The minimum size of an antenna for transmitting electromagnetic waves at $1500 \text{ MHz}$ is

$A$ transmitting antenna is at a height of $50 \ m$ and the receiving antenna is at a height of $5 \ m$. What is the maximum communication range in $km$? (Take $R = 6400 \ km$)

$A$ message signal of frequency $20 \, kHz$ and peak voltage of $20 \, V$ is used to modulate a carrier wave of frequency $1 \, MHz$ and peak voltage of $20 \, V$. The modulation index will be ..... .

Assertion $(A)$: Television signals are received through sky-wave propagation.
Reason $(R)$: The ionosphere reflects electromagnetic waves of frequencies in the range $(3-30)$ $MHz$.
Choose the correct option.

$A$ transmitting antenna covers a range of $12.8 \ km$. Find the height of the antenna in $m$. (Radius of the Earth $= 6400 \ km$)

The maximum amplitude of an $AM$ wave is found to be $20 \,V$ while its minimum amplitude is $4 \,V$. The modulation index is