CTK503 is a modular UHF transmitter rack unit for wireless intercom systems. It combines three CST50-D transmitters (300 mW each) mounted in the RST393 19-inch/1U rack frame with a mains power supply. Operating in the 400–550 MHz UHF band, additional frequency ranges can be provided on request. The configuration supports three parallel transmitter channels in a single rack frame for fixed installations and event use.