The SYST-A0004 is an antenna stack comprising DF-A0038 vertical polarisation DF and Omni, and OMNI-A0099 and OMNI-A0100 horizontal omni antennas. At the base it is supported by the MISC-A0047 support cage. All RF and control signals pass through the MISC-A0045 amplifier block which compensates for cable losses. Mounted on a mast, with vertical and horizontal polarisation monitoring, and interferometric direction finding, it provides a complete spectrum monitoring and DF solution. A DF switch can be mounted at the base of the DF array. Its cables can be fed back into the mast and routed through the omnis. The horizontal-polarisation omni antennas feature a cable duct for connection of antennas above, without distortion of their patterns. All monitoring antennas share a common axis, giving excellent radiation patterns.