Feed Image Lobes in Offset-Fed Reflectarrays: Diagnosis and Solution

The actual source of the “feed image lobe” in fixed-beam single-layer offset-fed reflectarrays composed of variable-size patch or dipole elements is diagnosed using a novel technique based on determining the individual contributions of the conduction currents on the reflectarray elements and the groundplane, to the reflectarray scattered fields. The behavior of the feed image lobe is studied over a range of frequencies about the center frequency, and this is used to establish the fact that both the elements and the groundplane are responsible for the presence of the feed image lobe. We show how to reduce or eliminate the feed image lobe at off-center frequencies in offset-fed reflectarrays through use of sub-wavelength reflectarrays instead of conventional reflectarrays with half-wavelength inter-element spacing. Measured and full-wave predicted results are used in all discussions.