The UHF Command Van at Carnarvon Tracking Station contained a high-power UHF transmitter which put out approximately 10 kilowatts of RF power into the antennas.
This transmitter was primarily used for all voice communications with the astronauts from both Acquisition of Signal (AOS) to Loss of Signal (LOS).
The transmitter was mainly made up of a massive water-cooled Klystron valve, driven by a 200-Watt modulator. The modulator fed into the Klystron valve which in turn fed 10 Kilowatts of UHF RF energy into the antennas located next to the UHF Command van.
During Skylab, if there was a failure of the Klystron valve, the 200-watt modulated RF was fed directly into the UHF antennas to maintain voice communication.
Recollection by Ian Murphy (1973-74) of UHF Command