Why A halo is simply a half-wave dipole bent into a near-circle with the ends close together but not joined. Because the dipole is horizontal and curved back on itself, the usual figure-eight pattern is smeared out into a roughly uniform pattern all around the horizon, giving horizontal polarization with no need to rotate the antenna. That is why halos are popular for VHF/UHF SSB and CW mobile work, where horizontal polarization is the convention.
Watch out Radiation broadside to the loop plane describes a full-wave quad loop or a driven loop array, not a halo; a halo has relatively little radiation along its axis.
Halo = bent dipole, horizontally polarized, hears all around the horizon. Think halo over your head, radiating outward in its own plane.