The lower dielectric body 16 has a lower base portion 161, and a lower connecting portion 162 protruded frontward from an upper portion of a front surface of the lower base portion 161. Two opposite sides of a bottom surface of the lower connecting portion 162 are recessed inward to form two lower lacking grooves 163 penetrating through two opposite sides of a front surface of the lower connecting portion 162. The upper base portion 151 and the lower base portion 161 are matched with each other. The upper connecting portion 152 and the lower connecting portion 162 are matched with each other. At least one portion of the top surface of the upper connecting portion 152 and the bottom surface of the lower connecting portion 162 are recessed inward to form at least one lacking groove 18 penetrating through at least one portion of the front surface of the upper connecting portion 152 and the front surface of the lower connecting portion 162.