Shure PA421B Antenna CombinerFEATURES The PA421B Antenna Combiner from Shure combines up to four PSM transmitters to a single antenna set. It can be used when multiple wireless systems are required for monitoring in sound reinforcement applications. The PA421B reduces rack clutter, optimizes RF management, and provides DC power to compatible IEM transmitters. LED indicators for signal presence and signal overload provide four input LED signals (green when active, off inactive).