Some uses of the Huygens' obliquity factor

It is shown how the effect of the mounting surface for a flush-mounted aperture antenna can be considered as a modification of the Huygens' obliquity factor, which represents the pattern of the radiation of an element of the aperture. The same method, in conjunction with Babinet's principle, applied to a small slit, gives the corresponding element pattern for an elementary dipole and lends credence to the method even when applied to very small apertures. An application is made to a solid-state laser configuration, and a result obtained by a more involved method is shown to correspond to the obliquity factor for an air-dielectric interface. A correction to the formula is also found, and an (apparently new) expression is found for the obliquity factor when there is a change of medium at the aperture.