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Find the angle at which nulls occur for the two element array antenna with separation λ/4 and phase difference is -π/2?

(a) 0

(b) π/2

(c) π/4

(d) π

This question was addressed to me during an interview.

The origin of the question is Factors topic in chapter Antenna Array of Antennas

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The correct choice is (d) π

The best I can explain: The normalized array factor is given by \(AF_n=\frac{AF}{2}=cos(\frac{kdcosθ+β}{2})\)

⇨ AFn=0

⇨ \(cos⁡(\frac{kdcosθ+β}{2})=0\)

⇨ \(\frac{(\frac{2π}{λ})(\frac{λ}{4})cosθ-\frac{π}{2}}{2}=±\frac{π}{2}\)

⇨ Cosθ=-1

⇨ θ=180 or π

So Nulls occur at 180°.

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