Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?

A Yagi-Uda antenna is a complex, highly interactive system of parasitic elements (directors and reflectors) and a driven element. The antenna's forward gain, front-to-back ratio, and SWR bandwidth are determined by the electromagnetic coupling between all of these components. Because the elements interact so closely, adjusting the physical length of the boom, changing the number of elements on that boom, or altering the spacing between the elements will all change the phase and magnitude of the currents in each element. Consequently, modifying any of these physical parameters will shift the antenna's performance characteristics. Antenna designers must carefully balance all of these factors, as optimizing for maximum forward gain often comes at the expense of a narrower SWR bandwidth or a worse front-to-back ratio.