Radiation-pattern improvement of patch antennas on a large-size substrate using a compact soft-surface structure and its realization on LTCC multilayer technology

The radiation performance of patch antennas on a large-size substrate can be significantly degraded by the diffraction of surface waves at the edge of the substrate. Most of modern techniques for the surface-wave suppression are related to periodic structures, such as photonic bandgap (PBG) or electromagnetic bandgap (EBG) geometries, which require complicated processes and considerable area. The concept of artificially soft surfaces has been proposed to suppress the surface-wave propagation since the 1990s. However, the typical corrugated soft surface only applies to a substrate whose thickness is one quarter guided wavelength. In this paper, a compact soft-surface structure consisting of a square ring of short-circuited metal strips is employed to surround the patch antenna for blocking the surface-wave propagation, thus, alleviating the effect of the edge diffraction and, hence, improving the radiation pattern. Since its operating frequency is determined by the width of the metal strip (about a quarter guided wavelength), the compact soft-surface structure is suitable for a substrate with arbitrary thickness and dielectric constant. More importantly, the compact soft surface can be realized on any via metallization incorporated packaging process, such as liquid crystal polymer (LCP), multilayer organic (MLO), or low temperature cofired ceramic (LTCC) technology. A numerical investigation for a patch antenna surrounded by an ideal compact soft surface is presented and the feasibility of its implementation on LTCC technology is demonstrated. It is shown that the gain at broadside of a patch antenna on a thick and large-size substrate can be increased to near 9 dBi through the use of the proposed compact soft-surface structure.