Int. J. Electron. Commun. (AEÜ) 65 (2011) 954–957 Contents lists available at ScienceDirect International Journal of Electronics and Communications (AEÜ) j our na l ho mepage: www.elsevier.de/a eue Asymmetrically loaded annular ring antenna Madhurika Mahajan a,∗ , T. Chakarvarty b , Sunil Kumar Khah c a Department of Physics, Chitkara University, Barotiwala, Distt-Solan (HP), India b EIS – MD & P, Tata Consultancy Services, Salt Lake Electronics Complex, Kolkata 700091, West Bengal, India c Department of Physics, Jaypee University of Information Technology, Waknaghat, Distt-Solan 173215(HP), India a r t i c l e i n f o Article history: Received 5 June 2010 Accepted 13 March 2011 Keywords: Microstrip antenna Cavity model Resonant frequency Asymmetric loading Symmetric loading a b s t r a c t A numerical model for resonant frequency using asymmetric loading is presented using cavity model. In asymmetric loaded antenna, the posts are not equispaced on annular ring antenna and resonant frequency is independent of the angular location of posts. The posts are located away from the centre of patch. The diameter of post is thin as compared to the diameter of annular ring antenna. The first few dominant modes calculated for asymmetric loading are compared to symmetric loading where the posts are to be located at specific angular locations. The theoretical model is compared to the simulated results of IE3D based on Method of Moments. The numerical model developed can also be verified further by the full wave analysis. © 2011 Elsevier GmbH. All rights reserved. 1. Introduction Antennas which are used in wireless local area networks (WLANs) needs operation at two or more discrete bands and an arbitrary separation of bands [1]. Loading can take various forms such as stub loading, slots, shorting posts, parasitic couplings, sub- strate loading, superstrate cover, resistors, capacitors and diodes. Loading can lead to polarization diversity [2], frequency agility [2,3] and radiation pattern control [4–6]. Shorting posts in microstrip antennas have recently come into use in order to take the advan- tage of different properties such as dual frequency, tunability and compactness [7–9]. Different geometries such as triangular and square patch microstrip antennas have been studied for dual band operation and to enhance bandwidth. A circular disk with off- centered shorting posts was first proposed by De [10]. Loading via shorting posts can be done symmetrically [11,13] or asymmetri- cally [12]. These two techniques are different in the aspect that the shorting pins are located at different positions with respect to feeding point. In this work the resonant frequency for asym- metric loading with two shorting pins at the same distance from the centre of annular ring is formulated and compared with the simulated results of IE3D software based on method of moments. The comparison of angular location of shorting pins for asymmet- ric loading and symmetric loading is also presented. The first few modes are calculated for both asymmetric and symmetric types of loading. In the even modes (TM m1 modes) the annular antenna ∗ Corresponding author. Tel.: +91 9418452888. E-mail address: madhurikam@gmail.com (M. Mahajan). acts as a good radiator and is a good resonator for odd modes (TM m1 mode) 2. Theory The basic ring geometry with two asymmetric loaded shorting posts is shown in Fig. 1. When the pins are located asymmetri- cally means the shorting posts are not equispaced on the circular disc. The annular ring of inner radius a and outer radius b is loaded with two passive conducting posts of radius  at any angular locations on the circumference of a concentric circle of radius ‘c’ where a < c < b. The annular ring will be divided into two regions. The field equations for the two regions are given in [12]. Application of vanishing tangential component of magnetic field H (1)  (a) = 0, H (2)  (b) = 0 in the two regions at the boundary edges lead to modification of field equations. The field equations satisfy the boundary conditions [11]. It is assumed that the diameter of the circular post is small. The wall admittance is also ignored for both the cases. In symmetrical loading the posts are equispaced along the cir- cumference of circle concentric with the patch radiator. For a single post, symmetry is maintained when  = 0 ◦ /180 ◦ , i.e. the post is located in the same line as that connecting the feed point and the centre of the patch. When two posts are applied they are applied at an equal distance from the feeding point. The general formula for N no. of pins is 2/N. The analysis of resonant frequency for symmetric loading is presented in [11] based on cavity model where variation of electric field within the substrate has only z- 1434-8411/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2011.03.007