Quadrifilar helical antenna comprising a conductive track with a meandering slit

A dielectrically-loaded quadrifilar helical antenna comprises first and second pairs of diametrically opposed antenna elements on a cylindrical dielectric substrate 12. At least one element of each pair includes a pair of mutually adjacent parallel conductive tracks 10BA, 10BB defining a channel 11B between them. The edges of the channel 11B are arranged such that they are longer than the edges of the tracks which do not define the channel. An alternative helical antenna, suitable for circularly polarised electromagnetic radiation, comprises first and second pairs of helical conductive track groups on or adjacent the outer surface of a cylindrical insulating substrate 12. The track groups each comprise at least a pair of helical tracks 10BA, 10BB where each track has one edge which is longer than another edge and each said helical track is nearer to one another than it is to the other groups of tracks. Also disclosed is a helical antenna which comprises four coextensive helical antenna elements, each of which is formed by at least two coextensive elongate conductors 10BA, 10BB separated by a slit 11B where the width of the slit is less than half of the spacing between one helical antenna element 10B and its neighbours 10A, 10C. The conductor boundary region of the slit 11B has an electrical length which is greater than that of the conductor regions not at the boundary of the slit. At least one of the helical conductive tracks may include at least one straight edge and at least one longer undulating edge such that a broadband of operational frequencies may be achieved by a compact antenna.