Cross polarization losses in periodic arrays of loaded slots

A periodic array of apertures in a conducting plane can act as a bandpass filter. With proper design the structure will be transparent at its resonant frequency but will have a transmission coefficient below unity at all other frequencies. These periodic surfaces are useful as radomes and in dual frequency antenna feeds. The resonant frequency of such an array may vary with incidence angle. This variation is undesirable for most applications, and can be greatly reduced by reactively loading the slots with the Babinet equivalent of a short circuited two-wire transmission line. It has been found, however, that singly loaded slot arrays have transmission loss due to cross polarized radiation when scanned in the -plane (perpendicular polarization). It is shown that this cross polarized radiation may be eliminated by using symmetrical loaded slots, which have the further advantage of transmitting waves of arbitrary polarization. In conjunction with this investigation, the modal matching method, previously applied to arrays of rectangular and circular slots, has been extended to the more complicated loaded slot shapes. Both the single loaded and 4-legged symmetrically loaded slots are treated, and good agreement with measured data is obtained.