Question

Consider the following statements:

The inter element spacing of

1. More than \(λ\) in array antenna will produce grating lobes in unscanned condition.

2. More than λ/2 but less than λ in array antenna will produce grating lobe under scanned condition.

3. Less than λ/2 in array antenna will not produce any grating lobe.

4. More than 1.5 λ in array antenna will not produce any grating lobe.

Which of the above statements are correct?

1. 1 and 2 only
2. 3 and 4 only
3. 1, 2 and 3 only
4. 1, 2, 3 and 4

Answer 1

Correct Answer - Option 3 : 1, 2 and 3 only

CONCEPT-

Grating lobes is the term for secondary main lobes (very strong sidelobes) in the antenna diagram. They have approximately the size of the main lobe . Grating lobes sometimes occur with phased array antennas. Grating lobes are a consequence of too large and uniform distances between the individual radiator elements in relation to the wavelength. With a good design of a phased array with an optimal spacing of the radiating elements, they do not occur.

Condition to Avoid Grating lobes-

The grating lobe can be avoided by avoiding multiple maxima in the pattern.

For maxima ϕ = 0, 2π, 4π, …

For Single maxima 0 ≤ ϕ < 2π

Analysis:

Since ϕ = α + βd cos θ

d = distance between adjacent antenna

0 ≤ α + βd cosθ < 2π

For maxima cos θ = 1

α = phase difference {max^m = π}

π + βd < 2π

\(d < \frac{\pi }{\beta }\)

\(\beta = \frac{{2\pi }}{\lambda }\)

\(d < \frac{\lambda }{2}\)

So when \(d < \frac{\lambda }{2}\) → Antenna will not produce any grating,

 \(d > \frac{\lambda }{2}\) → antenna will produce a grating lobe.

So 1, 2, 3 are correct.