Arab J Sci Eng DOI 10.1007/s13369-015-1819-2

RESEARCH ARTICLE - ELECTRICAL ENGINEERING

U-Shape Slot Antenna Design with High-Strength Ni54 Ti46 Alloy Adnan Kaya1 · Irfan Kaya2 · Haluk E. Karaca3

Received: 7 October 2014 / Accepted: 10 August 2015 © King Fahd University of Petroleum & Minerals 2015

Abstract In this paper, a compact, printed, capsuleU-shape ultra-high-strength Ni54 Ti46 antenna that can be used in biomedical and communication applications is presented. An accurate electromagnetic model of the shape memory alloy antenna is developed using CST Studio for numerical analysis. The 10 dB return loss bandwidth of the proposed Ni54 Ti46 antenna is 8 % GHz, which covers the recently proposed 802.11 and 802.15 applications. Radiation performance has been evaluated using FIT simulation to show that the proposed antenna can be used for ISM band telemetry applications. Ni54 Ti46 U-shape antenna operating at 2.4 GHz having 50 beamwidth and 6.96 dBi gain has been utilized as a reference antenna. The impedance bandwidth of the antenna has been enhanced from 4.2 to 8 % by using aged Ni54 Ti46 alloy. However, the conductivity increased after the aging process and antenna’s return loss and frequency levels are shifted. In addition, it has been shown that Ni54 Ti46 antenna significantly develops the radiation pattern. The computed and measured results showed good agreement for copper, aluminum and Ni54 Ti46 antennas. Keywords WiFi · Shape memory alloy · Bandwidth improvement · Antenna · NiTi

B

Adnan Kaya [email protected]

1

Department of Electrical and Electronics Engineering, University of ˙Izmir Katip Çelebi (A.O.S.B.), Mahallesi Havaalanı Sosesi ¸ No: 33/2 Balatçık, 35620 Çi˘gli, Izmir, Turkey

2

Department of Mechanical Engineering, Anadolu University, 26555 Eskisehir, Turkey

3

Department of Mechanical Engineering, University of Kentucky, Lexington, KY, USA

1 Introduction RFID and ISM (industrial, scientific and medical) band antenna design is quite challenging because of the constraints on its size, power consumption and compatibility. The environment temperature can also be added on these mentioned constraints. ISM band communication systems are being increasingly used in many commercial and military applications, such as imaging systems, medicine, high-resolution radar biotelemetry and mobile communication systems [1,2]. The ISM band frequencies are very susceptible to attenuation due to rain, snow and fog (depending on which part of the band). In this band, microstrip slot antennas have received considerable interest since they are low profile, cheap to manufacture and compatible with microwave integrated circuit (MIC) designs [3]. Although slot antennas suffer from low efficiency and narrow impedance bandwidth, they are very popular. Most often, the return loss is the main parameter limiting the antenna bandwidth. Wideband single-layer slot antenna design can be achieved by utilizing the multiresonance characteristics of single stubs by using active loading, shorting posts, recutting slots and active elements or adding lumped elements [2,4]. In ISM band communication applications, antennas may be used in harsh environments where replacing the current metals with a more suitable material increases the system reliability. Some antennas can dynamically adjust the antenna characteristics such as the far-field radiation pattern, center frequency or main lobe direction (squint) by changing the operating conditions such as an ambient temperature. For example, reconfigurable antennas that have the ability to alter its properties with working temperature can dynamically control their pattern to improve their reception, transmission or treatment by increasing gain, suppressing interference or combination of these [1]. Shape memory alloys (SMA) are

