Quadrantal error and dip are two separate errors.
Quadrantal error features in many text books and is caused by the cruciform shape of the aircraft. It occurs whether the wings are level or banked and is worst on signals from stations bearing 45°, 135°, 225° and 315° from the aircraft. Fortunately, most ADF tasks involve tracking directly towards or away from a beacon so quadrantal error is not a factor. However, when using the ADF for pure fixing stations on or near the quadrants should be avoided.
Dip is something different and does not affect all aircraft; it depends on the antenna, with small, combined antennae being particularly susceptable. It is caused (according to a man at Bendix/King) by shielding of the directional antennae by the sense antenna in a turn. It is worst when the station is directly ahead or behind you and is minimal for stations on the beam. The thing to remember is that the top of the needle (head or tail) always dips towards the low wing when you bank. The amount it dips is typically a maximum of 10°.