Experimental low power 3 GHz AM communication link from 1938 uses Barkhausen–Kurz tubes for both transmitting and receiving. The transmitting or receiving antenna is each time placed at the focus of the parabolic mirrors. Electrical diagrams of the transmitter and the receiver are shown in this figure. Picture in the public domain (Short-Wave and Television magazine's copyright was not renewed after 1978 and it is in the public domain.).