For a coaxial cable,the typical repeater spacing is approximately............$km$.

If the length of an antenna is $150 \,cm$,the transmission frequency is . . . . . . $MHz$. (Speed of light in vacuum $= 3 \times 10^8 \,ms^{-1}$)

$A$ carrier wave of peak voltage $60 \,V$ is used to transmit a message signal. Then the peak voltage of the modulating signal in order to have a modulation index of $90 \%$ is (in $\,V$)

$A$ transmitting antenna can cover an area of $6.4\pi \times 10^9 \ m^2$ when used for broadcasting. If the signal is transmitted to another antenna,which is at a distance of $100 \ km$ from this antenna,what should be the minimum height of the receiving antenna for satisfactory communication in line-of-sight mode (in $m$)? (Assume the radius of the Earth $R = 6400 \ km$)

The wavelength of the carrier waves in a modern optical fiber communication network is close to ........ $nm$.

The advantages of optical fiber communication over a two-wire transmission line or coaxial cable are . . . . . . .