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promising for antenna configurations since they have ability to undergo phase transformation with a change in temperature [5]. Their shape memory effect can be used to change their physical properties such as electrical resistivity [6]. They can also be used as self-sensing actuators [7,8]. Some recent studies have used various SMAs as replacements for metals [9]. SMAs are being used in the aerospace industry as a replacement for conventional metals because of their higher strength, lower cost and lighter weight [8]. The shape memory effect has been observed in many alloys such as CuAlNi, CuZnAl and AuCd, but NiTi, also known as Nitinol, is the most commonly used due to its good characteristics of corrosion resistance, high recoverable deformation, ductility, relatively high electrical conductivity and biocompatibility [9]. Nitinol wires are used to fabricate flexible whip antennas for mobile terminals, in the electromagnetic context [10]. Nickel-rich NiTi alloys have high strength and good corrosion resistance properties as compared to equiatomic NiTi SMAs. Recently, in a Ni54 Ti46 , shape memory effect was observed under an ultra-high stress level of 1500 MPa [11]. In [12], it was reported that shape memory effect causes changes in many internal properties of the SMA material, one of them being the electrical resistivity of the alloy. However, the complex dependence of the resistance on temperature and stress is generally not well understood. Thus, developing a model for the resistance and radiation behavior of SMA is the main focus of this paper. The important electrical properties of propagation media are relative permittivity, conductivity, dielectric loss tangent and penetration depth. It should be noted that relative permittivity and conductivity of media are frequency dependent [13–15]. Therefore, an antenna which is placed in media should be designed by considering the electrical properties of its environment. Because of the favorable mechanical and electrical characteristics, SMAs have been of interest in microwave antenna applications [16–18]. Moreover, the density of NiTi material is around 6.5 g/cm3 which is lower than that of copper (8.96 g/cm3) that could lead to fabrication of lighter antennas [18]. In this study, a novel U-shape slot antenna is proposed which is made of high-strength NiTi SMAs and works on short range with low complexity and very low power consumption. This article is organized into four main sections. The first section is concerned with the general knowledge on NiTi and microwave antenna applications. The second section is focused on the material properties of NiTi SMAs and preparation of slot antenna excited by microstrip line, and the third section is concentrated on revealing the temperature sensitivity of antenna. The fourth section reveals the NiTi U-Shape antenna design and experimental measurements. Conclusion is based on new-type antenna and is discussed in the last section of this paper.

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2 SMA Material and Method of Antenna Preparation There are various physical and mechanical properties of SMAs such as yield strain, hardness, Young’s modulus, bend resistance, roughness, expansion, electrical resistance, thermal conductivity and heat capacity that need to be considered for an antenna design [10]. The Ni54 Ti46 alloys are used to build the antennas with the effective resistivity and conductivity over the desired frequency range. The thin Ni54 Ti46 transmission line shows high electrical conductivity of 71,430 S/m. Due to its high conductivity, Ni54 Ti46 can replace the conventional metals used in various electronic applications. For antenna fabrication, Ni54 Ti46 was printed on a substrate and then cut by EDM to the desired antenna pattern. HP 8753D VNA was used for frequency response measurements of the designed antenna system by two por measurements. In this work, we have explored the potential of Ni54 Ti46 alloy for wideband ISM band antennas. A low-profile wideband coax-fed U-shape antenna that could operate over 2.4 GHz is designed, and its properties were investigated by experimental measurements and numerical simulation.

2.1 Substrate Plating FR4-glass epoxy with εr = 4.34 and tan δ = 0.0016 was used as the substrate of the antenna. The copper was removed from the FR4-glass epoxy substrate by etching with hydrogen peroxide/hydrochloric acid from both sides. Patches of 35 × 16 mm2 were cut out from a sample sheet.

2.2 Shape Memory Alloy Conductivity The Ni54 Ti46 ingots were homogenized at 1000 ◦ C for 4 h in argon atmosphere in evacuated quartz tubes, followed by quenching into water. After homogenization, they were aged at 550 ◦ C for 3 h. Aged SMAs were cooled down by furnace cooling (FC) where 2 ◦ C/min cooling rate was applied. Transformation temperatures of the material were determined with a cooling and heating rate of 10 ◦ C /min in helium atmosphere by PerkinElmer 1 differential scanning calorimeter (DSC) as shown in Fig. 1a [11]. Kelvin method was used for resistivity measurements as shown in Fig. 1b, c. Any voltage dropped across the main current-carrying wires will not be measured by the voltmeter that helps to avoid the errors caused by wire resistance. The electrical properties of various NiTi SMAs are measured and shown in Table 1.

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Fig. 1 a PerkinElmer Pyris 1 differential scanning calorimeter, b resistance measurement setup, c equivalent circuit, d geometry

2.3 Effects of Thermal Treatments and Composition

have a higher potential for antenna applications due to its low thermal expansion coefficient.

