$A$ message signal of $12 kHz$ and peak voltage $20 V$ is used to modulate a carrier wave of frequency $12 MHz$ and peak voltage $30 V$. Then,the modulation index is

In amplitude modulation,if a message signal of $5 \ kHz$ is modulated by a carrier wave of frequency $900 \ kHz$,then the frequencies of the side bands are

Digital signal

The layer of the atmosphere which efficiently reflects high frequency waves particularly at night is

$A$ $TV$ tower has a height of $160 \,m$. Its coverage range is nearly (Earth's radius,$R_e = 6400 \,km$):

If a message signal of frequency $f_{m}$ is amplitude modulated with a carrier signal of frequency $f_{c}$ and radiated through an antenna,the wavelength of the corresponding signal in air is ....... .