From OK2KKW webpage, a nice story about first OK QSO on 10 GHz:
Another piece, representing the level of our technology, they had with them at Klínovec OK1KAD – the first device for 10 GHz, equipped with a klystron and a superheat receiver. Unfortunately, due to an unfavorable coincidence, the connection failed to a greater distance. One of the facilities was located in Klínovec, the other was transported to Plešivec, QRB 10 km. However, the voltage in the network dropped to 180 V. Therefore, at least the connection was made to the distance allowed by the terrain on Klínovec and the length of the network cable – 100 m. cm) and thus CZ record.