| “Photonic Crystals” defined as periodic structures in one, two or three dimensions, prevent the propagation of an incident electromagnetic wave inside them, over a determined frequency range (bandgap) and for any propagation direction.; One of the main problems in the antenna field is the power transferred to the surface wave modes. This energy does not contribute to the main radiation of the antenna, considering this effect as a loss mechanism. Besides, this energy, depending on the dielectric constant of the substrate and its thickness, could be more than 70% of the total power. Even sometimes, this energy will be added in counter-phase to the main radiation pattern of the antenna making worse this pattern. The main idea is to use the “Photonic Crystal” structures as substrates for these antenna configurations leading to an increasing of the antenna efficiency and directivity, a more symmetric radiation pattern and a reduction of the back radiation.; The thesis can be divided into two parts: (1) Firstly, the Maxwell's equations for dielectric periodic structures are presented in Chapter 2. An eigensystem has been formulated to solve for the eigenvalues and eigenmodes of any “Photonic Crystal” providing the information to determine a full or a partial bandgap.; The gap behaviour of different structures as function of the geometry, dielectric constant contrast or fill factor of the unit cell has been analysed in Chapter 3. Besides, a new three-dimensional structure has been proposed. (2) Chapter 4 and 5 focus on using “Photonic Crystal” to avoid propagation of surface waves in microstrip patch antenna configurations, replacing conventional substrates for “Photonic Crystal” structures. A comparison between the conventional substrate features versus the “Photonic Crystal” is presented. The analysis reveals a decrease in coupled power to substrate modes and an improvement of efficiency, directivity, back radiation, pattern symmetry and mutual coupling.; In Chapter 5 the study of a dipole antenna on top of a woodpile structure at sub-millimetre wave frequencies is performed. A new fabrication method has been devised to fabricate a sample of a woodpile structure at 500 GHz. Transmission measurements of this sample for TE and TM polarisations at normal incidence show good agreement when compared with the simulations. If the structure is loaded with a dipole on its top, simulations reveal a highly symmetric main beam with very low back radiation. |
