Arse, was hoping it was either dodgy voltage or dodgy co-axial wiring.
Neither seem likely after your own tests.
Problem is, that the new aerial is shorter (I wouldnt be happy about this) but thats probably not the problem you're having.
You said the RX range went crap BEFORE replacing the aerial, so must be something else.
However, the new short aerial may give us the same lower range issue - so you wouldnt know if you've fixed the original problem or not.
My advice -, go back to the original aerial, fix the problem with that one, then replace for your new one and hopefulyl all should be well.
Sorry cant give more help mate, not up to speed on the internals of RX's just yet