The resistivity of NiTi alloys can be considered as having two components as:

3 Temperature Sensitivity of NiTi Antenna ρ = ρres + ρth

(1)

where ρres is the residual resistivity and ρth is the resistivity that depends on the temperature of the metal. An empirical equation is: ρ = ρ0 [1 + α0 (T − T0 )]

(2)

where ρ0 is the resistivity (in  cm) and α0 is the temperature coefficient of resistivity (in ◦ C−1 ), defined at a reference temperature. Table 1 shows the conductivity of selected NiTi alloys. The conductivity of Ni54 Ti46 is 71,430 S/m at room temperature, while it is 35,587 S/m at 140 ◦ C as shown in Table 1. The performance of the ∂ R A /∂ T for the Ni54 Ti46 is 0.9 (m/◦ C). The conductivity of Ni54 Ti46 and Ni50 Ti50 is 35,587 and 86,956 S/m at 140 ◦ C, respectively. The conductivities of as-received Ni50 Ti50 and as-received Ni54 Ti46 are examined, and the conductivity of Ni50 Ti50 is found to be higher than conductivity of Ni54 Ti46 at room temperature. However, Ni54 Ti46 SMA is selected for antenna design due to its superior strength [11]. Ni54 Ti46 was homogenized at 1000 ◦ C for 4 h and quenched in water, and its conductivity is measured at room temperature. The conductivity of Ni54 Ti46 increased to 78,740 S/m after homogenization. Following homogenization, Ni54 Ti46 SMA was aged at 550 ◦ C for 3 h, and better conductivity (117,647 S/m) was obtained. All these NiTi alloys suffer from the poor radiation characteristics, complexity and enlarged element size. The radiation efficiency depends on the geometry and material. Thermal expansion coefficient of Ni54 Ti46 is 11.3 × 10−6 /◦ C for austenite phase and 6.6 × 10−6 /◦ C for martensite phase, while the thermal expansion coefficient of copper is 17 × 10−6 /◦ C [6,10]. Compared to the copper, Ni54 Ti46 SMAs

The resonant frequency of a microstrip antenna (MSA) is sensitive to large temperature variations. Geometry and material type are the two major factors affecting the resonant frequency of a microstrip radiator exposed to large temperature variations [2]. In the proposed MSA, the radiating element is made of as-received, homogenized and aged Ni54 Ti46 alloys, and copper is used for the ground plane. 3.1 NiTi SMA Expansion or Contraction The thermal expansion/contraction of the NiTi radiating patch due to a change in temperature affects the resonant frequency. With an increase in temperature, the metallic patch expands, resulting in an increase in the effective resonant length and decrease in the operating frequency. The dependence of relative frequency on length may be expressed as [2]: 1 δL δf = −αd δT =− f0 2 L

(3)

where δ f is the change in resonant frequency, δL is the change in effective resonant length, αd is the thermal coefficient of expansion, and δT is the temperature change in ◦ C. 3.2 Effective Dielectric Constant Change Most of the substrates which are generally used for antenna applications (e.g., polytetra-fluroethylene-based materials, teflon/fiberglass-reinforced materials) exhibit a decrease in dielectric constant with an increase in temperature. The change in operating frequency of a microstrip antenna due to a small change in dielectric constant can be expressed as follows [1]:

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Conductivity (S/m)

263,200

71,430

78,740

117,647

0.00038

0.0014

0.00127

0.00085 10 2.85 4.9 0.159 1.0 0.24, 1.2 7.80 × 34.5 Ni54 Ti46 aged sample 550 ◦ C FC martensite +25

7 1.56 4.9 0.235 1.0 0.24, 1.2 7.80 × 34.5 +25

Ni54 Ti46 homogenized 4h WQ austenite

1000 ◦ C

7

7 1.43

1.42 4.9

4.9 0.115

0.148 0.9

6.45 × 21.3

1.1

0.25, 1.5

0.42, 1.2

4 × 40

Ni54 Ti46 as-received austenite +25

Ni50 Ti50 as-received martensite +25

t (mm), weight (g) Material Temp (◦ C)

Table 1 Electrical performances of different NiTi

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δf 1 δεr 1 =− = + αε δT f0 2 εr 2

(4)

where δεr is the change in εr , and αε is the thermal coefficient of dielectric constant. By combining (3) and (4), the change in resonant frequency due to a temperature variation can be expressed as:   1 δf = −αd + αε δT f0 2

(5)

4 Experimental Results

W×L (mm)

∂ R A /∂ T (m/◦ C) App.

