What is the minimum height of an antenna required to transmit a radio signal of frequency $20 \text{ MHz}$?

The amplitude of $15 \sin (1000 \pi t)$ is modulated by $10 \sin (4 \pi t)$ signal. The amplitude modulated signal contains frequencies of
$1. 500\,Hz$
$2. 2\,Hz$
$3. 250\,Hz$
$4. 498\,Hz$
$5. 502\,Hz$
Choose the correct answer from the options given below:

If a signal of frequency $3 \ kHz$ is to be transmitted via amplitude modulation,which of the following should be chosen as the carrier wave frequency?

The electron density of the ionospheric layer is $10^{11} \ m^{-3}$. Communication waves of frequency up to $f_c$ $MHz$ will be reflected by the ionospheric layer. Find $f_c$.

Which of the following is not a natural source of electrical noise?

An audio signal $v_{m} = 20 \sin(2 \pi \times 1500 t)$ amplitude modulates a carrier $v_{c} = 80 \sin(2 \pi \times 100,000 t)$. The value of percent modulation is..........