For effective radiation of a wave with a frequency of $20 \text{ MHz}$,what should be the length of the antenna in $m$?

Which of the following statements is $NOT$ correct?

In the process of space communication,the use of a modem is necessary. In which one of the following modes does a modem act as a modulator and a demodulator respectively?

The modulation frequency of an $AM$ radio station is $250\, kHz$,which is $10\%$ of the carrier wave. If another $AM$ station approaches you for license,what broadcast frequency will you allot? (Note: The interval between two carrier frequencies should be at least two times the modulation frequency to avoid interference.)

Which of the following statements is $NOT$ true?

$A$ carrier wave has a frequency of $1000 \, kHz$ and an amplitude of $20 \, V$. It is amplitude modulated by a signal wave of frequency $10 \, kHz$ and amplitude $10 \, V$. The modulation index and the frequencies of the $LSB$ and $USB$ are,respectively: