Conventional fast multipole algorithm is one of the most widely used integral equation method for its high efficiency. Unfortunately, it always come across a problem named subwavelength breakdown when the finest group size is smaller than 0.2 wavelength. In this article, singular value decomposition method is used to modify the translation operator of fast multipole algorithm. The finest group size will be smaller without subwavelength breakdown by this method, which leads to much reduction of the memory requirement. Furthermore, the time consumption is also reduced by applying the well-conditioned electric field integral equation. Numerical results prove the validity and efficiency of this method.ABSTRACT: Design of a high gain cavity backed slot antenna is presented with relatively small size. The large ground plane of the original design is cut 75%. Mushroom cells and ground plane orientations have been employed to improve the antenna gain. An 18.75-dB maximum gain is obtained with an average gain of 17.3 dB in the entire operating band.