Compact asymmetric fractal frequency and pattern reconfigurable monopole antenna

Int. J. Electron. Commun. (AEÜ) 111 (2019) 152915

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Regular paper

Compact asymmetric fractal frequency and pattern reconfigurable monopole antenna Anuradha A. Palsokar a,⇑, S. L. Lahudkar b a b

Resesrch Scholar, Rajarshi Shahu College of Engineering, Pune, India Professor, Imperial College of Engineering and Research, Pune, India

a r t i c l e

i n f o

Article history: Received 24 March 2019 Accepted 9 September 2019

a b s t r a c t A compact frequency and pattern reconfigurable monopole for LTE and WLAN applications is presented. The antenna consists of an asymmetric fractal patch which is derived from a bow-tie antenna and optimized using Pattern search algorithm. The antenna operates at 1.8 GHz and 5.2 GHz frequency with a different radiation pattern for each case. A single PIN diode is used to achieve frequency and pattern reconfigurability. The antenna is simulated using Ansoft High Frequency Structural Simulator (HFSS) software and fabricated on FR4 substrate with the relative permittivity of 4.4; having size of 30 mm * 24 mm and thickness of 1.6 mm. Ó 2019 Elsevier GmbH. All rights reserved.

1. Introduction Vast use of various standards in mobile phones and other wireless devices imposes the requirement of multifunctional antennas. Dynamically operated reconfigurable antennas provide a solution for such requirements [1]. Reconfigurable antennas have capability to operate at multiple frequencies and have varying radiation patterns or can provide various polarizations for the signal to be transmitted or received. Dynamic operation in reconfigurable antennas can be obtained with the use of electrically operated switches like radio frequency micro-electromechanical systems (RF MEMs), positive–intrinsic-negative (PIN) diodes, Varactor diodes etc. 1.1. Related work Literature evidence shows various antenna structures with different switching elements for simple and compound reconfigurability. Frequency reconfigurable antenna for WiMAX & WLAN is proposed in [2,3] where the antenna in [2] is reconfigurable for four frequency bands and the antenna in [3] is reconfigurable for three frequency bands. Pattern reconfigurable antenna for WiMAX & WLAN using multiple radiators and switchable director/reflector is presented in [4]. Frequency reconfigurable antenna for five bands using Koch fractal patch is designed in [5], it has used slotted ground with complementary split ring resonator. A novel compact hexa-band frequency reconfigurable monopole antenna is designed in [6]. Various antenna designs for compound reconfig⇑ Corresponding author. E-mail address: [email protected] (A.A. Palsokar). https://doi.org/10.1016/j.aeue.2019.152915 1434-8411/Ó 2019 Elsevier GmbH. All rights reserved.

urability were also observed in literature. A frequency & pattern reconfigurable antenna for WLAN is presented in [1,7]. Frequency and pattern reconfigurability using slots and six PIN diodes on ground structure is designed in [8]. Printed V shaped frequency and pattern reconfigurable antenna with extendable arms is designed in [9], whereas reconfigurability is achieved in slotted septagon in [10]. Antennas capable of reconfigurable in frequency and pattern are designed with slots and slot rings in [11,12] respectively. All these antennas have used PIN diode as switching element. This work presents a compact monopole antenna for LTE and WLAN applications which can dynamically change the frequency of operation and the radiation pattern. The antenna is designed with a single switching diode and minimum biasing requirements. Pattern search algorithm is used here to optimize the feed position of antenna. The paper is arranged in following manner. Section two gives the design and implementation details for the proposed antenna. Detail discussion of results is done in the third section. The concluding remarks are presented in the fourth section. 2. Antenna design & implementation The geometry of proposed monopole antenna is shown in Fig. 4. Initially a simple bow-tie antenna is designed; further the geometry is modified and converted to an asymmetric fractal antenna by inserting the koch into the bow-tie to obtain the required frequencies for LTE & WLAN applications. Frequency and pattern reconfigurability is obtained using a single PIN diode on the patch side. The feed position of the antenna is optimized using pattern search algorithm.

