

Antenna Design
To achieve some of the flexible wireless system requirements, the substrate should be chosen to achieve flexibility and mechanically robust, There are multiple flexible substrates for antenna applications including liquid crystal polymer (LCP), polydimethylsiloxane (PDMS), silicone, and many others [9].A Kapton polyimide substrate with a dielectric constant of 3.4 and a loss tangent of 0.002 is chosen.Polyimide Kapton is chosen as a substrate because it exhibits a good balance of physical, chemical, and electrical properties with a low loss tangent over a wide frequency range.Also, Kapton offers a very low profile (50.8 µm) yet, a very robust with a tensile strength of 165 MPa at 73ºF.A dielectric strength of 3500 -7000 volts/mil and a temperature rating of 65 to 150ºC [10].On the other hand, the monopole antenna is adopted for the design because has several attractive feature compared to the traditional patch antenna such as large bandwidth and high efficiency.The proposed antenna is fed by a Coplanar Waveguide (CPW) transmission line which offers a single-layer fabrication process.Both the radiating element and the CPW are printed on the same side of 27 mm × 26 mm Kapton polyimide substrate.The antenna geometry and dimensions are depicted in Figure 1.http://www.ispacs.com/journals/acte/2017/acte-00215/International Scientific Publications and Consulting Services The above design has been obtained through simulations and parametric study using CST Microwave Studio, the reflection coefficient, realized gain and the total efficiency of the antenna without using AMC are depicted in figures 2, 3, 4, respectively.

AMC Unit Cell Design
A Jerusalem Cross shape (JC) as one of the frequency selective surface (FSS) shapes is used to design AMC unit cell structure (JC-AMC).The dimensions of AMC unit cell are chosen and test to be operated at 2.45 GHz.There are a lot considerable work using AMC, JC-AMC is reported in [11] along with a vacuum as a substrate resonating at 5.56 GHz.In [12] the authors used ring and cross structure to design AMC with paper substrate and resonant frequency at 5 GHz.The model analysis of AMC present in this paper showed in section III following the algorithms reported in [13].

The Motivation of USEING JC-AMC Structure Integrated to an Antenna
The JC-AMC structure is chosen to obtain an ultra-low profile design by reducing the distance between the antenna and the ground plane, to avoid the traditional condition of a separation greater than λ/4, as shown in figures 5, 6, 7 [14]:   Also, the specific absorption rate (SAR), a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency (RF) electromagnetic field, which is one of the main consideration for the antenna to be used near human body is reduced [12].Finally, the AMC allows the enhancement of the frontto-back ratio which leads to increasing the realized gain.

Analysis Model and Simulation of the Proposed Structure
There are several types of FSS as shown in figure 8 which can be used to design AMC [11]:

The Model Analysis
The proposed structure and the equivalent circuit of the Jerusalem Cross shape (JC) based AMC is depicted in Figure 9.The surface impedance is given as [13]: where Z g is the FSS grid impedance, Z d is the impedance of the grounded dielectric slab and L g , L d , C g are the grid inductance, dielectric slab inductance and grid capacitance, respectively.The resonant frequency is: and the bandwidth is obtained as [13]: The reflection phase of a surface with impedance Zs [14]: where η is the characteristic impedance =120π .When Zs is low, the reflection phase is ± π .When Zs is very high, the reflection phase is zero.The phase crosses through ± π /2 when Zs is equal in magnitude to the impedance of free space [12].AMC possess very high impedance surfaces Zs and that lead to zero reflection phase.

Simulation of the Proposed Structure:
In this design a (22.68 mm x 22.68 mm) Roger 3003 substrate is used with a thickness of 1.6 mm, epsilon equal to 3 and loss tangent 0.001.Roger 3003 material has been chosen for its flexibility to meet the standard desirable for modern wireless flexible systems, the radiated element is assumed as perfect electric conductor (PEC).The proper dimensions of the AMC are obtained through a parametric study and simulations in CST program which it is based on the AMC equivalent circuit depicted in figure 9. and equations illustrated in [13,14].The final proposed structure is depicted in figure 10, and the dimensions of the elements are listed in Table 1.
Figure 10: Schematic of the AMC unit cell dimensions.
Table 1: Dimensions and values of the design (all dimensions in millimeter).
The results show that the structure has a good reflection coefficient (phase in degree) which make it a good candidate to be a backed for an antenna operating at 2.45 GHz as depicted in figure 11, and as aforementioned since the AMC possess very high impedance surfaces Zs, that will lead to zero reflection phase.

Antenna Design with Using AMC
The structure consists of three layers: the first layer contains 3 x 3 of AMC unit cells printed on a flexible Roger 3003 substrate with 1.8 mm thickness and dielectric constant = 3.The second layer contains a foam with air characteristic used as a spacer between the AMC structure and the antenna with 1.8 mm thickness because the proximity between the antenna and AMC surface increases the mutual coupling between them.The mutual coupling may introduce a non-uniform near field wave that may shift the operating frequencies of the antenna [15].The third layer contains the antenna structure with characteristics and dimensions mentioned in section II, and as depicted in Figures 12, 13.

