Radiation Field of a Conical Helix

Abstract

It is now well known that a cylindrical helix, when excited at frequencies corresponding to wavelengths comparable to the length of one turn of the helix, can radiate a sharp beam along the axis over a wide frequency range (about one octave). It is shown in the present communication that if the helix be conical instead of cylindrical (the diameter varying along the length of helix), then the axial mode of radiation can be maintained over a much wider band of frequencies. The radiation pattern of a conical helix, 60 cm diameter at the base, tapering linearly to 20 cm at the top in 10 turns within a height of 112 cm (with the ``ground'' provided by brass disk of 100 cm in diameter) has been studied experimentally. It is found that the axial mode of radiation is maintained from 150 Mc/sec to 450 Mc/sec. By increasing the number of turns, the band width can be considerably increased. Assuming a linear current distribution, theoretical expressions have also been deduced for Eφ and Eθ for a conical helix. Some modifications of the simple conical helix, such as may have special applications, are indicated.