Broadband Designs of a Triangular Microstrip Antenna with a Capacitive Feed

44 Broadband Designs of a Triangular Microstrip Antenna with a Capacitive Feed Mukesh R. Solanki, Usha Kiran K., and K. J. Vinoy * Microwave Laboratory, ECE Dept., Indian Institute of Science, Bangalore, India, 56 12 *Tel. +91(8) 2293 2853; Fax.: +91(8) 236 563, Email: kjvinoy@ece.iisc.ernet.in Abstract Broadband designs of a triangular microstrip antenna with a small capacitive-feed strip are proposed here. An SMA probe is attached to the feed strip which is capacitively coupled to the radiating patch on the same plane. The performance of an equilateral triangular microstrip antenna has been studied extensively. The location of the feed strip with respect to the triangle have also been investigated. Further, the effect of truncating the corners of the patch has been studied. The simulation results are validated experimentally. Index Terms Broadband antennas, Triangular microstrip antennas, Capacitive-feed. I. INTRODUCTION Broadband operation is becoming increasingly popular in several practical applications including next-generation wireless terminals. Broadband antennas that are small in size and simple in structure are typically demanded for such applications [1]. Microstrip patch antennas (MPA s) are widely preferred for wireless communication systems as they are of small size, light weight, low profile, low cost, and are easy to fabricate and assemble [2]. The triangular geometry drew the attention of MPA researchers as it is smaller than other patch geometries [3]. However, basic geometries of these antennas suffer from issues such as a small bandwidth. Several approaches for broadening the bandwidth of triangular MPA s have been proposed in the literature. Some of these approaches include using a multilayered configuration with a superstrate [3], loading the patch with properly arranged slots [4], [5], using an L-shaped probe [6] or loading with a chip resistor [7], [8]. Recently, broadband operation of the triangular patch antenna was achieved by placing two shorting walls at the opposite edge, truncating the tip and inserting a V-shaped slot. These modifications result in exciting two resonant modes simultaneously which together gives more than 25% impedance bandwidth [9]. A broadband configuration of triangular MPA employing a folded shorting wall is proposed by Li et. al. who reported a bandwidth of 28.1% [1]. Similarly a bandwidth of 53% is achieved by a planar inverted-f triangular patch antenna with V-shaped unequal arms and folded shorting wall [11]. However, these broadband designs have relatively complicated antenna structures.

45 In this paper, we present a simple design of broadband triangular patch antenna consisting of a triangular patch fed with a small coplanar rectangular capacitive feed placed very close to the radiator on a substrate. First, an equilateral triangular microstrip antenna fed with a small rectangular patch placed symmetrically and parallel to an edge has been studied. Later, effect of the location of the feed is studied by placing the feed close to a vertex of the triangle. Further, the effects of truncation of the tips of the triangle on the performance of these two antenna configurations are investigated. Four different configurations of triangular patch antennas (edge and vertex fed triangular patch antennas with and without truncations) are compared in terns of their impedance and gain bandwidths. Based on the simulations results a prototype is chosen for fabrication and experimental studies. II. ANTENNA DESIGN Fig. 1 shows the basic geometry of the proposed broadband triangular patch antenna with a small coplanar capacitive feed strip. The antenna is designed for a microwave substrate (thickness.787 mm, ε r = 2.2 and loss tangent.9) and spaced an air gap of height Δ above the ground plane with the size of 2 2 mm 2. a x θ Patch y (a) z Substrate Ground Air gap (b) SMA Δ h Fig. 1. Geometry of proposed triangular broadband patch antenna capacitively fed with a small coplanar strip. (a) Top View, b) Cross sectional View The antenna is excited by a small rectangular shaped feed strip of dimension (x y) mm 2. A coaxial probe of radius.6 mm is used for the excitation as shown. There is a gap (z) between the radiating triangular patch and the feed strip. Although the angle at the vertex of the triangle (θ) can be varied, we have used an equilateral triangle in this study.