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2.1. Geometry of antenna

2.2. Optimization of antenna

The design process started with a simple bow-tie antenna shown in Fig. 1(a), whose two arms are printed on two sides of substrate. The design of the bow-tie antenna is inspired from [3] but it is modified to obtain the required operation. Design equations for bow-tie antenna are followed from [3,13]. The side length of bow tie is calculated using:

The proposed antenna uses Pattern Search Algorithm (PSA) for optimization purpose. Pattern search is a direct, efficient and derivative free searching tool used for minimization of a given function [14]. As the impedance performance of antenna depends on feed position and matching with the 50 O connector, the feed position of the antenna is optimized using Pattern Search Algorithm to achieve perfect matching of the radiating patch with the connector. Fig. 5 shows the convergence curve for the algorithm. Pattern search algorithm converges for the optimized value of feed position for which the cost function is zero. Fig. 6 shows the graph of reflection coefficient for un-optimized feed position of antenna and optimized feed position of antenna using pattern search algorithm. The graph shows a significant improvement in the characteristics of antenna after the use of optimization algorithm. Without optimization algorithm the resonant frequency of antenna is 5.13 GHz, the reflection coefficient is 23.94 dB and impedance at this frequency is 56 O. After application of Pattern Search Algorithm for optimization of feed position the resonant frequency is 5.2 GHz, reflection coefficient is 37.89 dB and impedance is 50.6 O. Figs. 7 and 8 shows the smith chart used for calculation of impedance values of antenna before and after application of PSA. After optimization the required frequency of 5.2 GHz is achieved with 50.6 O impedance; which will lead to better impedance matching of antenna with the 50 O connector.

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2c m2 þ mn þ n2 pffiffiffiffi fr ¼ 3a er

ð1Þ

where Fr is resonant frequency m, n are the number of modes c is the velocity of light in free air a is the side length of bow tie strip Fundamental operating mode is considered for which m = 1, n = 0 or m = 0 and n = 1. This simple bow-tie antenna has a resonant frequency of 5.7 GHz, to which a right arm is added as shown in Fig. 1(b) and the dimensions are optimized, which changed the length of the antenna and the resonant frequency to 1.95 GHz, shown in Fig. 1(c). The antenna in step1 is a bow-tie antenna with each side of bow-tie of 10 mm. In second and third step simple bow-tie antenna is converted into a fractal antenna with a factor of 1/3 and 1/6. Finally the proposed antenna is designed having the left arm with a factor of 1/6 and right arm with a factor of 1/3 to get the desired multiband operation for LTE and WLAN applications. Geometry modification for various steps is shown in Fig. 2. Fig. 3 shows the simulated results for return loss of antenna in various stages. The final geometry of proposed antenna is shown in Fig. 4. Dimensions for the proposed antenna are as follows:

3. Results and discussion The prototype of proposed antenna is fabricated on FR4 substrate with relative permittivity constant of 4.4 and thickness of 1.6 mm with the loss tangent of 0.002. The footprints of the antenna are 30  24  1.6 mm3, the reduction in antenna size is achieved by introducing fractals in the bow-

Parameter

a

b

c

d

e

f

g

h

I

Value (mm)

3.89

3.62

4.53

4.2

4.45

3.58

2.71

6.5

4.1

Parameter

j

k

l

m

n

o

x

y

Z

Value (mm)

3.6

1.95

1.76

3.95

1

2.8

7.5

15.6

13.4

iasing elements used are L = 33 nH and C = 10 pF, S1 is PIN diode. In the subsequent step, a PIN diode and biasing elements are used to convert the antenna into reconfigurable element.

tie antenna. The antenna makes use of truncated ground as shown in Fig. 4.

Fig. 1. (a) Bow-tie antenna. (b) Bow-tie antenna with right arm. (c) Comparison of resonant frequency of antennas.

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stage 1

stage 2

stage 3

stage 4

Fig. 2. Various stages of antenna development.

Fig. 5. Convergence curve for pattern search algorithm. Fig. 3. Simulated return loss for different steps of proposed antenna.

3.1. Simulation and fabrication results The antenna is simulated using HFSS software. As per the biasing of PIN diode, there are two modes of operation for this antenna. When the PIN diode is turned on, the right side arm is attached to the left side arm and the antenna shows a resonant frequency of 1.8 GHz which can be used for LTE applications. When the PIN diode is off, the right side arm of the patch is detached from the left side arm hence the electric length of the antenna is changed which changes the operating frequency of the antenna to 5.2 GHz which is suitable for WLAN applications. Here frequency reconfigurability is achieved by changing the state of the PIN diode. The simulated gain of proposed antenna in both the modes is shown in following figure (see Fig. 9). Table 1 shows the summary of simulated results. When the switch is on, the gain obtained is 4.5 dBi and percentage bandwidth

Fig. 6. Reflection coefficient characteristics.

Fig. 4. Proposed antenna geometry (a) patch view (b) ground structure.

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Fig. 7. Impedance calculation using smith chart for the antenna before optimization.

Fig. 8. Impedance calculation using smith chart for the antenna after optimization.

