Contents I. Introduction
High-gain antennas are essential in various modern communications applications such as satellites, remote sensing, and 5G communications [1] . Transmit array antennas belong to the high gain antenna group and they combine the features of phased array and parabolic reflector but with low-profile and conformal design [2] . It consist of a feeding source element and a thin-transmitting surface of half-wavelength unit-cell elements. The feeding source is located at the equivalent focus of the transmit array. The unit-cell elements are designed with transmission coefficients adjusted to convert the spherical-waves from the feed to a plane-waves [3] . The selection of the feeding element is essential in transmit array performance. Recently, modern communication technologies are moving toward to the millimeter wave band such as 5G/6G system. The use of high frequency bands is characterized by unlicensed bands, using high directive antenna, secure transmission, and immunity to interference [4] . These communication technologies combine the analog, digital radio systems and the antenna together in the same chip at millimeter frequencies. The employment of classical technologies, such as microstrip lines and waveguides with these integrated modules is so difficult [5] . Hence, the researches have recently attracted the attention to develop new technologies with high performance, such as the ridge-gap waveguide (RGW) technology [6] . The basic theory of RGW technology is the use of artificial magnetic conductor (AMC) surface to create high impedance to guide the field only along desired paths, and stop it along other directions [7] . The RGW has three forms to guide the electromagnetic waves in the gab are ridge- gap waveguide, groove-gap waveguides and microstrip-gap waveguide [8] .