46 The equilateral triangular patch antenna is designed for the operational frequency band 4 to 6 GHz based on the design formula [2], [12], [13]: f 2 r = m + mn + 3a 2c eff ε re n 2 (1) where the effective side length, a eff, of the triangle is a eff h + Δ = a + (2) ε and the effective dielectric constant, ε re, for this 2-layer configuration (air and dielectric substrate) is obtained using the quasi-static expression re ε re ε r ( h + Δ) = h + ε Δ r (3) In (1) to (3), f r = Resonant frequency. a = Side of the triangle. 8 c = 3 1 m /sec h = Thickness of the substrate. ε r = Dielectric constant of the substrate. Δ = Height of the air gap. mn, Correspond to the mode of EM wave distributions in the patch. As it gives the smallest antenna, the TM 1 mode is typically used in the design of the antenna. The effects of various key parameters on antenna design are studied using IE3D 12. which is a method of moments (MoM) based electromagnetic (EM) software. III. PARAMETRIC STUDIES In this section, the design studies for various geometrical features of the proposed broadband triangular microstrip antenna are presented. A. Effect of Air gap In this study the patch dimensions are kept constant for a = 17 mm and the air gap is varied between 4 to 9 mm. It may be noted that increasing the air gap results in reduction of the effective permittivity for the patch and change in the feed reactance (increases the inductance of the feed pin and decreases the capacitance of the feed strip). Hence the dimensions of the feed strip are adjusted for the best impedance match for each case presented in Fig. 2. From Fig. 2, it is seen that maximum bandwidth is obtained for an air gap of 7 mm. Note that the patch dimensions are not changed in this study. Yet the relative placement of resonant frequencies changes with air gap. This indicates that the resonant frequencies observed in

47 Fig. 2 do not correspond to different resonant modes of the basic patch structure. It may be verified the antenna structure has a TM 1 resonance close to the higher null in the return loss characteristics. The return loss at lower frequency is improved by the nature of impedance matching provided by this feed arrangement, and this does not add any additional resonant modes to the antenna structure. -1 Return loss (db) -2-3 -4 4 5 6 7 8 9 1 Frequency (GHz) 4 mm 5 mm 6 mm 7 mm 8 mm 9 mm Fig. 2. Return loss characteristics for different values of air gap. B. Effect of Patch Dimensions Taking the air gap of 7 mm, the antenna dimensions were varied to arrive at a suitable design for the frequency range of 4 to 6 GHz. The return loss obtained for a range of values for the side of the triangle are shown in Fig. 3, and based on this study the optimum value for a is chosen to be 28 mm. It is observed that for a range of dimensions the antenna has a wide operational bandwidth. C. Optimum Value for the Air Gap Height As noted earlier, the dimensions of the patch for a desired frequency can be obtained using (1) to (3). However this requires a priori knowledge of the air gap height between the substrate and the ground. It has been concluded that in order to achieve the best bandwidth characteristics for an antenna of certain range of frequencies, an optimum air gap is needed. Based on the study we conclude that the air gap should be such that Δ.14λ h ε r (4) This expression agrees well with results for rectangular patch antennas on a monolithic substrate [16], [17]. D. Extent of Truncation It is widely known that current amplitude is minimum near the tips of a triangular patch. Yet the capacitance offered by these regions may affect the antenna bandwidth. Based on observations, the tip of the triangle (the corner opposite to the feeding edge) is truncated differently than the other corners. The truncation at the tip and the two other vertices are p and q respectively. In Fig. 4, we

48 kept p =.5 mm, while q is varied from to 2 mm. The maximum bandwidth of 36.4%is obtained for q = 1.8 mm. Again for this value of q, the effect of tip truncation is investigated for different values of p. Based on results in Fig. 5 it is concluded that p =.5 mm and q = 1.8 mm results in the best bandwidth characteristics for the antenna. -1 Return loss (db) -2-3 -4 3 4 5 6 7 8 Frequency (GHz) 2 mm side 24 mm side 28 mm side 32 mm side Fig. 3. Return loss characteristics for different side lengths of triangular radiating patch for air gap 7mm. -1 Return loss (db) -2-3 -4 4 4.5 5 5.5 6 q = mm q =.5 mm q = 1 Frequency mm q = (GHz) 1.5 mm q = 1.8 mm q = 2 mm Fig. 4. Return loss characteristics for different chopped edges (q). E. Effect of feed location The proposed capacitive feed configuration gives a bandwidth of about 35% [16]. We have further investigated the possibility of feeding the antenna near a vertex. The resulting antenna geometries are shown in Fig. 6. Further enhancement in the bandwidth has been obtained for vertexfed over edge-fed triangular patch antenna. The typical dimensions of the proposed broadband triangular antenna designs in Fig. 6 for optimum bandwidth are listed in Table 1. Fig. 7 shows the simulated return loss (S 11 ) for all the four proposed antennas in Fig. 6. It is clear that good matching conditions are obtained for all the antennas. The bandwidths for these antenna configurations are tabulated in Table 1. The bandwidth obtained in all these cases is quite comparable with the broadband triangular antenna available in the

