Defected Ground Structured Monopole Antenna for Broadcasting Satellite Communication Applications

Design of antennas for satellite communication applications like Global Positioning Systems, Geo Informatics Systems became crucial for real time data collection and transmission. A novel Defected Ground structure planar monopole antenna for broadcasting satellite communication applications is proposed in this study. The proposed model has a unique structure with the combination of rectangular, arrow lined and T-slotted inverted U-shaped structure as a radiating element on the topside of the substrate. The ground plane on the bottom side of the substrate consisting of a rectangular slotted defected ground structure. The designed antenna has a small overall size of 32X26X0.8 mm with polyster as a substrate material. The results show that the antenna gives monopole like radiation pattern and a good antenna gain over the operating bands. The proposed antenna performance characteristics are well suited for fixed and broad casting satellite communication applications.


INTRODUCTION
The design of an antenna with multi band and wide band characteristics systems has increased has increased in the communications systems with improvement in their compactness (Madhav et al., 2014a;Rafi and Shafai, 2004;Madhav et al., 2010;Prabhu Kumar et al., 2011).Day by day the demands of compact antennas with simple structures are increasing in the communication engineering.The antenna with various structures has become a familiar candidate because of its attractive characteristics including low cost, low profile and less weight and versatile structure for exciting wide impedance bandwidths (Baskaran et al., 2011;Madhav et al., 2013aMadhav et al., , 2013bMadhav et al., , 2013c)).The difficulty in designing antenna challenges engineers when the size of the antenna reduces and the number of operating frequency bands increases.Different structures with different configurations are proposed by the researchers in the literature (Boyes et al., 2012;Madhav et al., 2013d).Some of the promising feed structures such as probe, Microstrip and coplanar wave guide feeding.Generally in the monopole antennas large ground planes with different shapes of rectangle, circle, square or ellipse are usually adapted.This slotted ground planes with different structures forms Defected Ground Structures (DGS) (Yoon et al., 2007;Gupta et al., 2005).
Defected ground structures when integrated with planar feeds will suppress higher harmonics in the antenna design.Dum-bell shaped defected ground structures are proposed by the researchers to address the suppression of harmonics up to second level.Further improvement in the suppression of harmonics was done by Mandal and Sanyal (2006).To suppress up to 3 rd harmonic a pair of dum-bell shaped DGS were employed beneath the Microstrip feed (Madhav et al., 2014b).As per the design point of view the physical area of the defect appears to be an important aspect, especially in the field of microwave integrated circuits (Madhav et al., 2014c;Syam Sundar et al., 2014).Smaller defect in terms of the operating wavelength is always preferred to make the design compact leaving maximum area for integrating other devices.This study focus on the low profile Microstrip feed monopole antenna on defected ground structure for wideband operations.The radiating element used in this model is the combination of square, arrow shaped line and inverted U-shaped element.The proposed model is designed and simulated using commercial electromagnetic tool HFSS.The complete parametric analysis with its performance characteristics of the designed models are analyzed and presented in detail in this study.

MATERIALS AND METHODS
Two models are designed and analyzed in this study for wideband applications up to 12 GHz with first model and up to 23 GHz for second model.These two models are covering bandwidth of 16 GHz.The first model consisting of a circular slot on the rectangular step shaped radiating element with the combination of arrow shaped and inverted U-shaped elements as shown in Fig. 1.The entire radiating element with feed line stands on one side of the substrate and the bottom side of the substrate consisting of a wide slotted defected ground structure as shown in Fig. 1. Figure 2 shows same radiating element like structure in Fig. 1, without circular slot on the rectangular step shaped radiating element.These antennas are designed on a thin flexible substrate of polyster with the thickness 0.8 mm during the design process first the patches, substrate and ground plane for dimension using manual slot cutting EM-tool.The design process is complex by taking initial dimensions of pure rectangular Microstrip topologies with known procedure.Two rectangular patches are established to resonated two frequencies.These two resonances were overlapped by integrating them in single radiating structure.This can be achieved by using a common feed line connecting the patch elements.The proposed structure is relatively complex and was obtained as the result of an optimization process within the solver used.The detailed dimensional characteristic of the proposed models are tabulated in Table 1 and 2 (Fig. 3).

RESULTS AND DISCUSSION
Figure 4 shows the return loss characteristics of the antenna models in their operating frequency range.Antenna1 shows the narrow band characteritics at 2.5 GHz and wideband characterictics between 7-12 GHz.An impedance bandwith of 8% is showing fundamental at resonant frequency and an impedance bandwith of 56% is showing at second resonant frequency band.Antenna 2 is giving a huge bandwidth from 12 to GHz that means 11 GHz with impedance bandwidth of 58% in the operating band.The same characteristics of antenna1 and antenna 2 can be observed in the VSWR curve of Fig. 5.
The radiation characteristics of the antenna models are shown in Fig. 6 and 7. Figure 6a shows a passive omni directional radiation pattern of antenna 1 at GHz. Figure 6b shows the H-plane radiation pattern of antenna 1 in the form of 8 shaped radiation with nulling st 90° and 270°.Figure 7a and b shows antenna radiation charactertics at 12 GHz.
Figure 8 shows current distribution on the radiating element of the antenna models at 12 GHz.It is been observed that the current intensity is more on the radiating element for antenna 2 compared with antenna 1.This is because of the wide band characteristics of the antenna 2 at higher resonant frequency.Figure shows the current distribution of the antenna models on the ground plane at 12 GHz.From Fig. 9 we observed that the most of the current elements are focused around the slotted portion of the defected ground structure which is giving rise to the additional resonant frequency at higher bandwidth.Figure 10 shows frequency vs. gain plots of the antenna models it is been observed that the antenna-2 is giving gain more than 5.5 dB at higher resonant frequency and average gain of 3 dB at lower resonant frequency antenna1 is producing an average of 2.8 dB at its operating frequency.
Figure 11 shows frequency vs. directivity of the models.Peak directivity values of antenna 1 and 2 are giving similar results of peak gain values in Fig. 10. Figure 12 shows the efficiency of the antenna models at their respective operating frequency bands.A minimum frequency of 99% for antenna 1 at lower resonant frequencies is obtained and a maximum efficiency of 103% is achieved for the case of antenna 2, at higher resonant frequency the overall efficiency of antenna models at their respective resonant frequencies are almost 100%.
Figure 13 shows the fabricated prototype of the antenna model on 0.8 mm thickness substrate material.The substrate material is flexible and occupies less

Table 2 :
Patch dimensions Fig. 4: Return loss vs. frequency of the two models