Before revision: The "batwing" shape of the elements is used because it gives the antenna a wide bandwidth of approximately 20% of operating frequency at a VSWR of 1.1:1.
After revision: adapted from the butterfly antenna (a flattened biconical antenna),The "batwing" shape of the elements is used because it gives the antenna a wide bandwidth of approximately 20% of operating frequency at a VSWR of 1.1:1.