Pipes, cable and other buried objects pose serious problems when construction or maintenance work requires excavation. If their position can be exactly identified, this leads to effective construction work without trouble caused by the buried structures.
In the subsurface exploration of fine objects such as pipes, cables, and voids, etc., electromagnetic pulse and FM-CW radars are used. Detection of shallow fine targets requires an extremely short pulse or equivalently a large bandwidth FM-CW operation. The subsurface interface antennas are of the wide bandwidth type, i.e., monopoles, dipoles with end loading or distributed loading, biconical or insulated dipole, bow-tie, spiral TEM horn, and others.
Continuous wave (CW) multifrequency radar for subsurface exploration is advantageous over that of impulse in coping with the dispersion of the medium, the noise level of the receiving system, and the controllability of its frequency characteristics. It requires, however, the mutual coupling between the transmitting and receiving antennas, which determines the dynamic range of the system, as small as possible. It is relatively easy for the impulse radar with the time gating of the transmitted pulse.
Two planar dipoles embedded in a conductor-backed thin dielectric layer are suggested as a subsurface interface CW radar antenna of transmitting and receiving. It is shown that this dielectric layer of the transmitting and receiving antenna above the surface interface, where a thin air-gap layer separates the antenna from the interface, acts as a cut-off waveguide and by using the asymptotic evaluation of the wave integral that the coupling between the dipoles are due to the contribution of evanescent lateral waves and leaky waves. Since the dipoles are very close to the back conductors, the radiation efficiency is rather low but the larger isolation between dipoles plays more decisive role in providing the larger dynamic range.
The current density, the input impedance, and the radiation pattern of the planar dipole are numerically calculated by the method of moment. The coupling between two parallel dipoles and that between two collinear dipoles are calculated, respectively, and for different parameters of the antenna structures.
A scale-down measurement is carried out by building a water tank of 0.7×0.7×0.7㎥ in which the saline water solution of the relative dielectric constant, $ε_τ$ = 80, and the conductivity, σ = 0.1 S/m is filled. Suggested subsurface antenna is built by the two parallel planar dipole of 4.2×0.24 ㎠ separated by 7 cm and fixed in a conductor-backed polyethylene ($ε_τ$ = 2.3 and thickness of 0.2 cm) box of 50×15×0.7 c㎥ at the depth of 0.35 cm from the top conductor sheet. This antenna box is filled with another solution of $ε_τ$ = 78 and σ = 2 S/m and is placed on the top surface of the saline water solution in the water tank.
The input impedance and the voltage standing wave ratio(VSWR) of the planar dipole inside the conductor-backed dielectric layer is measured and compared with the numerical calculations in the frequency band from 200 MHz to 1000 MHz. Measured values are closely predicted by the calculations. Measured VSWR is less than 3 from 250 MHz to 900 MHz. The mutual coupling between two parallel planar dipoles are also measured by the network analyzer. About -60 dB coupling is measured over these frequency band, which is confirmed by the numerical calculations.
Since the radiation efficiency of the planar dipole closely located to the conducting plane is very low, its improvement is tried by bending the conducting plane at the back of the dipole into the grooves of semi-circle, elliptic, and rectangular shape. The improvement, however, is found rather small since the dimension of the groove is small compared with the wavelength.
