An optimised vehicular loop antenna for NVIS applications

Near vertical incidence skywave (NVIS) antennas intended for vehicle mounting offer limited scope for development given the usually severe size constraints that apply at typical NVIS frequencies (2-10 MHz). We present the results of research aimed at achieving an optimised antenna for this application when mounted on a typical "Land Rover"-sized vehicle, and particularly in producing a design procedure for such antennas. Previous work confirmed that a vertically orientated loop is the most effective antenna configuration, while Liu (see PhD thesis, University of Liverpool, p.123, 1999), and Austin and Liu (see Elec. Letts, vol.35, p.252-54, 1999) showed that appropriate capacitive loading of the loop has an especially desirable effect on its current distribution, and hence on the amount of energy radiated towards the zenith. Since the current distribution around the electrically small loop is akin to that on a short transmission line a suitable transmission line model of this antenna has been developed as an aid in its design.