Resistance ()

Rated current (Å)

Rated voltage (N)

Power consumption approx (W)

Resistivity ( cm)

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Ultra-high-strength Ni54 Ti46 alloys can be used as a conductor for microwave circuit. Here, Ni54 Ti46 microstrip line which is the simplest microwave circuit is fabricated to demonstrate its capability. Figure 2a displays the configuration of such a straight microstrip line printed on a FR4–glass epoxy substrate. Figure 2b shows the DSC result of the aged Ni54 Ti46 sample that is furnace-cooled after aging at 550 ◦ C for 3 h. The transformation temperatures were determined by using intersection method as: Rs = 56 ◦ C, Rf = 31 ◦ C, MsI = 20 ◦ C, MfI = 11 ◦ C, MsII = −3 ◦ C, MfII = −38 ◦ C, As = 32 ◦ C, AM f = ◦ C [11]. The Vickers hardness test was = 57 43 ◦ C, AR f conducted for homogenized and aged Ni54 Ti46 alloys. The hardness value for homogenized sample is 562 HV, while it is 620 HV for the aged alloy. It is clear that hardness increases with aging. In general, hardness is related to the material strength and formation of precipitates with aging can be responsible for increased strength of the material. Figure 2c presents the return loss (S11 ) of selected NiTi alloys. It is clear that return loss is a function of composition. The simulation results are in good agreement with the measured results. The S11 dip point for aged Ni54 Ti46 is simulated to be about 1.74 GHz, while the measured value is about 1.8 GHz. The measured insertion loss S21 and return loss S11 of the aged Ni54 Ti46 alloys are shown in Fig. 2d, respectively. When the temperature increases, the resonance point shifts to lower values. Aged Ni54 Ti46 has an attenuation factor of 1.3 dB/cm that makes them promising for ISM band transmission lines. The phase delay of both the aged Ni54 Ti46 microstrip lines is within 5± of difference. Therefore, the shape memory alloys’ straight microstrip lines can be used to design conformal antenna network circuits such as the microstrip array, dipole antenna. The S parameters, group delay, characteristic impedance, skin depth and phase delay of Ni50 Ti50 and aged Ni54 Ti46 with selected dimensions are summarized in Table 2. It should be kept in mind that Ni54 Ti46 has higher strength than Ni50 Ti50 that will increase the durability of antenna.

Arab J Sci Eng Fig. 2 NiTi microstrip line geometrical parameters: L = 50 mm, W = 5 mm, h = 0.2 mm. Substrate: thickness, h = 0.793 mm εr = 4.34 and tan δ = 0.0016. a Prototype of NiTi microstrip line, b measured DSC curve for aged Ni54 Ti46 , c S11 of NiTi alloys, d S parameters versus temperature for aged Ni54 Ti46

4.1 Results of the Temperature Sensitivity Study The temperature sensitivity study of U-shape slot antennas has been carried out for selected NiTi alloys. FR4 epoxy substrate with the following specifications is considered: εr = 4.34 and tan δ = 0.0016 and αε = 370 ppm/◦ C. For input impedance of 50 , the proper feed point is found to be 2.15 cm from the edge. The resonant frequency of NiTi alloy is determined at +25 ◦ C and +45 ◦ C. The effective parameters of the microstrip line at +25 ◦ C are W = 5 mm, L = 5 cm and ε = 2.5. The change in substrate thickness due to temperature variation is considered to be negligible. The results are tabulated in Table 2. From Table 2, it should be noted that the resonant frequency varies with temperature. When the temperature increased from 25 to 45 ◦ C, the return loss level was increased

from −19.8 to −21.8 dB for Ni54 Ti46 . However, it can be seen that return loss of Ni50 Ti50 is not changing with the temperature. The resonant frequency of the SMA antenna is sensitive to the temperature variations. 4.2 U-Shape Ni54 Ti46 Slot Antenna Design and Measurements Figure 3a shows the dimensions of the capsule-U-shape slot NiTi antenna, which is printed on an FR4–glass epoxy substrate. The antenna geometrical parameters are given as: L u = 21.60 mm, W = 2 mm, L = 35 mm, Wg = 16.2 mm, Wg1 = 1.8 mm, L b = 3.6 mm, L d = 10.8 mm, and total area is 700 mm2 . The substrate is FR4 with a thickness of 1.575 mm, εr = 4.34 and tan δ = 0.0016. Overall length and width of the antenna are 35 and 16 mm,

