JEE Main · 2021 · 16 Mar 2021 · Shift 1 · EM Waves

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A 25 m long antenna is mounted on an antenna tower. The height of the antenna tower is 75 m . The wavelength (in meter) of the signal transmitted by this antenna would be:

(A)100✓ Correct
(B)300
(C)200
(D)400

Answer: (A)

Solution

A straight antenna radiates efficiently when its length matches a quarter of the wavelength it carries,

ℓ = \frac{λ}{4}

. The radiating element here is the 25 m antenna; the 75 m tower only raises it above the ground and carries no signal. Hence

λ = 4 ℓ = 4 \times 25 = 100 m

The wavelength of the transmitted signal is 100 m.

JEE Main · 2021 · 16 Mar 2021 · Shift 1 · Single correct option

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