The Lunar Prospector helix antenna

Fourier analysis can be used to model an infinitely long helix antenna embedded in a layered cylindrical geometry. The source is expanded in cylindrical harmonics over the spectrum (v, /spl beta/), in which each component has behavior exp(jv/spl phi/-j/spl beta/z) in cylindrical coordinates. Due to periodicity in /spl phi/, v is an integer. A current on a helical tape is expressed as a sum over all n of (v/sub n/, /spl beta//sub n/) current sheets, and so produces a sum of (v/sub n/, /spl beta//sub n/) field harmonics. A helix antenna radiating a circularly polarized broadside beam was designed for the Lunar Prospector mission. A metal core keeps the energy bound along the axis to lengthen the effective aperture. The axial attenuation rate was designed by computationally adjusting the imaginary part of /spl beta/, which departs from zero when one or more /spl beta//sub n/ harmonics radiates in the visible region.