123

123

+25

+45 55

60 0.016

0.017 21.92

21.92 10

12 1.72–1.6

1.79–1.9 −7.34

−6.4 −22

−21.2 −0.1

−0.6 −23

−26

0.8 5 × 50 × 0.2

4.34, 0.0016 0.8 5 × 50 × 0.2

4.34, 0.0016

+45 55 0.018 21.92 12 1.8–1.6 −2.4 −21.8 −0.5 −26 0.8 5 × 50 × 0.2

4.34, 0.0016

−19.8 −0.5 −25 4.34, 0.0016 0.8 5 × 50 × 0.2

Ni54 Ti46 -aged

Ni50 Ti50

1.7–1.6 −2.4

S21 (dB) Meas. S11 (dB) S21 (dB) Sim. S11 (dB) Substrate FR4 epoxy-glass h (mm) εr tan δ W × L × t (mm)

Table 2 S parameters obtained from simulations and measurement for the operating at the first resonance

Fr (GHz) Meas-Sim

12

Td (ns) Meas.

21.92

Z c () Calc.

0.018

Skin depth @ fr (mm) Calc.

45

Phase (◦ ) Meas.

+25

Temp (◦ C)

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respectively. It is a planar dipole, which has been optimized using CST Microwave Studio and printed on as shown in Fig. 3. Ni54 Ti46 U-shape antenna is designed for a resonant frequency of 2.35 GHz. The connector is included in the simulation model in CST-MWS to better simulate the measurement setup. Antenna is fed by a 50- microstrip line. The SMA antenna circuit designed using the aged Ni54 Ti46 material with Rho = 6450 kg/m3 , Young Mod = 114, Poiss Ratio = 0.3, Thermal Exp = 10 [1e−6/K] [18]. This aged Ni54 Ti46 is cut into U-shape patch with equal length and width to get a symmetric pattern. The copper and aged NiTi U-shape antennas are fabricated as shown in Fig. 3a. The aged NiTi slot antenna is fabricated using the same method described in Sect. 2. The antennas are fed by 2.92 mm microwave connectors. Figure 3b shows the connector used in experiments and simulations. The simulated and measured bandwidth results are shown in Fig. 4a. The proposed aged NiTi U-shape antenna is designed for a resonant frequency of 2.4 GHz. The simulated and measured bandwidth results of aged Ni54 Ti46 are shown in Fig. 4b at selected temperatures. Impedance bandwidth of aged Ni54 Ti46 U-shape slot antenna is determined to be 8 % at room temperature. It is obtained that the bandwidth of the aged Ni54 Ti46 antenna is lower than that of the NiTi antenna operating over the 2–3 GHz frequency range. In fact, as the ohmic loss increases, the bandwidth increases and the Q-factor decreases. Aged Ni54 Ti46 antenna can be used in wide ISM band communication applications. The input impedance of an antenna tends to be sensitive to changes in frequency and temperature; hence, the deviation of the antenna’s input impedance from a real fixed value often determines the operational range of the antenna. The input impedance of a microstrip antenna depends on its geometrical shape, dimensions, temperature, feed type and material. Therefore, the antenna input impedance is a very important design parameter which controls the radiated power and the impedance bandwidth. The designed Ni54 Ti46 antenna operates at 2.4 GHz with |S11 | of −47.8 dB at the resonant frequency with simulation. The measured resonant frequency of an antenna did change with the condition of the Ni54 Ti46 when they are tested in aged, homogenized and asreceived conditions. At the 2.35 GHz, the impedance value of aged Ni54 Ti46 antenna is obtained as 49.5− j6.5 where the impedance of the connector is 50 , as shown in Fig 4c. Using an HP 8753 network analyzer, the phase of each antenna is measured and shown in Fig. 4d. Our proposed aged Ni54 Ti46 antenna is martensite at room temperature. It is observed that the bandwidth of the aged Ni54 Ti46 antenna (8 %) is narrower than that of the Ni50 Ti50 antenna (12 %) operating over the 2–3 GHz frequency range. When the radiation patterns are examined, significant difference in the H-plane and the E-plane patterns is observed at selected frequencies, as shown in Fig. 5a, b. The maximum

