Gain and bandwidth enhancement for array antenna by using multi substrate layers and EBG structure

The improvement of antenna parameters is very important. A 4 x 4 array antenna with improved parameters by using multiple substrate layers and Electromagnetic Band Gap (EBG) structure is proposed in this paper.

GAIN AND BANDWIDTH ENHANCEMENT FOR ARRAY ANTENNA BY USING MULTI SUBSTRATE LAYERS

AND EBG STRUCTURE

Nguyen Ngoc Lan*1, To Thi Thao2 and Vu Van Yem1

Abstract: Today, microstrip technology has been used widely thanks to its

advantages such as small size, lightweight, low cost and easy fabrication However, microstrip technology also has many challenges for antenna designers due to its disadvantages, for example: low gain, low efficiency and narrow bandwidth Therefore, the improvement of antenna parameters is very important A 4 x 4 array antenna with improved parameters by using multiple substrate layers and Electromagnetic Band Gap (EBG) structure is proposed in this paper The proposed antenna has some benefits: high gain (more 11.4 dBi) and large percentage of bandwidth (approximately 23% - 2.5 GHz for measurement result) and high efficiency (88%) Besides, the antenna has high directivity and low sidelobe level The antenna is designed at central frequency of 11 GHz The antenna is simulated and fabricated on FR4 substrate with thickness of 1.6 mm, εr = 4.4 and tanδ = 0.02 The simulation results are obtained using CST Microwave Studio software and are compared to measurement ones

Keywords: Array antenna, Electromagnetic Band Gap (EBG), Multi substrates, Microstrip antenna

1 INTRODUCTION

To meet the increasing demands of customers, the enhancement in service quality for wireless communication is essential Therefore, the improvement of antenna parameters is very important Currently, there are many different methods to improve antenna parameters, for example: Defected Ground Structure (DGS), Metamaterial Reflected Surface, multi substrate layers, Electromagnetic Band Gap (EBG) and so on With flexible characteristics, EBG can prevent or assist the propagation of electromagnetic waves; therefore, it is applied widely, especially in microwave and antenna fields EBG is artificial periodic (or sometimes non-periodic) objects that prevent/assist the propagation

of electromagnetic waves in a specified band of frequency for all incident angles and all polarization states [1] In general, EBG structures can be categorized into three groups according to their geometric configuration: three-dimensional volumetric structures, two-dimensional planar surfaces, and one-two-dimensional transmission lines In these structures, the uni-planar (UP) structure is the easiest one to be fabricated In addition, the 2D structure is apply the widest in antenna engineering because some of its advantages: low profile, light weight, and low fabrication cost Therefore, this paper proposes a uni-planar structure for EBG By using EBG structure, some parameters of antenna can be improved such as bandwidth, gain, directivity, efficiency

Besides, with some benefits such as high directivity and high gain, array antenna is used

in more and more applications such as mobile termination, satellite communication, radar Therefore, using array antenna is also one of the ways to improve parameters There are many published scientific papers about array antenna [2-4] However, there are some limitations in these papers In [2] although the antenna is designed at frequency of 35 GHz, the efficiency of antenna is only 67% With this efficiency, the antenna can not operate well

In another study [3], the 8x8 array antenna is designed at E-band (greater than 70 GHz), but the percentage of bandwidth of antenna is only approximately 2.6% and 1.7% In addition, the efficiency of antenna is only from 60% to 70% Similarly, although the 4 x 4 array

antenna is designed at frequency of 36 GHz, the bandwidth percentage is only under 6% [4] This bandwidth is not enough for applications at frequency band greater than 30 GHz This paper is going to present the 4 x 4 array antenna with improved parameters By using EBG structure, not only the gain of antenna is enhanced, but also the directivity and efficiency are improved In addition, by using multi substrate layer, the bandwidth of antenna is significantly improved The antenna is designed at frequency of 11 GHz and fabricated on FR4 substrate with thickness of 1.6 mm, εr = 4.4 and tanδ = 0.02

2 DESIGN OF ARRAY ANTENNA WITH EBG STRUCTURE

a) Proposed Electromagnetic Band Gap Structure

In the past, there was some papers which proposed EBG structures [1] [5] Based on these documents, the authors developed a new EBG structure The model and equivalent circuit of the proposed EBG structure are shown in Fig 1 In here, the gaps between the conductor edges introduce equivalent capacitance C In addition, the narrow strips, connected the two patches, introduce equivalent inductance L [5] The size of cell is 42 x

