A continuous, multi-frequency, cross-borehole, electromagnetic radar uses the transmitting and the receiving short dipole antenna. Since it requires a ultra wideband operation from 10 MHz to 80 MHz in the water-and-mud- filled borehole, the impedance matching of this insulated short dipole antenna to the borehole in the granite is a serious problem.
Insulated dipole of its diameter and length equal to 5 cm and 60 cm, respectively is put into the borehole at the depth of 30 m and its input impedance is measured. Measured impedance shows the strong capacitive reactance over the whole frequency band to have the voltage standing wave ratio(VSWR) greater than 5.
One may employ the L section of reactances and the broadband transmission line toroidal transformer to achieve VSWR less than 3 over the frequencies from 22 MHz to 76 MHz. The design and the implementation of this impedance matching circuits by using chip capacitors and inductors and the turn ratio of 9 to 4 for the transmission line toroidal transformer are presented and VSWR of 3 is confirmed by the measurement.
For the reflection type multi-frequency borehole radar, the coupling coefficients of two insulated antennas in one borehole are measured. The measured transmission coupling coefficients shows that coupling of -53 dB to -63 dB for 2 m apart insulated dipoles and -72 dB to -82 dB for 10 m apart are obtained for 10 MHz and 70 MHz operation, respectively.
