Abstract: A delay line is made by sandwiching a thin slice of ferroelectric between two “cladding” layers of relatively low-?, low loss material to form a dielectric slab waveguide, and may support the propagation of electromagnetic guided waves and hence be used as a source of electric-field tunable time delay. There is a frequency range within which a dielectric-slab delay line behaves like a homogeneous transmission line with an “average” dielectric constant that is much lower than that of the ferroelectric, thereby ameliorating the difficulty with the high dielectric constant of the ferroelectric material. The thin slice of ferroelectric material “expels” a large fraction of the wave electric field, causing most of it to occupy the low-loss cladding material, greatly reducing the propagation loss along the delay line while allowing delay time to be varied by applying an electric-field through the ferroelectric within the cladding structure.