The PDA3120-N (and -S) and PDA3140-N (and -S) are 20 ft and 40 ft extension cables used to extend the distance between the yagi antenna and the PDW30 or PDW90 wireless units.
The extension cable also comes with an antenna coupler to connect the PDW wireless device to the antenna extension cable, coax seal tape, and a 1/2" NPT cable gland.