The document discusses helical antennas, which consist of a conductor wound into a helical shape above a ground plane. Helical antennas provide circular polarization and are used at very high and ultra-high frequencies. The construction of a helical antenna involves parameters like length, number of turns, diameter, circumference, pitch angle, and distance from the ground plane. It can operate in either a normal mode or an axial mode, and the axial mode is generally used. Applications of helical antennas include AM broadcasting, reducing AC line noise, and low-frequency uses like RFID and airport door controls.