Question

Find the power received by the receiving antenna if it is placed at a distance of 20m from the transmitting antenna which is radiating 50W power at a frequency 900MHz and are made-up of half-wave dipoles.

(a) 23.65μW

(b) 2.365μW

(c) 236.5μW

(d) 4.73μW

The question was posed to me by my school principal while I was bunking the class.

I need to ask this question from Friis Transmission Equation topic in chapter Antenna Parameters of Antennas

Answer 1

Right answer is (c) 236.5μW

To elaborate: given d=20m, Pt=50W and f=900MHz

Gain of half-wave dipoles is 1.64

\(λ = \frac{c}{f} = \frac{3×10^8}{900Mhz} = \frac{1}{3} m \)

\(\frac{P_r}{P_t} = \frac{G_t G_r \lambda^2}{(4\pi R)^2} = \frac{1.64×1.64×1/3^2}{(4\pi ×20)^2}\)

Pr=236.5μW