In an amplitude modulator circuit,the carrier wave is given by $C(t) = 4 \sin(20000 \pi t)$ and the modulating signal is given by $m(t) = 2 \sin(2000 \pi t)$. The values of the modulation index and the lower sideband frequency are:

The maximum number of $TV$ signals that can be transmitted along a coaxial cable is

The maximum frequency of transmitted radio waves above which the radio waves are no longer reflected back by the ionosphere is . . . . . . ($N=$ maximum electron density of the ionosphere, $g=$ acceleration due to gravity).

The signal obtained at the output of a microphone is of which type?

$A$ signal is to be transmitted through a wave of wavelength $\lambda$,using a linear antenna. The length $l$ of the antenna and effective power radiated $P_{eff}$ will be given respectively as: ($K$ is a constant of proportionality)

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: