Folded substrate integrated waveguide (FSIW) has been proposed for further size reduction, while keeping the advantages of traditional SIW. FSIW-based horn antennas are very promising in millimeter wave applications. To improve the bandwidth and efficiency of such antennas, a thick substrate is preferred but the microstrip feeding line becomes unacceptly wide to obtain a reasonable impedance. This paper introduces compact microstrip feeding lines with elevated ground plane for thick FSIW. Arbitrary characteristic impedance can be achieved without increasing the width of the feeding line, which will be very useful for the integration with other millimeter circuit components.