GATE EC 2020 Set 1 — Question 53
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Electromagnetics → Antennas → Dipole & Monopole Antennas
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Question
For an infinitesimally small dipole in free space, the electric field in the far field is proportional to , where . A vertical infinitesimally small electric dipole is placed at a distance above an infinite ideal conducting plane, as shown in the figure. The minimum value of , for which one of the maxima in the far field radiation pattern occurs at , is
Correct answer
(A) λ
Solution
Image theory. A vertical electric dipole at height above a perfect conductor has an image dipole at carrying current in the same direction. The pair is a two-element array with spacing along .In the far field, the path lengths are and :The array factor reaches its maximum value 1 when( would require .) With and (), the smallest comes from :Check the other options:
- : , which is a null.
- or : the array factor is or , not a maximum.
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