A compact disk antenna for car-to-car communication
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The final goal of this document is the construction of multi-band terminal for the CVIS project to allow the communication among cars and between the cars and the roadside infrastructure. For the construction of this multi-band terminal, this document takes as starting point, a new compact disk antenna described in . It consists in a circular patch antenna shorted by a central metallic post. This allows reducing the dimensions of a classical circular patch antenna so it is very useful for our application given that the terminal is going to be place on the top of a car and it should be small enough to be attractive for the final users. Specifically, we are going to analyze the behaviour of this kind of antenna when it radiates in the TM01 mode, because the radiation pattern of this mode is particularly interesting for the applications of the CVIS project. This document is divided into two main parts. In the first one, a study in depth of the behaviour of the antenna proposed in  was performed. Firstly, through an analytical model and afterward with the help of two simulation tools (WIPL-D and EMDS), we analyze the influence of the main parameters of the antenna (outer and inner radiuses, height, electrical permittivity and the position of the feed) on its properties (resonant frequency, bandwidth, entrance impedance, the shape of the radiation pattern and so on). A general methodology for the design of this kind of antennas was proposed, and it was put in practice with the design of a prototype for a band around 2 GHz. In addition to the conclusions about the influence of the different parameters of the antenna, another important conclusion was done. It was discovered that the use of the simulation tool WIPL-D Lite was not suitable for the simulation of this kind of antenna. In the second part, this document tackles the construction of a dual-frequency antenna for the bands of 2.4-2.484 GHz and 5.75-5.95 GHz. For this, the present document studies the possibility of stacking two compact circular patch antennas, so we put the one which covers the higher band (the smaller) on the top of the one which covers the lower band (the lager). The two patch antennas have a coaxial feed and the feed of the upper antenna goes inside the central post of the lower antenna to minimize the influence of it on the radiation pattern of the lower antenna. This proposal works out not to be feasible, because we need a lower antenna with a large inner radius to allow the variation of the feed position of the upper antenna in a wide range. This is necessary to get a good matching for the upper antenna. The problem is that the radiation pattern becomes very asymmetric when we increase the inner radius of the patch antenna. To solve that, two alternative were analyzed in this document. The first consists in putting a second feed symmetrically placed with respect to the central post. It leads to a more symmetric radiation pattern so we can choose a larger inner radius. In addition, the introduction of the second feed increases the bandwidth of the antenna. The second alternative is a simplification of the first one. It consists in replacing the upper antenna with a monopole on the top of the lower antenna. It is simpler but it prevents the possibility of stacking other patch antennas to cover more frequency bands in a future. Due to this disadvantage, the first alternative was chosen. Finally, a proposal based on the first alternative, which fulfils quite well all the requirements which were raised in the wording of this master thesis, was presented and studied in depth.