42 mm The advantage of requiring no hole makes this EBG structure easy for fabrication

In addition, the band gap feature of EBG structures has found useful applications in suppressing the surface waves in microstrip antenna designs

a)

b)

Figure 1 The proposed model of structure: EBG structure and equivalent circuit (a),

Complementary-EBG structure and equivalent circuit (b)

In here, the equivalent circuit is constructed as follows The each microstrip transmission line corresponds with a inductor while the band gap between two microstrip tranmission lines corresponds with a capacitor

We know that the resonant frequency is given by:

1 2

re

f

LC



Then, the impedance of resonant LC circuit is calculated as follows

2

1

j L Z

LC







with   2 f 

The value of capacitor and inductor can be calculated by the following equations [1]:

cosh

r

C

g





g is the band gap between two adjacent EBG elements, W is the width of each EBG

element ɛ r is dielectric constant of metarial, ɛ 0 is dielectric constant of vacuum, µ and h

are the permeability and thickness of substrate, respectively

b) Apply EBG to antenna

To verify the proposed EBG structure, we will apply this structure to array antenna

Figure 2 The model of antenna with EBG (a), 4x4 array antenna (b), ground with EBG (c)

The model of antenna is illustrated in Fig 2 The proposed antenna model consists of

an array antenna on top, first substrate, second substrate and ground with EBG structure at the bottom In this case, both the first and second substrates are FR4 with thickness of 1.6

mm, εr = 4.4 and tanδ = 0.02 Although FR4 have a high dielectric loss, FR4 is the most popular substrate in Vietnam because of economic effect and its popularity Therefore, the author selected FR4 substrate

Table 1 Parameters of EBG structure

9

8

The array antenna includes 16 elements and 15 T-junction power dividers The size of each element is 15 x 12 mm while the distance between elements is approximately 32 mm (from center of antenna) The antenna is designed at frequency 11 GHz The size of antenna is 151 x 152 mm

By using formulas in [6], we can calculate size parameters of antenna as follow In here, the width and length of patch are given by, respectively:

0

1 2

2

r

c W





(5)

eff

in which:

0.412

eff

eff

W h

W h





 

0 2

eff

reff

c L

f 

1/ 2

1

eff

h W





In which, ɛ r and ɛeff are the dielectric constant and the effective dielectric constant of

substrate, respectively; h is the thickness of substrate; f 0 is the resonant frequency of antenna Finally, the size of an element is 15 x 12 mm while the width of microstrip line is given

by equations[7]:

ln

4

eff

h w Z



(10)

1.393 0.677 ln 1.444

eff

Z





(11)

of antenna is 50 Ohm, therefore, the width of microstrip line is approximately 3.05 mm

In addition, to match impedance for power dividers, the quarter-wave transformers are used Then, the impedance of quarter-wave transformers is given by [8]:

0

To enhance bandwidth for antenna, this paper uses method of multiple substrate layers

We know that the percentage of antenna bandwidth of is given by [9]:

0

% BW

r

L



In which: A = 180 if

0

0.045

r

h

   , A = 200 if 0.045 0 r 0.075

h

and A = 220 if

0

0.075

r

h

From above formula, we can see that to increase bandwidth, we can rise the thickness

of substrate by using multi dielectric layers

3 SIMULATION AND MEASUREMENT RESULTS

a) Simulation results

To verify the proposed EBG strucutre, in this section, we compare antenna parameters when the antenna is based on: one substrate layer, multi substrate layer, multiple substrate

layer with UP-EBG structure and with proposed EBG structure

Fig 3 shows the comparison between the reflection coefficient, gain and efficiency of antenna with UP-EBG strcuture [1] and proposed EBG structure, respectively It is clear that although the antenna bandwidth using UP-EBG structure is quite wide with approximately 1 GHz, this bandwidth is also smaller than bandwidth using proposed EBG structure In addition, at the frequency of 11 GHz, the gain of antenna with UP-EBG is not high Moreover, although the efficiency of antenna with UP-EBG is quite high with 87%, this value is also lower than ones with proposed EBG structure (88%) This shows that the proposed EBG structure improves parameters of antenna better This is due to a suppressing the surface wave when EBG structure is used Therefore, the gain and efficiency of antenna are enhanced

Fig 4 illustrates the reflection coefficients and gains of antenna in three cases: One substrate layer, two substrate layers and two substrate layers with EBG

