Calculation of the impedance of a rectangular waveguide aperture in the presence of a loaded dipole antenna embedded in a generally lossy material

The use of a combined embedded modulated scattering technique, utilizing a PIN diode-loaded dipole probe, and near-field microwave nondestructive testing technique, utilizing an open-ended rectangular waveguide, has been investigated as a means for determining the complex dielectric constant in which the MST probe is embedded. For this, the formulation of the forward problem of calculating the modulated reflection coefficient, measured at the aperture of the waveguide, is presented here. This formulation is based upon the reciprocity theorem for two antennae, and utilizes the near-field radiation pattern of a rectangular waveguide aperture and current distribution along the length of a dipole antenna. This formulation is verified with measured values as a function of location, load impedance and surrounding media. Finally, potential improvements, considerations, and the use of an inversion routine are discussed.