For an 8 feet (2.4 m) parabolic dish antenna operating at

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For an 8 feet (2.4 m) parabolic dish antenna operating at 4 GHz the minimum distance required for the far field measurement is closest to—

1. 7.5 cm
2. 15 cm
3. 15 m
4. 150 m

Correct Option: D

For measurement of far field or secondary we apply following relation

r ≥	2d²
	λ

d = antenna aperture, r = radius Substituting the value

r ≥	2 × (2.4)²	= 153.6 m
	0.075

So the closest answer is 150 m.