Fig. 9. Simulated gain for proposed antenna (a) switch on state (b) switch off state.

is 24%. When the switch is off, the gain obtained is 3.8 dBi and percentage bandwidth is 18%. Fabricated antenna as shown in Fig. 10 is tested using Agilent Technologies vector network analyser (VNA) N9923A. The simu-

lated and measured reflection coefficient characteristic and voltage standing wave ratio (VSWR) of the antenna for both operating modes is shown in Figs. 11 and 12 respectively.

A.A. Palsokar, S.L. Lahudkar / Int. J. Electron. Commun. (AEÜ) 111 (2019) 152915 Table 1 Simulation results for proposed antenna. State

I

II

PIN Diode Frequency Gain Efficiency %Bandwidth

ON 1.8 GHz 4.5 dBi 63.7% 24%

OFF 5.2 GHz 3.8 dBi 90% 18%

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The comparison of simulation and testing results in Table 2 shows a better matching of antenna parameters, the difference in operating frequency may be due to the fabrication errors, feed wires or imperfections of components. 3.2. Reconfigurability of antenna By changing the state of PIN diode the electrical length and current distribution of the antenna can be changed with which we can achieve frequency and pattern reconfigurability. A single BAR 64

Fig. 10. Fabricated antenna (a) patch view (b) ground structure.

Fig. 11. Reflection coefficient characteristics and VSWR for proposed antenna when switch is off.

Fig. 12. Reflection coefficient characteristics and VSWR for proposed antenna when switch is on.

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Table 2 Summary of antenna performance. Simulation results

Testing results

State

I

II

I

II

PIN Diode Frequency (GHz) S11(dB) VSWR % Bandwidth

ON 1.8 43.34 1.03 24%

OFF 5.2 34.89 1.03 18%

ON 1.87 24.11 1.26 28%

OFF 5.47 27.51 1.16 22%

PIN diode is used as a switch to achieve reconfigurability. In the simulation, the PIN diode is replaced with its equivalent circuit. During on state, the diode has an equivalent resistance as low as 2.1 O and during off state, it has an equivalent resistance of 3 kO in parallel with a capacitance of 0.17 pF. The series inductance of diode 1.8 nH is neglected for simplicity, but this may also be one of the reasons for slight difference in simulation and measurement results. PIN diode equivalents are simulated as RLC lumped components in HFSS. A simple biasing circuit is also provided with an

Fig. 13. Electric field distribution for the proposed antenna a) Switch on b) switch off.

Fig. 14. Radiation pattern for the proposed antenna when (a) switch is on (b) switch is off.

Table 3 Comparison of the proposed asymmetrical fractal antenna structure with the antennas designed in literature. Ref.

Size (mm3)

No of diodes

Operating frequency (GHz)

Gain for each band (dBi)

% Bandwidth

1 4 9 10 12 Prop.

60  50  1.6 36  45  1.6 66  60  1.6 30  30  3.1 100  100  2.5 30  24  1.6

6 4 4 5 4 1

5.2, 5.8 1.8–2.64 2.96, 3.26 3.9, 5.9 2.84, 3.84 1.8, 5.2

3, 2.5 3.7 3.93 Not provided 7.4, 5.7 4.5, 3.9

3.5, 3.59 34 1.68, 3.06 (Approximately) 8.9, 5.8 (Approximately) 1.76, 1.82 (Approximately) 24, 18

A.A. Palsokar, S.L. Lahudkar / Int. J. Electron. Commun. (AEÜ) 111 (2019) 152915

inductor of 33 nH and a DC blocking capacitor of 10 pF for proper biasing of PIN diode. As the switch condition is changed, the electric field distribution is changed. When the switch is on, maximum electric field strength is observed along the right arm of the patch and when the switch is turned off, the electric field is observed along the left arm of the patch as shown is Fig. 13. As the electric field distribution is changed, the current distribution and hence the radiation pattern is also changed, which is clearly observed from the changed E plane pattern for switch on and switch off condition shown in Fig. 14. Hence pattern reconfigurability is also achieved in addition to the frequency reconfigurability. Table 3 shows comparison of proposed antenna with the antennas discussed in literature. The size of the proposed antenna is comparatively small for similar range of applications. So the main advantage of proposed antenna is its compact size and use of single switching diode. As the no of diodes used are less, the biasing circuit is simple and operation of reconfigurable antenna will be more reliable. Moreover the novelty of antenna is the use of asymmetric fractal shape and use of PSA for optimization of antenna to make the antenna.

