How can the directivity of a half-wave dipole be increased?

A standard half-wave dipole radiates energy in a figure-eight pattern, sending signals broadly in two opposite directions. To increase directivity, meaning to focus the RF energy into a single, stronger beam, you must modify the antenna array. The most effective way to achieve this is by adding one or more parasitic elements (reflectors and directors) parallel to the driven element. This converts the simple dipole into a Yagi-Uda directional antenna, concentrating the signal exactly where you point it. Other options do not serve this purpose. Thickening the element increases bandwidth, adding traps allows multi-band operation, and lengthening the element simply lowers its resonant frequency.