Simulation Results
The reflection coefficient has been simulated to study the effect of the AMC integrated with the antenna, the simulated reflection coefficient (S 11 ) of the antenna with AMC decreases as a compare with the antenna without using AMC and this can be attributed to the impedance mismatching caused be AMC vicinity [16], as depicted in figure.14.Radiation patterns of the designed antenna at 2.45 GHz on the YZ plane (E-plane) and XZ plane (H-plane) are illustrate in figure 15.It can be noted that the radiation pattern is changed from omni-directional to unidirection with using AMC as a compare to the conventional monopole antenna because the increase in the front to back ratio realized by the in phase reflection of the AMC ground plane [16].The realized gain enhancement after adding the AMC to the final structure (increased from 0.821 dB to 2.61 dB) and the efficiency is also enhanced after adding the AMC ( increased from 78.56 % to 88.72%), these can be shown in figures.The final comparison of the total results (with/without) AMC cells are listed in Table 2.
Table 2: Comparison Parameters of the antenna with/without using AMC cells.

Human Body and SAR
Since, the structure designed for wearable applications.It is necessary to concern to the SAR value.As, aforementioned SAR values must not overtake the exposure guidelines fixed by the Federal Communication Commission (FCC).The maximum value of SAR allowed in Canada and USA is 1.6 W/kg rated over 1g of tissue [17], and this will be a standard for the SAR measurement in this paper.

𝑆𝐴𝑅 =
| 2 |  (6.6)where  is the tissue conductivity in S/m, E is the electric field in V/m, and  the tissue mass density in kg/ In practical, the SAR measured by position the antenna with / without AMC on a HUGO human model.HUGO is an functional 3D size and surface data set of the human body which is riley on the observable human data set formed by the National Library of Medicine [18].unfortunately, the HUGO human model is not available in the University of Basra and this led to find another method to achieve SAR calculations.
To calculate the SAR value we will try to build part of human body ( not whole of the human body ), arm for example for two reason: firstly, the antenna size is very small as a compare with the human body and the effect of the SAR will limited to a small region of the body.Secondly, the simulation of all human body will required computer with super properties [19].Following the producer reported in [20] to calculate the SAR value and as below: A three-tissue layer model has been built with considering them electromagnetic properties, the layers consisting of skin (1 mm thick), fat (3 mm thick) for the second layer and the third is muscle tissue (40 mm thick) with 100 mW input power as illustrated in Figure 18.
Figure 18: Three layer human body.
After that, positioned the antenna on these layer in two case: with and without AMC as depicted in Figure 19, then simulated the structure by CSTMWS and calculate the SAR value for these two ceases.The calculations show that the SAR value for the antenna before using the AMC is 5.218 W/kg which is exceeded the specified rate allowed by the FCC, while the antenna with AMC achieved 1.56 W/kg.Therefore, the structure with AMC achieved 70% reduction in SAR value.SAR value for the two condition illustrate in Figure 20.Design of flexible antenna based on AMC (JC-shape) structure is present.The antenna operating at 2.45 GHz with bandwidth of 110 MHz with gain of 2.61 dB.The results for the final structure show that the antenna realizes gain and efficiency are enhanced by using AMC while the bandwidth and the reflection coefficient are decrease but remain in acceptable range.The gain enhancement achieves 1.79 dB increases in value and also the total efficiency is increased by 10.16 %.The radiation pattern changes from omnidirection to uni-directional with an ultra-low profile (0.028 λ 0 ) which it suitable for wearable applications.Also, the SAR value as been calculated by simulation part of human body closely of the proposed antenna and the results shown redaction in SAR value by 70%.

Figure 3 :
Figure 3: Realized Gain of the proposed antenna.

Figure 4 :
Figure 4: Simulated efficiency of the proposed antenna.

Figure 5 :
Figure 5: An antenna lying flat against a ground plane.

Figure 6 :
Figure 6: The antenna which is separated by 1/4 wavelength from the ground plane.

Figure 7 :
Figure 7: A flush-mounted dipole on AMC ground plane.

Figure 11 :
Figure 11: Simulated S 11 (phase in degrees) of the proposed AMC.

Figure 12 :Figure 13 :
Figure 12: The component of the final structure.(a) 3 × 3 AMC unit cells with Roger 3003 substrate.(b) The foam layer.(c) The proposed antenna.

Figure 14 :
Figure 14: Simulated S 11 of the antenna with and without including AMC.

Figure 15 :
Figure 15: The radiation pattern of the antenna, the E-plane and the H-plane.(a) without using AMC.(b) with using AMC.

Figure 16 :
Figure 16: Realized gain of the antenna with and without using AMC.

Figure 17 :
Figure 17: Simulated efficiency of the antenna with and without using AMC.
Figure 19: Antenna structure positioned on three layer of human body.(a) without AMC.(b) With AMC.

Figure 20 :
Figure 20: SAR values for the proposed structure (a) without AMC.(b) with AMC.

3 . Parameter antenna without using AMC antenna with using AMC S11 -21.7 dB -11.68 dB
International Scientific Publications and Consulting Services