Arab J Sci Eng Fig. 3 a Fabricated NiTi SMA antenna configuration, b connector model (r1 = 0.18 mm, r2 = 0.38 mm, r3 = 1.22 mm)

radiation pattern is determined to be in the broadside direction when the resonance frequency is 2.4 GHz. The broadside direction has changed when the frequency is increased to 3 GHz. The radiation pattern of both the copper and the aged Ni54 Ti46 antennas was simulated in the same planes. The maximum of the radiation pattern is found to be in the broadside direction. It is observed that the aged Ni54 Ti46 antenna shows a stable radiation pattern over the frequency range of interest. The radiation pattern is found to be almost omnidirectional in the x y-plane, which is of most interest for wireless communication systems. Gain and beam-shape results are found to be promising in aged Ni54 Ti46 patch condition in E-plane pattern which is near the resonance frequency. Due to the higher conductivity of copper, the gain and radiation efficiency of NiTi-based antennas are lower than those of the copper-based antennas as shown in Fig. 5d. Promising gain and beamshape results have been obtained in E-plane pattern of aged Ni54 Ti46 alloys which is near the resonance frequency, as shown in Fig. 5c, d. For the aged Ni54 Ti46 loading antenna, better results in terms of the impedance bandwidth, return loss level or co-polarization radiation levels are obtained. The aged Ni54 Ti46 antenna shows stable gain and radiation patterns over the 2–3 GHz frequency range. The dispersion characteristics of the aged Ni54 Ti46 antenna make it suitable for communication systems. The E-plane pattern has been dramatically improved for the aged NiTi antenna. The relatively large peak in the pattern of the reference patch has been obtained near the 0◦ . The asymmetry probably occurs due to the surface wave effects in the copper antenna situation as shown in Fig. 5d. NiTi antennas have positive characteristics for sensing, therapy and short communication applications. The transfer function of the antenna is measured by making Tx/Rx setups of two identical antennas at a separation distance of 40 mm. Figure 6a shows the transfer functions between the Ni54 Ti46 and aluminum antennas. The simulation results of Ni54 Ti46 are also added to the graph. Figure 6b

shows the group delay of Ni54 Ti46 and Al antennas. It is revealed that the group delay highly depends on frequency. Group delay is obtained to be less than 6.5 ns in the operational band region. The fairly constant group delay indicates that Ni54 Ti46 antennas have low dispersion that is useful for ISM radio applications. At the same time, the reference aged Ni54 Ti46 antenna and the aluminum antenna have been simulated. The results for the antenna impedances, bandwidth, return loss and the gain parameters have been evaluated by using simulation and experimental measurements as shown in Table 3. Best results have been obtained with the NiTi antenna in terms of S11 and radiation pattern. When the aged Ni54 Ti46 is used as a patch, the bandwidth of 10 % and maximum gain of 6.96 dBi have been obtained with a small amount of shift in the resonant frequency to 2.35 GHz. The resonance frequency of the asreceived Ni54 Ti46 antenna is determined to be 2.4 GHz with increased impedance bandwidth of 18 %. Lastly, the performance parameters of the designed U-shape slot antenna with Ni54 Ti46 patch and copper have been compared using CST simulation program. It is evident that the calculated halfpower beamwidth is increased when Ni54 Ti46 patch is used. It is important that NiTi alloys with selected conditions can be used to optimize the impedance level and maximize the bandwidth. On the other hand, the improvement of the bandwidth depends on aged, homogenized and as-received circumferences and does cause a little shift in the resonant frequency. In addition to these, the minimum return loss value is −47.8 dB with NiTi. Hence, this material not only improves the mechanical properties but also lowers the deep point of the return loss characteristics at the resonant frequency. Due to the favorable mechanical and electrical properties, antennas with Ni54 Ti46 components are expected to receive increasing interest for microwave antenna applications in the near future. The proposed compact and lightweight antenna can also be employed as small implantable antennas for

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Fig. 4 a S parameters, b measurement results of aged Ni54 Ti46 U-shape antenna, c real and imaginary input impedances for (t = 0.2 mm), d transfer function phase (t = 0.2 mm)

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Arab J Sci Eng Fig. 5 Radiation pattern of aged Ni54 Ti46 U-shape antenna at a E-plane ( f = 2, 2.4, 3 GHz), b H-plane ( f = 2, 2.4, 3 GHz), c 3D pattern for Ni54 Ti46 , d 3D pattern for copper

