Patent ID: 8436783

Claim:
A dielectrically loaded helical antenna for operation at first and second frequencies above 200 MHz and with circularly polarised radiation, wherein the antenna comprises an electrically insulative dielectric core of a solid material that has a dielectric constant greater than 5 and occupies the major part of the interior volume defined by the core outer surface, a pair of feed coupling nodes, and an antenna element structure that includes a plurality of elongate conductive antenna elements and a common interconnecting conductor, the antenna elements being in the form of elongate conductors distributed around the core on or adjacent the outer surface thereof, wherein the antenna elements comprise a first group of at least four substantially co-extensive antenna elements extending from one of the feed coupling nodes to the common conductor and a second group of at least four substantially co-extensive antenna elements extending from the other feed coupling node to the common conductor, the said groups containing electrically short antenna elements associated with a circular polarisation resonance at the first frequency and electrically long antenna elements associated with a circular polarisation resonance at the second frequency, and wherein each of the said groups includes pairs of neighbouring antenna elements, with each pair comprising one electrically short antenna element and one electrically long antenna element, the arrangement of the elements being such that, in each group, the number of pairs in which, in a given direction around the core, the electrically short antenna element precedes the electrically long antenna element is equal to the number of pairs in which, in the said direction, the electrically long antenna element precedes the electrically short antenna element.