Broadband Proximity Fed Modified E-Shaped Microstrip Antenna Amit A. Deshmukh 1 , M. Parulekar 1 , S. Kadam 1 and Ameya Kadam 2 1. DJSCOE, Vile-Parle (W), Mumbai – 400 056 Email : amitdeshmukh76@rediffmail.com 2. TCET, Kandivali (E), Mumbai – 400 101 K. P. Ray 3 3. RFMS, SAMEER, I.I.T. Campus, Powai, Mumbai – 400 076, Email: kpray@rediffmail.com Abstract: Commonly used technique to realize broadband microstrip antenna is by cutting the slots at an appropriate position inside the patch. The slot introduces a mode near the fundamental mode of the patch and yields broadband response. The bandwidth of an E-shaped microstrip antenna is further increased by cutting the pair of rectangular slots on one of its edge. This increase in bandwidth is due to another mode introduced by additional pair of slots. In this paper, modal variations, for this modified E-shaped antenna are studied. It has been observed that the slot affects the resonance frequency of higher order mode, which along with modes of E-shaped patch, realize higher bandwidth. Also the broadband proximity fed variation of modified E-shaped antenna in 950 MHz frequency range is proposed. This antenna gives a larger BW of 47% with broadside radiation pattern with a peak gain of approximately 10 dBi. Key terms: Broadband microstrip antenna, E-shaped microstrip antenna, modified E-shaped microstrip antenna, proximity coupling I. INTRODUCTION The microstrip antenna (MSA) has several advantages like low profile planar configuration, however its applications gets restricted because of lower operating bandwidth (BW) [1, 2]. While maintaining the same antenna size, the broader BW is realized by cutting the slots of either half wave or quarter wave in length, having different shapes like, U-slot, V-slot, and pair of rectangular slots inside the patch [3 – 7]. The slot introduces a mode near the fundamental mode of the patch and realizes broadband response. The bandwidth (BW) of rectangular MSA (RMSA) is increased by cutting a pair of rectangular slots on one of the radiating edges, leading to an E-shaped MSA [8]. Further increase in BW of an E-shaped MSA is obtained by cutting additional pair of slots on one of its radiating edge [9]. This configuration is referred as Ψ- shaped MSA [9]. The increase in BW is reported to be because of mode introduced by the additional pair of slots which forms a tuning stub. However, it is not explained that which mode is leading to increase in BW. Also the Ψ-shaped MSA gives a peak gain very close to 10.5 dBi, which reduces to as low as 3.5 dBi towards the higher frequencies of the realized BW where mode due to the tuning stub is present. In this paper, to understand the increase in BW of Ψ-shaped MSA, the modal variations for the proposed antenna, were studied. It was observed that the additional pair of slots which forms the tuning stub alters the higher order TM 21 mode resonance frequency of the RMSA and along with the modes of an E-shaped patch, realizes larger BW. The radiation pattern at TM 21 mode is conical and it affects the radiation pattern of Ψ-shaped MSA towards the higher frequencies of the BW. Therefore, the Ψ-shaped MSA shows higher cross-polar radiation pattern (reduced gain) at those frequencies. Also, the broadband proximity fed design of Ψ-shaped MSA is proposed in the 950 MHz frequency band. The proximity fed antenna gives a measured BW of 524 MHz (47.7%) with a peak gain very close to 10 dBi. The modal variations for Ψ-shaped MSA and the proximity fed Ψ-shaped MSA were analyzed using IE3D software [10]. Further, the experiment was carried out to validate the result for proximity fed MSA. The proximity fed Ψ-shaped MSA is analyzed using the air substrate to realize maximum radiation efficiency. II. Ψ-SHAPED MICROSTRIP ANTENNA The Ψ-shaped MSA is shown in Fig. 1(a, b). The dimensions as shown in the figure are in cm. The patch is fabricated on microwave substrate (ε r = 2.50, h = 0.33 mm, tan δ = 0.002) and it is suspended above the ground plane using a foam substrate (ε r = 1.06, h = 6 mm, tan δ = 0.0002). In this configuration, the horizontal pair of slots is cut on one of the edges of the E-shaped MSA. This configuration on infinite ground plane gives a BW from 3820 to 7100 MHz, which is more than the BW of equivalent E-shaped MSA [9]. The radiation pattern is in the broadside direction with a peak gain very close to 10.5 dBi. However, towards the higher frequencies of the BW, gain reduces to less than 4 dBi. The increased BW in Ψ-shaped MSA is attributed to improved control of the current distribution on the patch by removal of the bottom side conductors (horizontal slots), resulting in a tail part [9]. This tail part acts as a tuning stub and introduces a mode near 7000 MHz and increases the BW [9]. However, a clear description of which mode is getting introduced by this stub is not been presented. To study this increased BW, the modal analysis of broadband Ψ-shaped MSA against that of RMSA is presented in the following section.