49 literature [6]. However, since the feed and the radiating patch are on the same side of the substrate, the proposed antenna design is simple-to-fabricate. Furthermore, it is clear from Fig. 7, that vertex-fed triangular patch antenna gives more bandwidth than the edge fed antenna. Minor improvement in the return loss characteristics is observed in both cases while truncating the corners of triangle [17]. -1 Return loss (db) -2-3 -4 4 4.5 5 5.5 6 Frequency (GHz) p= p=2 mm p=.5 mm p=.8 mm p=1.1 mm Fig. 5. Return loss characteristics for different chopped edges (p). Edge fed antennas Vertex fed antennas Triangular patches without truncation a c Antennas with truncation p p b q d q Fig. 6. Geometries of the proposed broadband antennas. The simulated gains of the four proposed broadband triangular microstrip antennas are as shown in the Fig. 8. It is clear that the gain is comparable till about 4.5 GHz. Based on these observations it is concluded that the vertex-fed triangular microstrip antenna [Fig. 6(c)] gives the best possible performance and has been selected for experimental validation.

5 TABLE 1. TYPICAL DIMENSIONS AND SIMULATED PERFORMANCE OF THE PROPOSED BROADBAND TRIANGULAR ANTENNAS. Parameters Edge-fed (without truncation) Edge-fed (with truncation) Vertex-fed (without truncation) Vertex-fed (with truncation) Air gap (Δ) 7 mm 7 mm 7 mm 7 mm Length of the feed strip (x) 1.2 mm 1.2 mm 1.2 mm 1.2 mm Width of feed strip (y) 6.2 mm 6.2 mm 9. mm 9. mm Separation between radiating patch and the feed strip (z).4 mm.4 mm.4 mm.4 mm Truncation at tip (p) -.5 mm -.5 mm Truncation at corners (q) - 1.8 mm - 1.5 mm Start of band (S 11 <-1dB) 4.16 GHz 4.17 GHz 4. GHz 4.6 GHz End of band (S 11 <-1dB) 5.87 GHz 6.1 GHz 6.3 GHz 6.6 GHz % Bandwidth 34.13 36.14 4.47 39.52 Return loss, db -1-2 -3-4 4 4.5 5 5.5 6 Frequency, GHz Edge-fed (without truncation) Edge-fed (with truncation) Vertex-fed (without truncation) Vertex-fed (with truncation) Fig. 7. Simulated return loss characteristics for the antenna geometries in Fig 6. 1 8 Gain (dbi) 6 4 2 4 4.5 5 5.5 6 Frequency (GHz) Edge-fed (without truncation) Vertex-fed (without truncation) Edge-fed (with truncation) Vertex-fed (with truncation) Fig. 8. Simulated gain characteristics of antennas in Fig. 6.

51 IV. EXPERIMENTAL VALIDATION A prototype of the vertex-fed triangular microstrip antenna which gives highest bandwidth is fabricated and tested experimentally. The fabricated antenna is shown in Fig. 9. The return loss characteristics of this antenna is measured using Agilent PNA (N523A) and is shown in the Fig. 1. The measured bandwidth (S 11 < 1dB) ranges from 3.57 GHz to 5.65 GHz. The experimental results appear to be slightly different from the simulated results. This may be attributed to fabrication issues such as bumps caused by the solder, slight curvature of substrate due to its flexible nature, and the minor changes in the air gap height. The radiation patterns are measured for this antenna over its bandwidth in a microwave anechoic chamber. Good broadside radiation patterns are observed over the entire range (Fig. 11). As expected for most wideband designs of similar configurations, the cross-polarization level of this antenna is slightly high. This may be due to the long SMA pin required in this 2-layer, air-dielectric substrate configuration. Fig. 9. Fabricated triangular patch antenna with a vertex-feed. -5-1 S11(dB) -15-2 -25-3 3.5E+9 4.E+9 4.5E+9 5.E+9 5.5E+9 6.E+9 6.5E+9 Frequency Fig. 1. Experimental results of Vertex-fed triangular patch antenna.

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