For a deep underground radiation of electromagnetic wave, an insulated antenna in the borehole in used. For a pulsed electromagnetic wave radiation, the ringing of the pulse exitation in the antenna has to be minimized in order to minimize the artificial reflection and differentiation due to the delayed radiated pulses from the transmitting antenna.
One may design the ultra wide band pulsed electromagnetic wave antenna useable for the pulse for 5 nanosecond pulse width by changing the cross section fo the insulated dipole in the borehole.
The ending part of 150 millimeters of a short dipole antenna of its total conductor length 265×2 millimeters is tapered from 50 millimeter diameter to 20 millimeter diameter, linearly at both ends of the dipole antenna with its insulating acetal case. This antenna is fabricated and its mutual coupling $(S_{21})$ in the borehole 10 meter apart is measured in the frequency domain from 10 MHz to 210 MHz by network analyser. This data is used in synthesizing the pulse response, which shows the delayed pulse -15dB in its amplitude compared to the first received pulse.
