Let me elaborate.
1. On this picture we have unbalanced line connected to balanced antenna. So we have 3rd element in 2 element antenna. WRONG. Coax is now part of the antenna which is cause of many problems.
2. Antenna is mounted on metallic pole. So we have 4th element in two element antenna. WRONG. We don't want a pole to be antenna part, neither feedline right?
2 things: 1:1 or choke balun to cure #1 issue.
Insulate antenna mount from pole or use dielectric mast to cure #2 problem.
Mike
p.s. if antenna connected with black banana plug is insulated the same way as antenna on the left, we can skip #2