SB144R2: Made of high quality aluminum. This support stacking boom element is suited for A144 Yagi series parallel stacked VHF antennas only. The SB-144 R DIAMOND holder is of the appropriate length to ensure the correct distance between two directional antennas for the 2m band. Good quality aluminum holder SB144R is used to move the directional antenna away from the mast. The KB type holders are used for single antennas, while the SB type holders are for mounting double antennas. When using a dual antenna system (dual antenna), an SS type coupling transformer is required.