4. Conclusion This paper presents a frequency and pattern reconfigurable monopole antenna using a single PIN diode as a switch. The antenna operates at 1.8 GHz and 5.2 GHz frequency hence it can be used for LTE/WLAN applications. The advantage of the antenna is its compact size and use of a single switching element. A prototype is fabricated on FR4 and the performance verification is done through the correlation of simulation and testing results which shows a good agreement.

Declaration of Competing Interest The author(s) declare(s) that there is no conflict of interest.

Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.aeue.2019.152915.

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References [1] Selvam YP, Elumalai L, Alsath MGN, Kanagasabai M, Subbaraj S, Kingsly S. Novel frequency- and pattern-reconfigurable rhombic patch antenna with switchable polarization. IEEE Antennas Wirel Propag Lett 2017;16:1639–42. https://doi.org/10.1109/LAWP.2017.2660069. [2] Thao HTP, Son TV, Doai NV, Duc NT, Yem VV. A novel frequency reconfigurable monopole antenna using PIN diode for WLAN/WIMAX applications. In: 2015 International conference on communications, management and telecommunications (ComManTel), DaNang; 2015. p. 167–171. doi: 10.1109/ ComManTel.2015.7394281. [3] Li T, Zhai H, Wang X, Li L, Liang C. Frequency-reconfigurable bow-tie antenna for bluetooth, WiMAX, and WLAN applications. IEEE Antennas Wirel Propag Lett 2015;14:171–4. https://doi.org/10.1109/LAWP.2014.2359199. [4] Alam MS, Abbosh AM. Wideband pattern-reconfigurable antenna using pair of radial radiators on truncated ground with switchable director and reflector. IEEE Antennas Wirel Propag Lett 2017;16:24–8. https://doi.org/10.1109/ LAWP.2016.2552492. [5] Ali Tanweer, Fatima Nikhat, Biradar Rajashekhar C. A miniaturized multiband reconfigurable fractal slot antenna for GPS/GNSS/Bluetooth/WiMAX/X-band applications. AEU-Int J Electron Commun 2018;94:234–43. https://doi.org/ 10.1016/j.aeue.2018.07.017. [6] Shah IA, Hayat S, Basir A, Zada M, Shah SAA, Ullah S, Ullah S. Design and analysis of a hexa-band frequency reconfigurable antenna for wireless communication. AEU – Int J Electron Commun 2019;98:80–8. https://doi.org/ 10.1016/j.aeue.2018.10.012. [7] Selvam YP, Kanagasabai Malathi, Gulam Nabi Alsath M, Velan Sangeetha, Kingsly Saffrine, Subbaraj Sangeetha, et al. A low-profile frequency- and pattern-reconfigurable antenna. IEEE Antennas Wirel Propag Lett 2017;16:3047–50. https://doi.org/10.1109/LAWP.2017.2759960. [8] Sahu NK, Sharma AK. An investigation of pattern and frequency reconfigurable microstrip slot antenna using PIN diodes. Progress in electromagnetics research symposium – spring (PIERS), St. Petersburg; 2017. p. 971–6. doi: 10.1109/PIERS.2017.8261884. [9] Tawk Y, Costantine J, Christodoulou CG. Reconfiguring the frequency and directive behavior of a printed V–shaped structure. IEEE Trans Antennas Propagat 2017;65(5):2655–60. https://doi.org/10.1109/TAP.2017.2679347. [10] Varsha J, Sumi M. A novel pattern and frequency reconfigurable antenna. International conference on inventive computation technologies (ICICT), Coimbatore; 2016. p. 1–5. doi: 10.1109/INVENTIVE.2016.7830118. [11] Majid HA, Rahim MKA, Hamid MR, Ismail MF. Frequency and pattern reconfigurable slot antenna. In: IEEE transactions on antennas and propagation, vol. 62, no. 10; 2014. p. 5339–43. doi: 10.1109/ TAP.2014.2342237. [12] Li W, Ren Z, Shi X, Hei Y. A frequency and pattern reconfigurable microstrip antenna using PIN diodes. In: 2014 IEEE antennas and propagation society international symposium (APSURSI), Memphis, TN; 2014. p. 1449–50. doi: 10.1109/APS.2014.6905050. [13] Rahim MKA, Abdul Aziz MZA, Goh CS. Bow-tie microstrip antenna design. In: 2005 13th IEEE international conference on networks jointly held with the 2005 IEEE 7th Malaysia international conference on communications, Kuala Lumpur; 2005. p. 17–20. doi: 10.1109/ICON.2005.1635425. [14] Gunes Filiz, Tokan Fikret. Pattern Search Optimization with application to linear antenna array. Expert Syst Appl 2009:4698–705. https://doi.org/ 10.1016/j.eswa.2009.11.012.