A Study of the Field Distribution at an Axial Focus of a Square Microwave Lens

The field distribution near the focus of a microwave lens is studied both theoretically and experimentally. The relation between vector-field diffraction theory and scalarfield diffraction theory is discussed for this particular problem, and it is shown that in the case in question the principal component of the electric field vector can be evaluated with sufficient accuracy from the scalar theory. Measurements of the transverse component of the electric field have been made by a perturbation method using a spinning dipole. The experimental results are in good agreement with the theory. The often-discussed phase-shift of 180° in the passage of a wave through the focal plane is considered in some detail and the related change in wavelength near the focus is verified experimentally.