Resistively loaded insulated antennas for narrow borehole radar

The transmission-line model for an insulated symmetric dipole in a lossy medium has been extended for the case of an insulated asymmetric dipole with a conductive arm and a uniformly resistive arm. The model is used to improve the understanding of the characteristics of a narrow borehole impulse radar recently developed. It predicts that sufficient loading of the resistive arm will dampen the antenna suitably for pulse transmission, despite the presence of the conductive arm. The model is partly verified by laboratory experiments. Examples of pulses received in situ in marble by the radar are presented.