Mean optical power launched into an $8 \ km$ fibre is $120 \ \mu W$ and mean output power is $4 \ \mu W$,then the overall attenuation is ....... $dB$ (Given $\log 30 = 1.477$)

The bandwidth of a video signal is approximately ........

We do not transmit low frequency signals to long distances because:
$(a)$ The size of the antenna should be comparable to signal wavelength,which is impractical for a signal of longer wavelength.
$(b)$ Effective power radiated by a long wavelength baseband signal would be high.
$(c)$ We want to avoid mixing up signals transmitted by different transmitters simultaneously.
$(d)$ Low frequency signals can be sent to long distances by superimposing them with a high frequency wave.
Therefore,the most suitable options are:

The height of a transmitting antenna at the top of a tower is $25 \; m$ and that of the receiving antenna is $49 \; m$. The maximum distance between them for satisfactory communication in $LOS$ (Line of Sight) is $K \sqrt{5} \times 10^{2} \; m$. The value of $K$ is $\dots$ [Assume radius of Earth is $64 \times 10^{5} \; m$] (Calculate up to the nearest integer value).

Choose the correct statement for amplitude modulation.

To radiate a wave of frequency $1 \text{ kHz}$ efficiently,what should be the minimum length of the antenna in $km$?