Here's a photo of the one's I installed. Got them from my usual all things avionics supply store,, SteinAir, for about 17 bucks each. Used RG400 and BNC connectors. Stein also sell's a converter that connects the BNC/RMA for the plug on the SkyRadar D2. These were the most expensive component, about 18 bucks each.
These work great in this placement. Where I live with the stock D2 black antennae's I would get a couple stations through about 2000 feet AGL. Now I get 5 at 1000 feet.
These are mounted on the left wing fairing, just behind the left gear leg. Spaced about 4 inches apart in line fore and aft.