RADIOENGINEERING, VOL. 19, NO. 4, DECEMBER 2010 633 Sierpinski-Based Conical Monopole Antenna Petr VŠETULA, Zbyněk RAIDA Dept. of Radio Electronics, Brno University of Technology, Purkyňova 118, 612 00 Brno, Czech Republic xvsetu00@stud.feec.vutbr.cz, raida@feec.vutbr.cz Abstract. Planar Sierpinski monopole exhibits a multi- band behavior, but its parameters in operation frequency bands are not optimal. By mapping the Sierpinski mono- pole on a conical surface, a symmetrical three-dimensional (3-D) structure is obtained. In this way, a larger bandwidth and a better radiation pattern is achieved. The symmetrical 3D Sierpinski-based monopole is an original contribution of this paper. In the paper, different versions of the conical Sierpinski- based monopole are designed, and results of simulations performed in CST Microwave Studio are mutually com- pared. Then, the simulated versions of the conical mono- pole are optimized according to specified criteria. The optimized conical Sierpinski-based monopole is manufac- tured and its properties are experimentally verified. Results of measuring the Sierpinski-based conical monopole antenna are published here for the first time. Keywords Sierpinski monopole, multi-band antenna, conformal antenna, fractals, conical monopole. 1. Introduction In today’s communication devices, multi-band anten- nas play a relevant role. The multi-band behavior of the antenna can be obtained by applying self-similarities of fractals [1]. The number of operation frequency bands depends on the number of fractal iterations then [2]. In this paper, ways of converting a planar version of the Sierpinski monopole to the conformal, conical antenna are discussed. Following the described way, two versions of the conical monopoles can be created. The designed antennas are modeled in CST Microwave Studio and their properties are mutually compared. The first kind of the conical monopole antenna was published in [3], and the second one is an original contribution of this paper. The designed conical Sierpinski-based monopole is optimized using particle swarm optimization (PSO) and the Nelder-Mead simplex algorithm to reach a proper imped- ance matching in specified frequency bands. The optimized antenna is measured and results are compared with simula- tions. In Section 2, properties of a planar Sierpinski mono- pole and the modified gasket monopole antenna are briefly reviewed [4]. In Section 3, planar versions of antennas are projected to the conical surface [4]. Section 4 deals with the optimization of designed antennas, and Section 5 pre- sents experimental results. Section 6 concludes the paper. 2. Planar Sierpinski Monopole The planar Sierpinski monopole of the third order (Fig. 1) is created by 3 self-similar elements (equilateral triangles). The antenna is attached to the perfectly electri- cally conducting ground plane. At the antenna input, the SMA connector is assumed [4]. Fig. 1. Sierpinski monopole. In frequency response of the return loss (Fig. 2), the multi-band behavior can be observed (the first band reaches |S 11 | = –9.74 dB, the next three bands exhibiting |S 11 | < –10 dB for the reference impedance 50 Ω) [4]. The lowest operation frequency is determined by the dimensions of the basic bowtie monopole. The higher operation frequencies are determined both by the basic bowtie structure and the triangular slots (the higher fractal iterations) [4]. In the left-hand part of Tab. 1, magnitudes of S 11 at the input of the planar Sierpinski monopole at the operation frequencies are summarized. Obviously, S 11 does not reach the optimal values and bandwidths are narrow.