You are constructing an HF delta loop antenna. It is oriented with the bottom element parallel to the ground. Where should you locate the feed point for horizontal polarization?

Antenna polarization is determined by the direction of the radiating electric field, which follows the dominant orientation of the currents in the antenna. For a loop antenna, placing the feed point in the center of a horizontal section results in horizontally polarized waves. When your delta loop is mounted with its bottom element flat and parallel to the ground (an "apex up" configuration), feeding it directly in the center of that horizontal bottom wire ensures the radiation is horizontally polarized. If you wanted vertical polarization from the exact same physical structure, you would move the feed point to one of the angled side elements, typically one-quarter wavelength down from the top apex.