Lossy helical antennas for ground penetrating radar at low frequencies

Helical antennas, used in the normal mode and horizontally polarised, are modelled using a moment method above typical lossy ground. Distributed resistive loading based on the Wu-King form was used. The centre frequency of 1m dipoles was reduced from 250MHz for the straight resistive wire to 50MHz for a helix of pitch 2.5cm and diameter 5cm. An efficiency reduction of 12dB was required to maintain the fractional bandwidth for this helix at unity. This conforms with the theory for small antennas in free space. The results were also verified by comparing measurements performed on a monopole resistively loaded helical antenna in a watertank with the numerical model used