Suppresion of log-periodic dipole antenna spurious radiation by lumped element loading for radioastronomical application

Log-Periodic Dipole Antennas have been known to provide good performance over large fractional bandwidths. However, optimized designs of log-periodic antennas, in particular for radioastronomy and remote sensing, need to tackle the spurious radiation of secondary dipole harmonics that can compromise the gain smoothness and therefore the ability to detect natural spectral features emitted by the probed sources. A new, low-cost method applicable also at a post-design stage is proposed in this paper, which relies on loading some of the dipoles with appropriate lumped circuit elements in order to avoid this unwanted feature while preserving the desired performance characteristics. Application is found on the log-periodic dipole antenna selected for the low-frequency instrument of the SKA (Square Kilometer Array), and general guidelines are described for other designs. An experimental test using a transmitting VHF antenna and the antenna under test with new RLC loaded dipoles confirms our solution.