A New Triple Band Microstrip Fractal Antenna for C-band and S-band Applications

Recently research show that some parameters such as the shapes of antenna patch and the ground plane when geometrically altered produces changes in the current density distribution of the planar structure and consequently in the resonant modes. This paper presents a new microstrip fractal antenna using the technique of inserting slots of shape fractal in ground plane in order to increase the bandwidth and insertion discontinuities in the feed line to reach specific behaviors in three resonant modes. The FR-4 substrate with dimensions 85.0 x 85.0 x 1.57 mm3 is used. Also, it used different techniques of impedance matching in feed line of antenna with changes of the width of the transmission line in order to obtain a variation in the current distribution and consequently of the impedance bandwidth for S11 ≤ -10dB for C-band (3.625 GHz – 4.2 GHz) and S-band (2.0 GHz – 4.0 GHz). Good agreement between measured and simulated results is achieved. Proposed fractal microstrip antenna can be easily designed, built and applied in wireless communication.


I. INTRODUCTION
Nowadays, the microstrip antennas are especially attractive because of its low cost, low profile and ease of integration with other circuit elements.The patch and the ground plane may have various geometric configurations and input impedances are usually 50 Ω or 75 Ω.Some devices in wireless communications require broadband transmission and even ultra-broadband.In order to obtain the desired resonant modes is employed the addition of slots in microstrip antenna, both in the patch and the ground plane [1][2][3].The removal of ground plane parts for various applications has been studied, as curve fractal-shaped for communication systems [4].Studies of monopole antennas have been considered in order to find resonant modes for WLAN/WiMAX applications [5].Investigation of radiation characteristics for microstrip antennas using geometries such as the island and the curve of Minkowisk as well as fractal-shaped slots in the ground plane is shown in [6].Several studies have shown that the impedance bandwidths for printed antennas can be controlled by coupling between the patch and the slot in the printed ground plane [7][8][9].In [10], the author suggests variation in width of A New Triple Band Microstrip Fractal Antenna for C-band and S-band Applications Edwin L. Barreto 1 , Laércio M. Mendonça 2 , 1 Federal Rural University of the Semi-Arid Av.Francisco Mota, 572, CEP:59600-97, Mossoró, RN, Brazil 2 Federal University of Rio Grande do Norte, Dept. of Communication Engineering Av.Salgado Filho, 3000 CEP: 59072-970, Natal, RN, Brazil edwinbarreto@ufersa.edu.br 1 , laercio@ct.ufrn.br 2 the antenna's feed line, in order to obtain other resonant modes.Reference [11] shows different levels fractal geometry can increase the electrical length of the slot sides that help in controlling the resonant modes and makes the antenna resonates between 2 GHz and 6 GHz.Slots with fractal-shape and the use of fractals geometry defects in ground plane producing two resonant modes in 2.5 GHz and 5.5 GHz is reported in [12].Studies of microstrip patch antenna with defected ground structure (DGS) for WLAN/WiMAX applications are shown in [13].
In and WiMax (3.5 GHz / 5.5 GHz).In order to validate the technical proposals is realized a comparison of the optimized antenna in this paper with another of the literature [13], observing best performance for the antenna here reported.
The remainder this paper is organized as follows.Section 2 describes the proposed antenna with resonant modes in terms of the discontinuities of feed microstrip line, patch of antenna and defects in ground plane.Parametrization analysis is presented and analyzed in Section 3. Section 4 shows the results and discussion for simulated and measured data are presented and analyzed for the resonant modes, the linear current density, the radiation patterns and the gain.The conclusion is given in Section 5.

II. PROPOSED ANTENNA: ANALYSIS AND DESIGN
In this section are analyzed the microstrip fractal antenna techniques and the effects of partial ground plane removal and of discontinuities of feeding microstrip line on the transmission characteristics of the proposed antenna.Some tests were performed with various forms of ground plane and patches in analysis of the antennas and it was choosing the shaped-fractal for these structures.Fractal geometry offers almost unlimited ways of describing, measuring and predicting the resonant modes for planar structures of transmission.In computational numerical simulations was used Ansoft HFSS Vs. 13 software using finite element method, for optimization of planar microstrip antennas.After the optimization, the antennas were built and measurements carried out in UFRN laboratories using an analyzer Vector network of Rohde & Schwarz.The topics following describe the main parameters in the proposed antenna design.In order to do a comparative analysis, we noted that the case with the rectangular patch, it isn't satisfactory in terms of impedance matching, because it does not reach the insertion loss of -10 dB for operation of the printed antenna without discontinuities.However, for the case with patch fractal and using discontinuities results the appearance of resonance modes in 4.3 GHz and 8.1 GHz.  Figure 5 shows the gain of antennas for variations of frequencies between 1 GHz and 10 GHz, and values less than 0 dBi.This implies in no antenna radiating.Therefore, we have that considering the insert of defects in the ground plane and realize the analysis on the gain of the antennas.

