The optimum length for a dipole antenna is a multiple of half the wavelength that it is designed to receive or emit. Why is this? If an electromagnetic wave has E in the x-axis, B in the y-axis, and propagates along the z-axis, it will be generate the optimal current in a dipole antenna positioned along the x-axis.

However, the wavelength of the EM wave is measured in the z-axis. Of course, the fluctuations of the E field occur along the x-axis, which is why the antenna is positioned this way, but then why does the length of the antenna in the x-axis matter?