Shure PA421BX Four-Way Antenna Combiner (865-960 MHz)Shure PA421B Antenna Combiners are complementary accessories for Shure PSM Personal Stereo Monitor systems. When multiple systems are required for monitoring environments, the PA421B combiners quietly and efficiently reduce rack clutter and optimize RF management for up to four PSM900 transmitters or two PSM1000 transmitters when racked in a central location. Features: Combines multiple PSM transmitters to a single antenna set Improves RF networking