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 (1964) form was used. The centre frequency of 1 m dipoles was reduced from 250 MHz for the straight resistive wire to 50 MHz for a helix of pitch 2.5 cm and diameter 5 cm. An efficiency reduction of 12 dB 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.