Figure 3 The comparison of antenna parameters between UP-EBG and the proposed

EBG structure

Figure 4 The reflection coefficients (a) and gains (b) of antenna in three cases: one

substrate layer, two substrate layers and two substrate layers with EBG

From Fig 4(a), we can see that the bandwidth of antenna with one substrate layer and two substrate layers is only approximately 900 MHz and 1 GHz, respectively While the bandwidth of antenna with two substrater layers and EBG is 1.4 GHz It is clear that the bandwidth of antenna is significantly improved when using EBG structure Moreover, the impedance matching of antenna is also improved We can see that through using multiple substrate, the bandwidth of antenna is also improved Moreover, using EBG structure creates consecutive cavity resonators and this leads to the fact that the bandwidth is

enhanced dramatically From Fig 4(b), it is clear that the gain of antenna is the greatest at frequency of 11 GHz While in the other cases, the highest pick of the gain reaches at frequencies of 13 GHz and 10 GHz, respectively We can see that by using EBG structure, not only the bandwidth of antenna is improved, but also the gain of antenna is enhanced The current re-distribution helps the antenna to create many in-phase currents and this helps to reinforce the gain for antenna

Fig 5 shows the bandwidth, efficiency and gain of the proposed antenna while Fig 6 illustrates radiation pattern of 3D and polar We can see that the bandwidth of antenna is 1.4 GHz and the percentage of antenna bandwidth is more 12.7% Normally, the bandwidth percentage of microstrip antenna is only 5-6% This shows that the bandwidth

is remarkably improved Moreover, the gain and the efficiency of antenna are 11.4 dBi and 88%, respectively Normally, the efficiency of microstrip antenna is only 60% This shows that the efficiency of antenna is significantly improved

Figure 5 The parameters of the proposed antenna: reflection coefficients (a);

gain and efficiency (b)

Figure 6 Radiation pattern: 3D (a) and polar (b)

From Fig 6, we can see that the directivity of antenna is very high and the angular width of 3dB is very small (16.8 degree) In addition, the side lobe level is also small (-4.5 dB) Therefore, the antenna can satisfy well applications which require high directivity By using EBG structure on the ground plane, the surface current re-distribution for antenna is implemented and this leads to the fact that there is the enhancement of in-phase currents Therefore, the gain of antenna is improved

b) Measurement results

tanδ = 0.02 and the simulation and measurement results The size of antenna is 151 x 152

mm The antenna includes 16 elements and 15 power dividers The antenna model consists

of an array antenna on top, first substrate, second substrate and ground with EBG structure

at the bottom

We can see that although there is a small difference between simulation and measurement results, the frequecy range for operation is still guaranteed Therefore, this result is acceptable In addition, the bandwidth of antenna is very large (2.5 GHz)

corresponding to 22.7% of bandwidth percentage This bandwidth is wide enough for applications in X-band

Figure 7 The model of the fabricated antennna: Top (a), bottom (b) and the simulation

and measurement results of antenna (c)

Compared to some announced research results, we can see the followings In [10], the array antenna of 16 elements is designed frequency of 10.5 GHz, but the gain of antenna is only 10.3 dBi With applications in X band, this value of gain is too low In another research [11], the percentage of bandwidth is only 6% although the array antenna consists

16 elements and is designed at frequency 11 GHz This bandwidth is not large enough for applications in X band Similarly, the array antenna of 64 elements is designed at 60 GHz, but the efficiency of antenna is only 70% [12] With this efficiency, the antenna can not operate well

4 CONCLUSIONS

A new EBG structure is proposed in this paper and it is successfully applied for array antenna of 4 x 4 The antenna is designed at frequency of 11 GHz based on FR4 thickness of 1.6 mm, εr = 4.4 and tanδ = 0.02 By using new EBG structure, the parameters of antenna are improved such as bandwidth, gain, efficiency The proposed antenna has wide bandwidth (2.5 GHz for measurement result), high gain (11.4 dBi) and high efficiency (88%) The size of antenna is 151 x 150 mm while the distance between elements is 32 mm By limitting some disadvantages for microstrip technology through using proposed solution, the microstrip antenna becomes very useful With some advantages such as simple and easy fabrication and low cost, the proposed EBG structure can be widely applied

REFERENCES

[1] F Yang and Y Radmat-Samii, “Electromagnetic band gap structures in antenna

engineering”, NY: Cambiridge Press, 2009

[2] A Mavaddat, S H M Armaki and A R Erfanian, “Millimeter-Wave Energy

Harvesting Using 4x4 Microstrip Patch Antenna Array,” in IEEE Antennas and

Wireless Propagation Letters, vol 14, no , pp 515-518, 2015

[3] L Wang, Y J Cheng, D Ma and C X Weng, “Wideband and Dual-Band High-Gain

Substrate Integrated Antenna Array for E-Band Multi-Gigahertz Capacity Wireless Communication Systems,” in IEEE Transactions on Antennas and Propagation, vol

