The attenuation in optical fibre is mainly due to

$A$ message signal of frequency $f_m$ is used to modulate a carrier of frequency $f_c$. If the sidebands are $f_1$ and $f_2$, then the ratio $\frac{f_c}{f_m}$ is:

The height of a transmitting antenna is $h \ m$ and the height of a receiving antenna is $45 \ m$. The distance between them is $40 \ km$. For satisfactory $Line\, of\, Sight$ $(LOS)$ communication between the two antennas,the height of the transmitting antenna $h$ must be ....... $m$. (Radius of the Earth $R = 6400 \ km$)

The maximum distance up to which $TV$ transmission from a $TV$ tower of height $h$ can be received is proportional to

$A$ $TV$ transmission tower of height $h$ covers a range of distance $d$. By how much will the range change if the height is increased to $3/2 h$?

$A$ carrier wave of peak voltage $20 V$ is used to transmit a message signal. For getting a modulation index of $60 \%$, the peak voltage of the modulating signal is (in $V$)