I was confused about dip error as well so I e-mailed Bendix/King who make the KA44 antenna which is fitted to vast numbers of single and twin pistons. They tell me that the dipping of the needle towards the low wing is caused by a shielding effect in the antenna which uses a set of coils in an X shape plus a single coil across the fore-aft axis of the aircraft (Bellini-Tossi system???). Strangely enough older antenna systems are not affected by dip error so, if you've got an old 'towel rail' type antenna, you'll be OK. Even on large aircraft, some suffer from dip and some don't - the RAF's C-130s do, the Canberra doesn't, the DC-10 doesn't.