If $f_a$ and $f_f$ represent the carrier wave frequencies for amplitude and frequency modulations respectively,then

$A$ bandwidth of $6 \, \text{MHz}$ is available for $A.M.$ transmission. If the maximum audio signal frequency used for modulating the carrier wave is not to exceed $6 \, \text{kHz}$,the number of stations that can be broadcasted within this band simultaneously without interfering with each other will be ..... .

If the sum of heights of transmitting and receiving antennas in line of sight communication is $h$,then the height of the receiving antenna,to have the maximum range,is

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

$A$ transmitting antenna covers a range of $12.8 \ km$. Find the height of the antenna in $m$. (Radius of the Earth $= 6400 \ km$)

The process of superimposing low-frequency signals onto high-frequency carrier waves is called ..........