Modal analysis of the `gap effect´ in waveguide dielectric measurements

In waveguide measurements on dielectric slabs, small air gaps between the guide walls and the dielectric sample are found to be capable of radically altering the complex reflection and transmission coefficients of the excitation mode. The modal-analysis representation is used to compute these coefficients for low-loss and high-loss samples with air gaps. The `gap effect´ is explained quantitatively by considering the influence of the dominant `slab mode,´ which focuses its energy into the dielectric slab, and the dominant `gap mode,´ which focuses its energy into the air gap. An experimental approach, which consists of filling the air gap with conducting paste, is shown to essentially correct the problem