Compact Microstrip Rectangular Edge Fed Antenna ...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 10, Number 10 (2015) pp. 24331-24348 © Research India Publications http://www.ripublication.com

Compact Microstrip Rectangular Edge Fed Antenna with DGS Structure 1

V Narasimha Nayak, 1B T P Madhav, 2R Sai Divya, 2A Nava Sai Krishna, 2K Rohith Ramana, 2D Mounika 1

Faculty, Department of ECE, K L University, AP, India Engineering Project Students, Department of ECE, K L University, AP, India

2

Abstract A compact Rectangular Microstrip antenna with edge fed technique on the top side and Defected Ground Structure on the bottom side is proposed in this work. Without DGS structure antenna is resonating at 5.54GHZ and with defected ground structure antenna is resonating at 3.3GHZ. The proposed defected ground structure in this model improved the overall performance of the antenna by decreasing its electrical length. We got a miniaturisation up to 40% when compared with conventional rectangular microstrip antennas in the literature. A comparative study on antenna with DGS and without DGS is analysed with respect to its performance characteristics along with the placement of DGS structure. The overall size of the antenna is around 72X41X1.6mm. A prototype of the proposed antenna is fabricated on FR4 substrate material and tested for validation. Keywords: Compact, Defected ground structure (DGS), FR4 substrate, Rectangular edge fed.

1. INTRODUCTION: Microstrip patch antennas are becoming very popular now a day because of lot of advantages like lighter weight, lower cost and smaller dimensions [1-12]. Microstrip patch antennas which operate in dual-band and multi-band applications for dual or circular polarization are easy to design [13-18]. These are used in several applications like medical, satellites and military systems [19-28]. In order to reduce the size of the microstrip antennas, recently several methods have been proposed such as using a dielectric substrate of high permittivity, defected microstrip structure, defected ground structure (DGS) at the ground plane or a combination of them [29-34].

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DGS is an etched periodic or non-periodic cascaded configuration defect in the ground of a planar transmission line (e.g., Microstrip, coplanar, conductor backed coplanar waveguide).It does not require large area and is easy to implement [35-38]. Thus this structure acquires a great relevance in microwave circuit design. Microstrip antenna with DGS is used for different applications such as cross polarization, mutual coupling reduction in antenna arrays and harmonic suppression and widely used in the development of miniaturized antennas [39-42]. DGS in the ground plane of the microstrip antenna disturbs its shield current distribution. This disturbance affects the transmission line characteristics such as the line capacitance and inductance [43-44]. Actually it will increase the effective capacitance and inductance which in turn influences the input impedance and current flow of the antenna. Therefore antenna size is reduced with respect to given resonant frequency. In this paper we employed DGS in the ground plane of microstrip antenna. We vary the position of the DGS in the ground of microstrip antenna and we proposed antenna with all optimised parameters such as bandwidth, resonant frequency, Return loss, antenna efficiency, peak gain and peak directivity.

2.

ANTENNA DESIGN AND GEOMETRY:

Fig.1. Rectangular edge-fed microstrip antenna

Compact Microstrip Rectangular Edge Fed Antenna

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Fig.2 Defected Ground Structure

Figure1 shows rectangular edge fed microstrip antenna model and Figure2 shows the defected ground structured modified model. The basic rectangular edge fed model is designed based on standard design equations with resonating frequency of 5.5GHZ taking in to the consideration. The modified DGS model is constructed after examining so many defected ground structures, finally taking this particular model after optimisation using electromagnetic HFSS tool.

 k z   2 0 eff  eff    2   0 0 0  k0  2

 reff (Effective Dielectric constant) =  reff Where

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 = angular velocity,  0 = permeability and  0 = permittivity w When > 1 (where w = width, and h = height of the substrate) h 1  r 1  r 1  h 2  reff   1  12  2 

2

W

We observed that at lower frequencies, the  reff is slowly increasing and when moving towards higher frequency the  reff is almost constant even though there is change in permittivity.

L  0.412 h



reff



reff

W   0.3   0.264  h  W   0.258   0.8   h 

Where L is change in length.

v0

 f rc 010 

v0

 reff

2  L  2L 

g

r

2L

Where g = fringe factor (length reduction factor) Leff  L  2L

 f r 010   f rc 010 

1 2 L  r 0 0 1

2 Leff  reff

0 0





v0 2L  r

v0 2  L  2L   reff

The resonant frequency with fringing is given by


 f r 010 

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v0 2L  r

Because of fringing the effective distance between the radiating edges seems larger than L by an amount of ∆L at each edge. This causes the actual resonant frequency to slightly less than f ro by a factor q. Thus

 f rc  

v0 v0 q 2( L  2L)  reff 2L  r

2.1. Defected Ground Structure Defected ground structure are became performance improvement elements in the modern antenna design systems. Researchers are proposing different defected ground structures to improve gain, bandwidth, directivity and radiation efficiency etc. A defected ground structure consisting of periodic slotted sections on the ground plane to excite additional resonant frequencies. In the current design we used E shaped defected ground structure connected back to back as shown in the fig 2. This particular shape is chosen because of its easiness in the design as well as the effectiveness of performance enhancement.

