Heissischer Rundfunk is a public broadcasting network serving the German state of Hesse. Its radio network consists of several audio streams. They broadcast on FM on numerous transmitters throughout Hesse and medium wave on 594 kHz on two different transmitters (Frankfurt and Hoher Meißner.) The MW transmission was easily audible in Southern Germany. We heard the FM outlets in our July 1989 trip to Northern Germany. The main station in Frankfurt responded to my reception reports with the QSL cards seen below. Their website is here.
Click on the thumbnails for full views of the QSLs.