Question

What is the amount of Electric field present at a distance of 10km for an isotropic radiator with radiating power 3kW?

(a) 30mV/m

(b) 60mV/m

(c) 15mV/m

(d) 10mV/m

I got this question in examination.

My question comes from Isotropic Radiators topic in portion Antenna Parameters of Antennas

Answer 1

Right option is (a) 30mV/m

Explanation: For isotropic radiator, power per unit area = \(\frac{P_t}{4πr^2}\) — (1)

From pointing theorem, \(P=\frac{\mid E_{max}^2\mid}{2\eta}\), where η=120π. —– (2)

Equating (1) & (2) and Erms=Emax/√2, we get

\(E_{rms}=\frac{\sqrt{30P_t}}{r}=\frac{\sqrt{30×3000}}{10000}=30mV/m.\)