C. Defect on the Ground Plane
The defects on ground plane are used with the optimized patch antenna in order to investigate the effects of fractal slots in the characterization of printed antennas in search of better behavior.We tested different shaped-slots on ground plane, as seen in Fig. 6.In  The fractal antenna seen in Fig. 6(c) was used for the physical construction and measurement.Figure 9 shows the gain peaks in dB in range between 1 GHz and 6 GHz for the proposed antenna.
In this case, fractals elements inserted by Minkowisk curve changed the formation of the fields in antenna and therefore the insertion of resonant modes for antenna operation to frequencies band of WLAN and adjusting the gain for a better propagation of the signal.

III. PARAMETRIZATION ANALYSIS
Variations were performed to investigate the relationship of influences on the antenna response as function of the L 1 e W 2 parameters.The variation of parameters L 1 and W 2 have direct influence on the impedance ratio given by 1/4 wavelength transformer, which causes direct change in the reflection coefficient and control of frequency.For this test were considered the values of W 2 = 0.9 mm; 1.0 mm; 1.1 mm.For L 1 , were used 7 mm; 9 mm; 10 mm; 11 mm; 13 mm, as shows the Table II.First it was considered L 1 = 10 mm and variations W 2 as seen in Fig. 10.After this, it was done simulations considering W 2 fixed in 1.0 mm and variations in L 1 as seen in Fig. 11.

TABLE II. PARAMETRIZATION
Fixed Parameter Variety Parameter

C. Radiations Pattern and Gain
The simulated radiation patterns for the resonant frequencies of the proposal antenna are shown in Fig. 17.Simulations for far fields E (ZX plane) and H (ZY plane) were obtained using the HFSS software.Fig. 18 shows gain plot for measured antenna.

D. Comparative Analysis
Figure 19 shows the comparing the simulation results of the antenna proposed in this work and results in [13].It is evident the difference between a dual-band antenna and the antenna in this work that has three resonant modes operating in 2.1GHz, 3.8 GHz and 5.3 GHz.In this analysis the simulated bandwidths of proposed antenna were BW 1 = 1.912GHz-2.115GHz, BW 3 = 3.55 GHz-4.39GHz, BW 4 = 5.39 GHz-5.49GHz; and the gain of 1.4 dBi, 4.8 dBi and 2.9 dBi for 2.1GHz, 3.8 GHz and 5.3 GHz, respectively, while the antenna proposed by Mohammad et.Al [13], presented BW 2 = 2.276 GHz-2.553GHz and BW 5 = 5.144 GHz-5.90GHz bands with gain 2.053 dBi and 4.52 dBi for resonant modes 2.43 GHz and 5.52 GHz, respectively.
Note that for the two antennas in comparison were used similar techniques such as transformer 1/4 wave, fractal defects and patch geometries.
In table III is shown numerical parameters between the proposed antenna this paper and the proposed antenna in [13].The maximum return loss of −11 dB, −13 dB and −23 dB is obtained for these resonant modes.The enhancement of transmission characteristics, the small size, the light weight and the cost effectiveness of the proposed antenna makes strong candidature for small and slim wireless devices in telecommunication systems.

Figure 1
Figure 1 shows a microstrip feed line with discontinuities, where the line changes the width, being shown in Fig. 1 (a).The Fig. 1(b) depicts fringing electric field and the Fig. 1(c) shows equivalent circuit.This change in the width of the microstrip line is widely used for network configuration, transformers and ¼ wavelength couplers.The main characteristics these lines are: (i) the parasite effect or fringe fields effect (due to capacitance associated with the widest line of discontinuity); (ii) the phase-shift related to the discontinuity; (iii) the parasitic effect of a step-junction that is similar to open-end circuit.These characteristics are parameters used in the analysis of microstrip antenna.

Fig. 1 .
Fig. 1.Characteristics of discontinuities for microstrip line: (a) change in width; (b) appearance of fringe electric field; (c) equivalent electric circuit with two Ls inductances and one Cp capacitance.

Fig. 2 .
Fig. 2. Evolution of geometry of microstrip patch antenna with full ground plane: (a) rectangular patch; (b) patch fractal;(c) patch fractal with discontinuity.

Figure 3
Figure 3 shows the simulation of the return loss characteristics for the structures shown in Fig 2(a)-(c).In order to do a comparative analysis, we noted that the case with the rectangular patch, it isn't

Fig. 3 .
Fig. 3.Return loss for the proposed patch antenna.

Figure 4
Figure 4 shows the radiation pattern for E-plane and H-plane for the structures of Fig. 2. Fig. 4(a) depicted the planes ϕ = 0 o and θ = 90 o with geometry of full ground plane of the rectangular patch antenna and resonant mode of 10 GHz, whereas for the Fig. 4(b) shows the fractal patch antenna with resonant mode of 8.2 GHz.Fig. 4(c) shows the patch fractal with discontinuity and resonant modes in 4.3 GHz and 8.1 GHz.

Fig. 6 .Figure 7
Fig. 6.Evolution of the geometry of printed ground plane, for: (a) geometry with full ground plane; (b) rectangular-shaped slot on the ground plane; (c) fractal-shaped printed slot on ground plane.

Fig. 7 .
Fig. 7.Return loss for the tested cases of defect ground plane.

Figure 8
Figure 8 shows the measured radiation patterns in H and E planes of the proposed antenna for the resonant modes in 4.3 GHz and 8.1 GHz with geometry full ground plane (a), with resonant mode in 6.8 GHz for rectangular-shaped slot on the ground plane (b), and resonant modes in 2.1 GHz, 3.8 GHz and 5.3 GHz for fractal-shaped printed slot on the ground plane (c).

Fig. 8 .
Fig. 8. Radiation patterns of E-Plane and H-plane for cases of defects on ground plane for its resonant modes.

Figure 10
Figure 10 shows the variation of W 2 , considering fixed L 1 = 10 mm.Thus, it can change the simulated reflection coefficient and the bandwidth for S11 < -10dB practically unchanged.It happens for resonant modes in 2.2 GHz, 3.7 GHz and 5.0 GHz.

Fig. 14 .
Fig. 14.Photograph of the fabricated antenna proposal: (a) microstrip patch; (b) ground plane.A.Return LossThe behavior three bands have remained, as seen in Fig.15.Simulations and measurements were performed to assess the behavior of antennas in terms of the S 11 scattering parameter.For the analysis

Fig. 15 .
Fig. 15.Return loss measured and simulated.B.Current Density DistributionFigure16shows the distribution of current density J (A/m) for optimum antenna at frequencies of 2.1, 3.8 and 5.3 GHz.It can be observed that the antenna impedance changes due to the resonant properties of the discontinuities and ground slot shaped.The surface current concentrated on the edges of the interior and exterior of the fed by microstrip discontinues.The slits and indentations as well as the use of discontinuities in microstrip lines is much studied in order to parasitic effects and structures for linear current distributions analysis.Then observed a radiation pattern similar to a conventional patch rectangular antenna, in which the maximum radiation in the far field occurs in the direction perpendicular to the radiating element in type broadside.

Fig. 19 .
Fig. 19.Comparison between the dual band antenna proposed in[13] and the triple band antenna proposed in this paper.
this paper, a new triple band antenna fractal microstrip for C-Band and S-Band applications is proposed.The methodological technique consists initially design and optimize a microstrip patch antenna with full ground plane fractal with resonate modes at 4.3 GHz and 8.1 GHz.As a second step, it's inserted two defects in ground plane and then the microstrip patch fractal antenna adds two resonant modes at 5.0 GHz and 6.8 GHz, besides having controlling of resonant modes and impedance in the operating range of interest.In this way, the antenna with fractal defect and feed line discontinuity (¼ wavelength transformer) starts to resonate in three modes: 2.2 GHz, 3.7 GHz and 5.0 GHz for dimensions of specific parameters.The behavior of the resonant modes of the structure and the influence they suffer due to the level of fractal geometry and defects in the ground plane structure are important factors to be considered in this paper.

TABLE I .
OPTIMIZED PROPOSAL ANTENNA