Four Element Cylindrical Dielectric Resonator Antenna Array With Annular

Shaped Microstrip Feed Gourab Das, Anand Sharma, Ravi Kumar Gangwar

Department of Electronics Engineering Indian School of Mines Dhanbad, Jharkhand gourabecehit@gmail.com,anand@ece.ism.ac.in, ravi.gangwar.ece07@itbhu.ac.in

Abstract: In this paper four element cylindrical dielectric resonator antenna array with annular shaped corporate feed is presented. The modern communication system requires high gain and large bandwidth antennas which are providing better performance over a wide range of frequency spectrum. This requirement leads to the design of DRA array with microstrip feeding. By optimizing the distance between the dielectric resonators, they are loaded over the microstrip corporate feed line. With this arrangement approximately 27% of bandwidth has been achieved for the proposed DRA array. The proposed antenna array gives the appreciable gain and better radiation at resonant frequencies. To validate the proposed design, prototypes of the proposed antenna array is fabricated according to the optimized dimensions. Simulation has been performed using Ansys HFSS simulation software and a close agreement between simulation and measured results were observed.

Key Words- Cylindrical DRA, Partial Ground plane, Corporate feed, wideband

I. INTRODUCTION Due to the attractive radiation characteristics, Dielectric Resonator Antenna (DRA) has been widely used in the microwave and millimeter frequency. DRA offer several advantages such as small size, light weight, wide bandwidth, low loss, high radiation efficiency, no excitation of surface waveetc. There are different shapes of DRA have been presented in the literature such as rectangular, triangular, hemispherical, elliptical, pyramidal etc [1-3]. The cylindrical shaped DRAs provide advantages over hemispherical DRA as with CDRA, the designer can select the suitable aspect ratio to realize the desired operating frequency and bandwidth. With proper feed arrangements, the dielectric resonator elements can be used to form DRA arrays which offersdirectional radiation pattern with high gain and directivity [4-5]. The performance of DRA array depends on geometry of dielectric resonator elements, feed orientation, modes of operation, anddistance between the DR elements etc.

In the proposed work, an annular shaped corporate microstrip linefeeding network is used to excite the cylindrical dielectric resonator antennaarray. In this article, a four way divider is designed to satisfyall the design requirement such as sufficient suppression of reflection, power division and proper phase distribution.A modified copper feed line is printed on a single face of a copper grounded substrate which can be etched easily.The proposed antenna array showsnearly27% impedance bandwidth with 7.95dB peak gain. The proposed DRA array has been fabricated and the measured results are compared with the simulated results.

II. ANTENNA GEOMETRY The geometry of the annular-shape microstrip linefeed with a cylindrical dielectric resonator antenna (DRA) array is presented in Fig. 1. Fig. 1(a) shows the top view of the proposed DRA. The panoramic view of the proposed antenna array is shown in Fig. 1(b). Alumina Ceramic (ɛr=9.8) has been used as a Dielectric Resonator. The CDRA has been placed on a substrate having dielectric constant (ɛr=4.4) with 1.6mm thickness. For minimum mutual coupling the distance between two DR elements is kept almost 0.5λ. Instead of full ground plane, partial ground plane has been usedwhichprovide better impedance matching of the antenna by reducing back reflection. Archetype of proposed antenna design is shown in Fig.2.

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(a)

(b) Fig. 1. Geometry of the proposed antenna (a) top view (b)

panoramic view

Fig. 2Prototype of four element CDRA array with annular shaped microstrip feed

Fig. 3Annular shape parallel microstrip feed network

Design approach of annular shaped microstrip feed- The prototype of the annular shape parallel microstrip feed network is shown in the Fig. 2. A quarter wave (λ/4) impedance

transformeris used to split the power by transforming line impedances through each junction.The separation between two adjacent microstrip junctions is set to far away to avoid the unnecessary interference. In this case, network theory may be applied to cascade individual junction and to provide output current distribution at the output port. One of the main features of corporate feed antennas is that the corporate feed are consists of several power splitter based on transmission line theory to provide specified amplitude and phase distribution of the output currents [6-8].So in order to match feeding network, λ/4 transformer is used.A well known equation for impedance transformation in transmission lines is given as equation (1).

Z0= (Zin * Za )1/2 (1) So it can be stated from equation (1) that if transmission line with characteristics impedances Z0 is employed, input impedance can be matched to the load impedance efficiently. Though some amount of losses occur in each junction of the feeding network but due to absence of metallic losses sufficient power is transmitted to each arm of the feeding network to excite loaded Dielectric Resonator. To calculate the width of 50Ω, 70Ω and 100Ω microstrip line, following relation has been used [9].

