A wideband focal plane array (FPA) covering the full K-band (18 - 26.5 GHz) is proposed for use in passive imaging and microwave astronomy. Focal plane arrays are revolutionizing microwave and millimeter-wave imaging by replacing conventional single-pixel feed horns with arrays of planar elements, decreasing the survey time dramatically. The extent and element spacing of a rectangular array is determined by comparing classic and encircled power techniques. A prime focus reflector-feed configuration is considered with f/D=0.45. Far-field performances of arrays of simple Gaussian feeds and also imported Vivaldi feeds, which include mutual coupling between elements, are evaluated by PO/PTD techniques.