G C ( p ) = 1 N ??? i = l N ei max ( p ) ei max ( p ) = max R min ??? n ??? R max { 1 n - R min + 1 ??? j = R min n cos ?? i j ( p ) } ( 25 ) where p is an operating point (OP), and N is the number of the symmetric directions used for computing the response (FIG. 21 a).