I think that effective range is actually their base range + 3000 or something (base range is the range given in the stats in the tooltips). It is an odd and confusing mechanic that is a holdover from older versions where this was needed for some obscure reason. Keith has expressed interest in removing this mechanic at and have the tooltips report their actual ranges, after bumping up all non-zero ranges by 3000 of course, to keep things the same.
So basically, the Radar jammer does not half the reported range in the tool-tips, but halfs the effective range. This means that when a ship is under the influence of a Radar jammer (Mk. I or II), its new range is actually "base range"/2 + 1500.
Hope this explains it.