Nature and optimisation of the ground (lateral) wave excited by submerged antennas

The radiation of a dipole immersed in a conducting dielectric halfspace bounded by air is examined by employing a representation which resolves the field into a spacewave, a lateral wave and lower-order diffraction terms. At points away from the interface between the two media, the far field is given primarily by the spacewave which appears as a geometric optical-ray contribution. However, at points close to the air-dielectric boundary, the field (referred to as a groundwave) is dominated by the lateral wave, the quasioptical properties of which are discussed in detail. It is shown that the groundwave may be enhanced by orienting the dipole in a direction which optimises the excitation of the lateral wave. The optimum condition is obtained as a function of the properties of the lossy medium, and it is shown that, for small conductive losses, the groundwave is strongest when the free-space radiation of the dipole is directed at an angle close to the critical angle of reflection in geometrical optics. For large losses, the optimum orientation is realised when the dipole is nearly parallel to the interface, in agreement with previous results. Quantitative results are presented for the fields produced under these optimum conditions; calculations are given over wide frequency ranges for representative cases of grounds consisting of dry earth, moist earth or seawater.