Fig. 4 Equivalent geometry of a quasi-TEM microstrip line [9]

For w/d < 2 we know that

�� = ������� (2)

Where

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A = �� �∈���

+ ∈���∈��� �0.23 + .��

∈� � (3)

Table 1. Optimize Dimensions of Proposed Antenna Array

Structure Sl no

Parameter Dimension Used

1 Length of the substrate (Ls) 115mm 2 Width of the substrate(Ws) 80mm 3 Height of the substrate(Hs) 1.6mm 4 Radius of the DRA(r1) 7mm 5 Height of the DRA(Hr) 4.7mm 6 Distance between two DRA(S) 27.5mm 7 Length of the Partial ground Plane(Wp) 55mm 8 Distance between the center of the DR and

inner portion of the annular feed(r2) 7.5mm

9 Width of the 50Ω microstrip line(w1) 3.09mm 10 Width of the 70Ω microstrip line(w2) 1.96mm 11 Width of the 100Ω microstrip line(w3) 0.71mm

III STUDY OF ANTENNA ARRAY

In some cases antenna characteristics like high gain, beam scanning or steering capability are not achieved with single antenna element. So for finding these antenna characteristics discrete radiators are combined to form arrays [10]. The elements of an array are spatially distributed to form linear array. A linear array consists several antenna elements located finite distances along in a straight line. The radiation pattern of an antenna of N-identical elements evaluated at a location in the far field can be approximated by the product of the radiation intensity of the element and the array factor using [11]: Radiation Pattern = 20log(Radiation Intensity ofthe element

× ArrayFactor) A uniform array is defined by uniformly spaced antenna elements of equal magnitude with linearly progressive phase from point to point that is shown in Fig. 5.

Fig.5 linear array on the z axis of N element

A general form for the array factor in which antenna elements placed along z axis for linear array is given by [12]

AF = 1 + e�(��������) + e�(��������) + … + e�(���)(��������) (4) k=2π/λ0, N is the no of antenna element, and δ is the phase shift. This paper can be seen as a one dimensional symmetric linear array of N isotropic elements that is positioned along the z axis and the antenna elements are separated by a distance d. The array factor of N elements can be written as [11, 12]

��(!) = ∑ �#$%(#��)(&�'*,-�/)4#5� (5) Anis the complex voltage excitation. The term(6789:! + ;) can be written as Ψ, so the array factor becomes

��(!) = ∑ �#$%(#��)<4#5� (6) Where θ is the angel between thearray axis and the vector from the origin to the observationpoint. For broadside array the phase shift δ=0 such that all the antenna element current are in phase.

This array is called broadside array. This can also be seen far field radiation pattern of proposed antenna array which is described in the next section.

Fig. 6. Linear antenna array with equal spacing

In this paper a uniform amplitude excitation (A1=A2=……..=An=A0) is chosen in order to get maximum directivity. Then the array factor is simplifies to

AF = >� �?@BC�

�?@C�

(7)

IV ANTENNA ANALYSIS

Simulation of the proposed DRA array has been performed using Ansys HFSS simulation software. Fig. 7 shows the simulated return loss characteristics of the four element cylindrical DRA array with annular shaped microstrip feed.

Y

X

Z

r1

r2

r3 rN

1 2

3

d d

θ

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Fig. 7 Simulatedinput reflection coefficient S11(dB) of the proposed antenna

From Fig. 7, it can be observed that there are two resonant frequencies. The first resonant frequency observed at 3.8 GHz and the second resonant frequency observed at 4.6 GHz.The first resonance at 3.8 GHz occurs due to annular shaped microstrip feed line and the second resonance at 4.6 GHz occurs due to Cylindrical Dielectric Resonator. In the proposed feed for the cylindrical DRA, the wideband characteristics are obtained due to the resonances of both microstrip feed and the DR.When the microstrip feed is in resonance, the dielectric resonator acts as a high permittivity load and provides wideband characteristics [13]. From Fig.8 it has been observed that the first resonant frequency at 3.8 GHz occurs due to the only microstrip feed.

Fig. 8 Simulated surface current distribution of proposed cylindrical DRA array with annular- shaped microstrip feed at

3.8 GHz.

At 4.6 GHz the resonance observed due to cylindrical DR that is also proved by modal analysis. In order to find the mode Fig. 9 and Fig. 10 shows the electric field and magnetic field distributionat 4.6 GHz. From Fig. 9 and Fig.10 it is clear that at 4.6 GHz, HE11δ mode is generated.