Fig. 6 a Transfer function of U-shape antenna in Tx/Rx configuration of R = 40 mm (aluminum and Ni54 Ti46 ), b group delay

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123 <6.2 ns FR4/ Engineering foams, Copper, Al

<6.5 ns FR4/Engineering foams, NiTi

<6.5 ns FR4/ Engineering foams, Ni54 Ti46 SMA–female 35 mm × 20 mm × 1.15 mm brackets 2.6 g

Group delay variations

Construction

Connector

Dimensions weight

SMA–female

SMA–female

2.42 Sim.

2.45 Meas.

2.44 Sim

2.42 Sim

2.43 Meas.

2.35 Meas.

Freq. (GHz)

1.25 Meas.

Resonance

1.12 Meas.

15 % Sim.

16 % Sim.

8 % Meas. 10 % Sim.

1.2 Meas.

Meas.

Meas.

VSWR (Sim.)@ fr

12.03 %

7.9 %

Bandwidth (@−3 dB)

Sim. 40.3 ◦

Sim. 45 ◦

Sim. 50

8.35 dB

7.65 dB

6.96 dB

Sim.

48 − j4

Sim.

49.64 − j8.9

Sim.

−32 dB

Meas

−30 dB

Sim.

U-shape slot antenna Copper/Al

Sim.

Beamwidth HPBW (@−3 dB)

Gain @ fr

49.5 − j6.5

−47.8 dB

−36.8 dB Meas.

Meas

Meas Meas.

−19.5 dB

−17.5 dB

Input impedance @ fr

Sim.

Sim.

Return loss @ fr

U-shape slot NiTi antenna Thickness t = 0.2 mm As received

U-shape slot Ni54 Ti46 antenna Thickness t = 0.2 mm Aged

Parameters

Table 3 Comparison of simulated and measured performance of NiTi antennas

SMA–female

FR4/Engineering foams, Ni54 Ti46

<6.5 ns

2.42 Sim

2.4 Meas.

1.1 Meas.

16 % Sim.

Meas.

4.2 %

Sim. 50

6.4 dB

Sim.

49 − j4.5

Meas.

−42.8 dB

Meas

−17.5 dB

Sim.

U-shape slot Ni54 Ti46 antenna Thickness t = 0.2 mm As received

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high-power systems. The antenna impedance at the resonant frequency is a complex number having both real and imaginary parts as shown in Table 3. It is shown that a compact, printed and capsule-U-shape antenna can be fabricated and utilized for wide range of applications, particularly in biomedical and communication industries. The dispersion characteristics of the SMA antenna make it suitable for ISM band communication systems. Good agreement between computed and measured results is shown for both copper, aluminum and NiTi antennas.

5 Conclusion In this paper, an accurate electromagnetic model of the Ni54 Ti46 antenna has been developed using CST for numerical analysis. U-shape slot antenna made of ultra-highstrength aged Ni54 Ti46 shape memory alloy is designed to have a band of 2.4 GHz for telemetry and short-range applications. The operation and design guidelines are presented. Measured and simulated results show that the aged Ni54 Ti46 antenna has good performance over a bandwidth of 2.2–2.45 GHz and aluminum element antenna shows similar results. Moreover, the Ni54 Ti46 antenna shows lowdispersion characteristics over the frequency range of interest. The material effect on the antenna performance is investigated, and it is shown that the NiTi antenna is much less affected than the copper antenna. In addition, effects of aged Ni54 Ti46 loading on the bandwidth and efficiency of a U-shape antenna have been investigated. The simulation and measurement results show that Ni54 Ti46 can improve the bandwidth from 4 to 8 % with utilizing the aged process, and the good radiation pattern in E- and H-planes has been obtained. These pattern shapes are similar to the reference antenna, but results show that the radiation patterns of the Ni54 Ti46 antenna with NiTi case are significantly improved compared to conventional copper and aluminum antennas. The gain is decreased from 8.35 dB for copper to 6.96 dB for aged Ni54 Ti46 antenna resulting in improved E-plane pattern for the NiTi antenna. The proposed antenna design method is feasible for small implantable antennas for high-power medical and communication systems. This antenna module is expected to have high radiation efficiency within a widerange communication. Due to the favorable mechanical and electrical properties, Ni54 Ti46 alloys are expected to receive increasing interest for microwave antenna applications in the near future. The NiTi-based antenna shows stable gain and radiation patterns over the frequency range of 2–3 GHz. Computed and measured results showed good agreement for copper, aluminum and Ni54 Ti46 antennas.

