Theory of a directional borehole antenna

The theory of a directional antenna constructed for the RAMAC borehole radar system is described. The radar was developed for the International Stripa Project and is presently widely used for mapping rock in mines and tunnels. The radar operates in the frequency range 20-100 MHz and the antenna can locate fracture zones and other reflectors from single borehole measurements. The antenna design is determined by the small diameter of the antenna compared with wavelength. Four different signals are measured and digitally combined to produce signals corresponding to an ideal directional antenna, a dipole antenna and a checksum testing the function of the system. Tests have shown that the azimuth of fracture zones can be determined within 3/spl deg/.