Fig.9 E field distribution of the antenna array at 4.6 GHz

Fig.10 H field distribution of the antenna array at 4.6 GHz

It can also be proved by using following empirical formula of calculating resonant frequency of 4.6 GHz. [14]

DE = '∗�.HJKLMɛO� [0.2 + 0.36 � L

S� + 0.02( LS) (8)

where a and h is the radius and height of the dielectric resonator respectivelyand c is the velocity of light.By using equation (8) resonant frequency for HE11δ mode is found to be 4.8GHz, which is closer to the desired frequency.So it is proved from modal analysis that at 4.6 GHz the resonance occurs due cylindrical dielectric resonator.

IV. RESULTS AND DISCUSSION This section mainly focuses on comparison of final simulated results on HFSS as well as measured results of antenna array prototype using Agilent E5071C vector network analyzer. Comparison between measured and simulated result of S parameter (S11 in dB) versus frequency curve of the proposed four element CDRA array is shown in Fig. 12.

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Fig.12 Comparison between measured and simulated input reflection coefficient characteristics of the proposed four

element CDRA array

Table 2.Comparison between measured and simulated result for the four element CDRA array

1st Resonant frequency

2nd Resonant frequency

Operating frequency Range

Bandwidth

Simulated 3.8 GHz 4.6 GHz 3.5 GHz-4.72 GHz

27%

Measured 3.75 GHz 4.35 GHz 3.5 GHz-4.72 GHz

27%

Table 2 shows the comparisonbetweenmeasured and simulated input reflection coefficient of four element CDRA array. From Fig. 12 and Table. 2, it can be observed that there are slightly mismatch between measured and simulated resonant frequencies. This may be occurring due to some fabrication errorsand alsodue to adhesive material (Fevi-quick).From input reflection coefficient curve i.e. from Fig. 7 and Table 2 it has been observed that there are two resonant peaks. The first resonance at 3.8 GHz occurs due to annular shaped microstrip feed and the second resonance at 4.6 GHz at occurs due to Cylindrical Dielectric Resonator. The simulated and measured far field radiation patterns at 3.8 GHz & 4.6 GHz of the proposed antenna is shown in Fig. 13 and Fig. 14 respectively.

(a)

(b)

Fig. 13Radiation pattern characteristics of proposed four element CDRA array at 3.8GHz (a) E-Plane, (b) H-Plane

(a)

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(b)

Fig. 14Radiation pattern characteristics of proposed four element CDRA array at 4.6GHz (a) E-Plane, (b) H-Plane

Fig. 13 and 14 shows the Co-polarization and Cross Polarization radiation pattern in two principle plane at two resonant frequencies, measured in echo free Anechoic Chamber (Spherical near Field Measurement). The good agreement has been observed between co-polarization and cross- polarization level at the E plane and the H plane. At 3.8 GHz and 4.6 GHz, in the E-Plane the difference between co-polarization level and cross polarization level is more than 20 dB in the broadside direction.From Fig. 13, it can also be observed that the measured half power beam width in E and H-plane are 67.71 7$UV$$ and 64.11 7$UV$$ respectively. Similarly from Fig. 14, that the measured half power beam width in E and H-plane are 80.79 7$UV$$ and 75.11 7$UV$$ respectively. The values of Gain at 3.8 and 4.6 GHz is found to be 7.7 dB and 6.2 dB respectively. The measured and simulated value of the gain at two resonant frequencies is given in Table 3. From Table 3, it can be observed that the simulated gains are in agreement with measured value of gain. Table 3. Comparison between simulated and measured gain of

the propose antenna array Gain at 3.8 GHz Gain at 4.53 GHz Simulated 7.95 dB 6.7 dB Measured 7.7dB 6.2dB

V. CONCLUSION

Here an annular-shape microstrip feeding technique is proposed to excite four element cylindrical dielectric resonator antenna array for S & C band applications. The measured results demonstrate that the proposed DRA array achieves an impedance bandwidth of 27%, covering the frequency range from 3.5–4.75 GHz with peak gain of 7.95 dB. Thus, it can be a good candidate for wideband communication systems.

ACKNOWLEDGEMENT First author, Ravi Kumar Gangwar wishes to acknowledge to the FRS Project, Indian School of Mines, Dhanbad for awarding financial aid.

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