Continuous wave(CW) radar for subsurface exploration requires the direct coupling between the transmitting and receiving antennas, which determines the dynamic range of the system, as small as possible. Two planar dipoles embedded in a conductor-backed thin dielectric layer were suggested as a subsurface interface CW radar antenna of transmitting and receiving. It was shown that the dielectric layer acts as a cut-off waveguide. -60dB coupling between transmitting dipole and receiving dipole was measured from 250MHz to 900MHz. Since the transmitting planar dipole is closely located to the conducting plane, the radiation efficiency is rather poor.
It is proposed that the radiation efficiency may be improved without increasing the decoupling by using planar dipoles in the feeding waveguides coupled through a cut-off dielectric waveguide. Proper location of dipoles is searched by measurement, where the feeding rectangular waveguide has two cross sections, 15cm×1.5cm and 15cm×3.0cm with length 3.0cm and 6.0cm respectively. The voltage standing wave ratio, direct coupling, radiation pattern, and radiation power of the planar antennas with the rectangular waveguides are measured in the frequency range from 200MHz to 1000MHz, and compared with those of planar antenna. The measured result shows that the proposed antennas can improve the radiation efficiency by about 10dB in the frequency range from 500MHz to 1000MHz.
