Two planar dipoles inside an air waveguide backed by a perfect-magneticconducting (PMC) plane are suggested as a highly isolating transmitting-and--receiving subsurface continuous-wave-radar antenna. Its coupling mechanism is shown to be the contributions of the evanescent lateral wave and the leaky wave. It is shown asymptotically as well as numerically that the mutual coupling by the lateral wave decrease while that by the leaky wave increase as the operating frequency increase for the fixed antenna dimension, which makes it possible to design the optimum frequency and its bandwidth yielding the minimum mutual coupling.
Numerical integration technique integrating the wave integrals in the spectral domain is developed for the calculation of the input impedance and the mutual coupling of the antenna, having planar transmitting and receiving antennas inside the air waveguide backed by an infinite plane of either perfect or magnetic conductor or slab of other materials such as the ferrite, and its radiation fields inside the ground. The effects of various design parameters, the width and the length of the dipole, the separation of two dipoles, the thickness of the air wave guide, the permittivities and conductivities of the ground, and the back plate materials, are calculated and shown.
A practical highly decoupled transmitting and receiving antenna of planar dipoles of 0.4m by 0.05m located 0.002m from the back ferrite slab of its thickness 0.005m pasted on the copper plate of 1.5m by 0.5m inside the air waveguide of 0.14m thick is designed and fabricated and its isolation is measured by less than -60dB over the frequencies from 200MHz to 600MHz (possibly well beyond this frequency which is not measured) when the antenna is above the ground having its relative permittivity of about 15 and conductivity of about 0.003S/m. Numerical calculation of the isolation of the antenna having the same dimensions except the finite back ferrite slab shows -80 dB. Commercial program HFSS shows that this degradation of the isolation is due to the finite size of the antenna and one may obtain the isolation up to -80dB over this bandwidth by extending the ferrite slab pasted on the copper plate to the side walls of the antenna. This ferrite backed antenna shows higher isolation more than 40 dB and lower gain about 5 dB compared to the same planar dipole in the half space.
