Interdigitated photoconductive antennas are powerful and easy-to-use sources of terahertz radiation for time-resolved spectroscopy. However, the emission of unwanted echoes, resulting from reflections of the emitted pulse in the antenna substrate, inherently limits the spectroscopic frequency resolution. A novel interdigitated photoconductive antenna that suppresses unwanted echoes from the substrate, without power losses, is proposed and demonstrated. This is realized through a buried metal geometry where a metal plane is placed at a subwavelength thickness below the surface antenna structure and GaAs active layer. In a reflection geometry this effectively eliminates echoes, permitting high resolution spectroscopy to be performed. As a proof-of-principle, the 1(01)-2(12) and the 2(12)-3(03) rotational lines of water vapor have been spectrally resolved with the new buried metal antenna, which are unresolvable with a standard antenna. In addition, as no THz field is lost to the substrate and reflections, the terahertz peak electric field amplitude is enhanced by a factor of three compared to a standard design in the equivalent reflection geometry.