Of course, the two basic methods of coupling a grid dipper to an antenna are
capacative and inductive. For capacative, couple one side of the dipper coil
to the end of the antenna by 2 to 5 pf. You may be changing the resonant
frequency by stray coupling.
For inductive, make a coil shaped like a wire coathanger, size or more than
one turn to get to the right frequency. couple the long side of this "coil"
to the center of the element.
bob w4mb