When the helix circumference is near the wavelength of operation, the antenna operates in "axial mode". This is a nonresonant traveling wave mode, in which instead of standing waves, the waves of current and voltage travel in one direction, up the helix. Instead of radiating linearly polarized waves normal to the antenna's axis, it radiates a beam of radio waves with circular polarisation along the axis, off the ends of the antenna. The main lobes of the radiation pattern are along the axis of the helix, off both ends. Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated in a flat metal sheet or screen reflector to reflect the waves forward.