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We show here the design of a linearly polarised flat antenna. The main goal is getting a low cost antenna to use for the reception of Hispasat satellite emissions in the 12 GHz band. The gain needed for the antenna is around 30dBi. A rectangular array of slots in the upper side of a parallel plate wave guide form the main antenna structure. The guide is excited by one planar feeding structure consisting of an array of rectangular microstrip patches.Parallel plate slot antennas have been widely studied by Ando and Hirokawa [1], from 12 GHz to Millimeter Wave frequencies. We also modify the feeding network, the design procedure and the materials used to build of the radiation surface. PARALLEL PLATE WAVE-GUIDEThe antenna main structure is made with two metallic plates, and filled by two kind of dielectric materials: a low density foam dielectric and an epoxy fibre glass. The ground plane is a 3 mm aluminium plate and the upper plate is a copper plated, low thickness fiberglass sheet. The upper plate will support the slots printed antenna, while the ground plane supports the printed feeding structure at both sides of the antenna. Fig. 1 shows a schematic section if the antenna structure.The shape of the antenna is square of 36 cm x 36 cm, with the slots printed in the upper plate, all of them parallel to the others to give linear polarisation. To get a plane feeding wave inside the guide an array of rectangular patches were placed in the side of the antenna and fed with equal amplitude and phase. Fig. 2 shows a schematic drawing of the antenna. FEEDING STRUCTURE AND PLANE WAVE EXCITATIONThe feeding network has been printed in a 0.254 mm thick teflon-glass substrate. It is based on an array of rectangular resonant patches fed by a microstrip line to get an homogeneous field in a plane wave inside the guide. The total number of patches is 24. With this number the ripple of the field inside the guide (amplitude and phase) is really small. The feeding structure is a parallel feeding network printed in the same substrate as the resonant patches and made with a set of two and three way power dividers and quarter wavelength transformers. SLOT COLUMNSThe radiating structure is formed by columns of slots printed in the upper plate. Each column acts as a full radiating element in a travelling wave antenna, and we can associate to it a radiated power and a set of transmission and reflection parameters in the waveguide. The radiated power depends on the slot length, that is constant for all the slots in a column, and we can adjust this length to control the excitation along the antenna