Acknowledgments This work is supported by the ˙Izmir Katip Çelebi Üniversitesi BAP (Scientific research projects) Department, Project No: 2014-1-MUH-18, TUBITAK 114E078 and 114E490 Projects.

References 1. Bernhard, J.T: Reconfigurable Antennas. Synthesis Lectures on Antennas, pp. 1–66 (2007) 2. Balanis, C.A.: Modern Antenna Handbook. WileyInterscience, Hoboken, NJ (2008) 3. Joong, H.Y.; Young, C.R.; Jang, Y.K.: Compact monopole antenna design for WLAN/WiMAX triple-band operations. Microw. Opt. Technol. Lett. 54, 1838–1846 (2012) 4. Kaya, A.; Kaya, I.; Karaca, E.H.: Radio frequency U-shape slot antenna design with NiTi shape memory alloys. Microw. Opt. Technol. Lett. 55, 2976–2984 (2013) 5. Jalali Mazlouman, S.; Mahanfar, A.; Menon C.; Vaughan, R.G.: A pattern reconfigurable axial-mode helix antenna using shape memory alloys. IEEE Trans. Antennas & Porpag. 59(4), 1070–1077 (2011) 6. Funakubo, H.: Shape Memory Alloys. Gordon and Breach Publishers, New York (2008) 7. Otsuka, K.; Wayman, C.M.: Shape Memory Materials. Cambridge University Press, Cambridge (1999) 8. Shahrzad, J.M.; Alireza (Nima), M.; Carlo, M.; Vaughan, G.R.: Square ring antenna with reconfigurable patch using shape memory alloy actuation. IEEE Trans. Antennas Propog. 60(12), 5627–5634 (2012) 9. Hornbogen, E.: Review thermo-mechanical fatigue of shape memory alloys. J. Mater. Sci. 39, 385–399 (2004) 10. Niti Shape Sheet-Properties [Online]. http://web.archive.org/web/ 20030418012213/http://www.sma-inc.com/Ni-TiProperties.html 11. Karaca, H.E.; Kaya, I.; Tobe, H.; Basaran, B.; Nagasako, M.; Kainuma, R.; Chumlyakov, Y.: Shape memory behavior of high strength Ni54 Ti46 alloys. Mater. Sci. Eng. A 580, 66–70 (2013) 12. Kai, Y.; Chengin, G.: Design and control of novel embedded SMA actuators. J. Electr. Electron. Eng. 2, 513–520 (2002) 13. Yu, H.; Irby, G.S.; Peterson, D.M.; Nguyen, M.T.; Flores, G.; Euliano, N.; Bashirullah, R.: Printed capsule antenna for medication compliance monitoring. Electron. Lett. 43, 41–44 (2007) 14. Ryckaert, J.; Desset, C.; Fort, A.; Badaroglu, M.; De Heyn, V.; Wanbacq, P.; Van der Plas, G.; Donnay, S.; Van Poucke, B.; Gyselinckx, B.: Ultra-wide-band transmitter for low-power wireless body area networks: design and evaluation. IEEE Trans. Circuits Syst. I 52, 2515–2525 (2005) 15. Soontornpipit, P.; Eurse, C.M.; Chung, Y.C.: Design of implantable microstrip antenna for communication with medical implants. IEEE Trans. Microw. Theory Tech. 52, 1944–1955 (2004) 16. Stefano, C.; Cecilia, O.; Gaetano, M.: Multi-chip RFID antenna integrating shape-memory alloys for detection of thermal thresholds. IEEE Trans. Antennas Porpag. 50(7), 2488–2494 (2011) 17. Soora, S.; Gosalia, K.; Humayan, M.S.; Lazzi, G.: A comparison of two and three dimensional dipole antennas for an implantable retinal prosthesis. IEEE Trans. Antennas Propag. 56, 622–629 (2008) 18. Shabalovskaya, S.A.: Surface, corrosion and biocompatibility aspects of Nitinol as an implant material. Bio-Med. Mater. Eng. 12, 69–109 (2002) 19. Kalidindi, S.R.; Anand, L.: Large deformation simple compression a copper single crystal. Metallurg. Trans. A 24, 989–992 (1993)

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