The waves which are reflected back to the earth by the ionosphere are:

$A$ telephonic communication service is working at a carrier frequency of $20 GHz$. Only $20 \%$ of it is utilized for transmission. If each channel requires a bandwidth of $5 kHz$,then the number of telephonic channels that can be transmitted simultaneously are

$A$ $TV$ transmitting antenna is $98\,m$ high and the receiving antenna is at the ground level. If the radius of the earth is $6400\,km$,the surface area covered by the transmitting antenna is approximately $.........\,km^2$

For transmission of e.m. wave of audible frequency,these waves are superimposed with waves of

$A$ transmitting antenna is kept on the surface of the earth. The minimum height of the receiving antenna required to receive the signal in line-of-sight at $4\,km$ distance from it is $x \times 10^{-2}\,m$. The value of $x$ is (Let radius of earth $R = 6400\,km$).

The size of the antenna required for a carrier wave of frequency $3 \text{ MHz}$ is: (in $\text{ m}$)