Transportable High Frequency Transmit Array with High Gain and Wide Bandwidth

This paper describes a high-gain wideband transportable high-frequency (HF) transmit array designed at MIT Lincoln Laboratory that is suitable for supporting ionospheric research, propagation, and Over-the-Horizon (OTH) radar experiments. Horizontally polarized log-periodic dipole antennas (LPDAs) are mounted on transportable erectable towers set at a height to maximize the antenna gain at the desired elevation angle. To reduce grating lobes in azimuth, the log-periodic antennas are overlapped or reduced in width by loading the longer elements with coils. Unlike vertically polarized transmit arrays, this horizontally polarized array requires no ground screen and does not present radiation hazards at ground levels when transmitting at moderate power levels. This work describes the array design, including a consideration of grating lobes and array height for daytime vs. nighttime experiments, and the LPDA element design, including the modification to reduce the size of the antennas without changing its performance. An alternate approach where full-length antennas are used but the antenna heights are staggered to avoid mechanical interference is also considered. Future work will include electromagnetic models of the LPDA antennas before and after they are shortened, as well as the VSWR performance of the whole array as a function of frequency and scan angle.