# Minkowski Island and Crossbar Fractal Microstrip Antennas for Broadband Applications
## 1 Introduction  
Antennas are a key element in wireless communication systems and one of the important problems in antenna design is to provide wideband characteristics. Telecommunication systems, especially those intended for military applications, should be equipped with an antenna capable of supporting a wideband or even ultra-wideband (UWB) frequency range. In addition, for some systems, such as cognitive radio, the antenna should allow for dynamically and autonomously emitting and/or receiving signals in the desired frequency ranges [1]. The efficiency of a radio broadcasting system is directly related to the characteristics of its antenna. Like other designs before them, microstrip antennas revolutionized antenna technology. Attractive properties of microstrip patch antennas--low profile, light weight, compact and conformable structure, and easy fabrication, make them good candidates for wireless systems. However, microstrip patch antennas have a narrow bandwidth, which is a significant drawback that limits their usefulness. The conventional microstrip antenna consists of a metal strip conductor (patch) as an upper layer of a grounded substrate. The performance of microstrip antenna, in terms of bandwidth and radiation characteristics, is determined by its shape, dimensions and the dielectric constant of the substrate. With the development of new materials called left-handed metamaterials (LHM), it is possible to create antenna designs with new properties. The microstrip antennas based on composite LHM should be characterized by simultaneously negative permittivity and permeability. Such materials exhibit counter-intuitive electromagnetic phenomena, such as reversed refraction. As a result, they can be referred to as negative refractive index (NRI) materials, since the wave vector forms a left-handed triplet. Structures based on split ring resonators (SRR), with concentric rings separated by gaps in the ground plane, can yield to a material with simultaneously negative permittivity and permeability [2]. Such a metamaterial-based antenna can be smaller despite having improved properties. However,the antennas based on SRR and wire strips cannot achieve a sufficiently wide frequency range. This is why many different attempts have been proposed to widen the bandwidth of microstrip antenna using left-handed metamaterial structures. Among these, we can find a broadband planar antenna composed of unit cells and a dipole element having metamaterial property [3]. This configuration achieved a bandwidth of 1.2 GHz. Planar left-handed metamaterial structures were proposed by [PERSON] et al. [4]. The presented structures consisted of 2D periodic arrays of unit cells on both the upper and lower layers. On the upper layer, 45-degree rotated square metal cells were placed, while, on the lower layer, four isosceles triangles were laid out and surrounded by a square frame. This configuration allowed for obtaining a frequency range, which is several times wider than one achieved by the same patch antenna without the periodic pattern. Taking into account the fact that the main motivation of this work was to design an antenna with the widest possible bandwidth, the structure of the microstrip antenna was motivated by the advantages of using fractals in antenna engineering. The proposed antenna geometries were based on crossbar and Minkowski fractals, as an increased number of resonances over a given or fixed frequency band was reported for such antennas [5]. Fractal antennas are designs inspired by structures found in nature, and take up highly convoluted shapes. Fractal shapes applied in antenna designing can have various shapes and forms [3; 6]. One can mention the following mathematically described shapes of fractals: Sierpinski carpet and gasket, Minkowski gasket, Koch snowflake, Mandelbrot set, crossbar, etc. These are the deterministic fractals, which can be used to create a scaled-down design with a recursive algorithm.