62, no 9, pp 4602-4611, Sept 2014

[4] B T P Madhav , V G K M Pisipati , S Susrutha Babu , S Suparshya, “K15 Liquid

Crystal Substrate Based 4X4 Array Elliptical Patch Antenna Operating At 36 GHz Band,” International Journal of Recent Trends in Electrical & Electronics

Engineering, Sept 2011, ISSN: 2231-6612

[5] Huynh Nguyen Bao Phuong, Dao Ngoc Chien, Tran Minh Tuan, “A Novel Compact

Triple-Band Electromagnetic Bandgap (EBG) Structure,” International Journal of

Advances in Engineering & Technology, Vol.5, Iss 2, pp 253-262 , 2013

[6] Constantine A Balanis, “Antenna theory - Analysis and Design,” 4th Edition, John

Wiley & Son, INC, 2016

[7] Pozar, D M., “Microstrip Antennas: The Analysis and Design of Microstrip Antennas

And Arrays”, IEEE Press, New York, USA, 1995

[8] David M Pozar, “Microwave Engineering,” 4th Edition, Wiley, 2012

[9] D G Fang, “Antenna Theory and Microstrip Antennas,” CRC Press 2009, ISBN:

978-1-4398-0739-2

[10] D Hua, S S Qi, W Wu and D G Fang, “CPW-Fed Printed Antenna Array With

Conical Beam,” in IEEE Transactions on Antennas and Propagation, vol 64, no 3,

pp 1096-1100, March 2016

[11] S Casas-Olmedo, J L Masa-Campos and P Sánchez-Olivares, “Design and

characterisation model for a linearly polarised patch array fed by serial rectangular waveguide network,” in IET Microwaves, Antennas & Propagation, vol 8, no 14, pp

1204-1210, 11 18 2014

[12] Y Li and K M Luk, “A 60-GHz Wideband Circularly Polarized Aperture-Coupled

Magneto-Electric Dipole Antenna Array,” in IEEE Transactions on Antennas and

Propagation, vol 64, no 4, pp 1325-1333, April 2016

TÓM TẮT

TĂNG CƯỜNG BĂNG THÔNG VÀ TĂNG ÍCH CHO ANTEN MẢNG BẰNG CÁCH

SỬ DỤNG NHIỀU TẦNG ĐIỆN MÔI VÀ CẤU TRÚC EBG

Ngày nay, công nghệ vi dải được sử dụng rộng rãi bởi các ưu điểm của nó như kích thước nhỏ, trọng lượng nhẹ, chi phí thấp và dễ dàng chế tạo Tuy nhiên, công nghệ vi dải cũng đặt ra nhiều thách thức cho những người thiết kế anten do một số những nhược điểm của chúng như tăng ích và hiệu suất thấp, băng hẹp Do đó, việc cải thiện các tham số cho anten là rất quan trọng và cần thiết Một anten mảng 4 x

4 với các tham số được cải thiện bằng cách sử dụng nhiều tầng điện môi và cấu trúc EBG được đề xuất trong bài báo này Anten đề xuất có một số ưu điểm: tăng ích cao (hơn 11.4 dBi) và tỉ lệ phần trăm băng thông lớn (23% - 2.5 GHz cho kết quả đo lường) và hiệu suất cao (88%) Bên cạnh đó, anten có độ định hướng cao và mức búp sóng phụ thấp Anten được thiết kế tại tần số trung tâm 11 GHz Anten được mô phỏng và chế tạo trên lớp điện môi FR4 với độ dày 1.6 mm, hằng số điện môi 4.4 và suy hao 0.02 Các kết quả mô phỏng thu được từ phần mềm CST và được so sánh với kết quả đo

Từ khóa: Anten mảng, Dải chắn điện từ (EBG), Nhiều tầng điện môi, Anten vi dải.

Nhận bài ngày 20 tháng 3 năm 2017 Hoàn thiện ngày 31 tháng 7 năm 2017 Chấp nhận đăng ngày 18 tháng 8 năm 2017

Author affiliations:

1Hanoi University of Science and Technology;

2Posts and Telecommunications Institude of Technology

*Corresponding author: nnlan@moet.edu.vn.