Fig 3: Equivalent circuit

The equivalent admittance of the parallel resonance is given by Equation (1) Y

1 1    j 2  Cf  2 Lf  R 4  

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V Narasimha Nayak et al Since Z=1/Y, then the equivalent impedance is given by

Z  1/

 1 1   j 2  Cf  2  R 4 Lf  

In order to compute the equivalent circuit parameters (R, L, and C), we use the following expressions

S21 

2Z 0 1  1  2Z 0    j 2  Cf  2   4 Lf    R

1

Supposing that R>> Z 0 S21 

2Z 0   1  2Z 0   j 2  Cf  2   4 Lf    

1

At -3 dB corresponding to the cut off frequency f c we have

S 21 

2Z 0   wc  4 Z 02    C  w02  wc2    

2



1 2

Where wc is the cut-off angular frequency and w0 is the resonance angular frequency. Using Equations (5) and (6), we conclude the capacitance and the inductance of the equivalent circuit.

C

wc 2Z 0  wo2  wc2 


L

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2Z 0  wo2  wc2  wc w02

Resistance R in the equivalent circuit is best fitted around the resonance frequency. In this case, the equivalent impedance Z e and then we have: S21 |w w0 |

2Z 0 2Z 0 | 2Z 0  Z e 2Z 0  R

Then,

R



2Z0 1  S21|w w0



S21|w w0

3. RESULTS AND DISCUSSIONS: The simulated results are constructed from HFSS tool and presented in this work. Fig 4 shows the return loss curve for the antenna model without DGS. The basic rectangular edge fed antenna is resonating at 5.5GHZ with poor return loss and bandwidth.

Fig.4 Return loss without DGS

Fig5. Shows the return loss curve for defected ground structure antenna model. It has been observed surprisingly that the resonant frequency is shifted towards lower band which rises the miniaturisation of the antenna as per the electrical dimensions are concerned. The modified DGS model is also raising additional resonant frequency at 6.3GHZ but return loss is poor from fig 5.

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Fig.5 Return loss with DGS

The position of the DGS structure is also taken in to account and we examined the antenna parameters with respect to the defected ground structure position on the ground plane from the origin. Fig.6 shows distance Vs bandwidth of the defected ground structured antenna model. It is been observed that when DGS is placed at a distance 5-9mm from the center the band width is improved. When DGS is placed at a distance more than 9mm the bandwidth is reduced. The bandwidth is increased with the distance because of the separation between DGS and the radiating element on topside.

Fig.6 Distance Vs bandwidth


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Fig.7 shows distance Vs magnitude of the DGS structured antenna model. In this case also we observed better results when DGS is placed at 5-9mm in particular at 7mm.

Fig.7 Distance Vs magnitude

Fig.8 shows peak directivity curve of the DGS antenna with respect to distance. The peak directivity is higher when the distance from the origin to the DGS is 1mm and peak directivity is low when the distance is 7mm. It is been observed that peak directivity and bandwidth results with respect to distance are not in correlation with each other.

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Fig.8 Distance Vs peak directivity

Fig 9 shows distance Vs peak gain of the DGS antenna which is also showing similar kind of results like directivity.

Fig.9 Distance Vs peakgain


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Fig.10 shows antenna efficiency of the antenna with respect to distance of the DGS structure. The antenna efficiency is increased by increasing the distance from 110mm. The efficiency increased because the effect of DGS on the ground plane is less at 10mm compared to 1mm from the origin.

Fig.10 Distance Vs Antenna efficiency

Fig.11 shows the radiation pattern of the proposed antenna at its resonant frequency in E and H plane. In E plane antenna is showing quasi-omni directional radiation pattern with low cross polarization level less than -29dB. In H plane antenna is showing directive pattern rather than omni- directional pattern.

Fig.11 Radiation Pattern

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Fig.12 shows the current distribution plot of the DGS model at its fundamental resonant frequency. The current distribution is more on feed line rather than radiating element. On the ground plane an uniform distribution of the current elements with equal magnitude is observed over the entire surface.

Fig.12 DGS Current distribution

Fig 13 shows the current distribution over the surface of the antenna and it has been observed that current density on the patch surface lower and on the feed line it is higher. The feed line contributes towards the radiation rather than patch surface.

Fig.13 Patch current distribution


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4. MEASUREMENT RESULTS

Fig 14. Fabricated Prototype of antenna (a) Top View, (b) Bottom View

Fig 15. Measured S11 of Fabricated Antenna using ZNB 20 VNA

Fig 16. Measured Phase of Fabricated Antenna using ZNB 20 VNA

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Fig 14 shows the Prototyped antenna which is fabricated on FR4 substrate with thickness 1.6 mm. Figure 15 shows the measured S11 parameter with ZNB 20 vector network analyzer and Fig 16 shows the measured phase of the antenna. The simulation and measurement results are in good agreement with other corresponding to its resonant frequencies.

5. CONCLUSION A Defected ground structured antenna is designed and its performance characteristics are analysed in this work. Initially antenna is designed to work at higher frequency band and after adding DGS on the ground plane, the resonant frequency is shifted towards lower frequency band. By incorporating DGS we observed improvement in the antenna characteristics and the electrical length of the antenna is also decreased. Finally a compact antenna is proposed with DGS structure and we achieved more than 40% size reduction compared to conventional antenna. The measurement results also giving support to the simulation results when tested with ZNB 20VNA.

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