Question

At a distance 10km from a half wave dipole antenna in air the maximum field strength is 5V/m then the average power density(in$mW/m^2$) 40km from antenna is ________

• A. 2.07
• B. 3.07
• C. 4.14
• D. 1.03

Answer 1

The correct option is A 2.07

$E\alpha \frac{1}{r}$

$\frac{E_1}{E_2}=\frac{r_2}{r_1}\Rightarrow \frac{5}{E_2}=\frac{40}{10}$

$E_2=\frac{5}{4}V/m$

${\rho }_{avg}=\frac{1}{2}\frac{E^2}{\eta }=\frac{1}{2}{\left(\frac{5}{4}\right)}^2\left(\frac{1}{120\pi }\right)=2.07\times {10}^{-3}W/m^2$