In the two-element endfire array of Example 14.4, consider the effect of varying the operating frequency, f, away from the original design frequency, f₀, while maintaining the original current phasing, ξ = −π/2. Determine the values of ϕ at which the maxima occur when the frequency is changed to

(a) f = 1.5 f₀;

(b) f = 2 f₀.