A transmitting antenna is kept on the surface of the earth. The minimum height of receiving antenna required to receive …

Given, \(\mathrm{dr}=4\mathrm{km}\)
For receiving antenna. \({d}_{r}=\sqrt{2{h}_{r}R}∴{h}_{r}=\frac{{d}_{r}^{2}}{2R}\)
\(\mathrm{R}=6400\mathrm{km}=6.4\times {10}^{6}\mathrm{m}\\ =\frac{{\left(4\times {10}^{3}\right)}^{2}}{2\left(6.4\times {10}^{6}\mathrm{m}\right)}=\left(\frac{1}{0.8}\right)\mathrm{m}=1.25\mathrm{m}=125\times {10}^{-2}\mathrm{m}\)