Optimum designs of broad and narrow band parabolic reflector antennas fed with log-periodic dipole arrays

The Log-Periodic Dipole Antenna (LPDA) is attractive as a broadband feed for paraboloid reflectors despite its tendency to cause defocusing owing to axial phase-center movement with frequency. The gain loss due to defocusing in a parabolic reflector is only a function of the defocusing in terms of wavelengths and not dependent on the diameter of the reflector. Hence, antennas such as the LPDA which have a frequency independent phase center location in terms of wavelengths from some geometrical point (virtual apex) will experience a frequency independent defocusing loss over its bandwidth if the virtual apex is placed at the focal point. The optimum feed design for maximum gain will change with varying reflector f/D (reflector focal length to diameter ratio) since spillover, aperture illumination, and defocusing loss are functions of f/D, while primary pattern beamwidth and symmetry, gain and VSWR are functions of the feed parameters.