Suppression of parasitic currents on feed lines of colinear dual antenna systems

Investigations are reported of an antenna system consisting of a pair of colinear dipoles with a feed line "isolator section" which provides independent operation of the dipoles over an octave frequency range. The isolator section acts as a band elimination filter for the parasitic currents induced on the outer surface of the transmission line of the upper dipole. Suppression of these currents eliminates distortion of the antenna radiation patterns and significantly reduces coupling between the dipoles. Over the 200-400-MHz range, an interdipole isolation greater than 35 dB has been achieved between the colinear dipoles separated by 1.5 m.