The transmitting antenna placed at the top of a tower has a height of $45 \ m$ from the ground. The distance between receiving and transmitting antennas is $40 \ km$ and the radius of earth is $6400 \ km$. The minimum height (in $m$) at which the receiving antenna is to be placed for satisfactory communication in $LOS$ mode, is

This question has Statement-$1$ and Statement-$2$. Of the four choices given after the statements, choose the one that best describes the two statements.
Statement-$1$: Sky wave signals are used for long-distance radio communication. These signals are, in general, less stable than ground wave signals.
Statement-$2$: The state of the ionosphere varies from hour to hour, day to day, and season to season.

$A$ transmitting antenna is at a height of $50 \ m$ and the receiving antenna is at a height of $5 \ m$. What is the maximum communication range in $km$? (Take $R = 6400 \ km$)

For an amplitude modulated wave,the modulation index is found to be $0.5$. If the maximum amplitude is found to be $10.0 \ V$,then the minimum amplitude is: (in $V$)

In communication with the help of an antenna,if the height is doubled,then the range covered,which was initially $r$,would become:

The government has divided the frequency range from $30\ MHz$ to $60\ MHz$ between $20$ stations,which are independently beaming their signals. Which of the signals can be beamed by each of the stations?