Which of the following options is NOT correct | vedclass

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(A) Modulation is required for the following reasons:$1$. Reducing the size of the antenna: The height of the antenna should be at least $\lambda/4$.

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Reducing the time lag between transmission and reception in the information signal.

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(A) Modulation is required for the following reasons:$1$. Reducing the size of the antenna: The height of the antenna should be at least $\lambda/4$. For low-frequency signals,$\lambda$ is very large,requiring a huge antenna. Modulation shifts the signal to a higher frequency,reducing $\lambda$ and thus the antenna size.$2$. Reducing fractional bandwidth: By increasing the carrier frequency,the ratio of the signal bandwidth to the center frequency decreases,which improves the quality of transmission.$3$. Increasing selectivity: Modulation allows for multiplexing,enabling multiple signals to be transmitted simultaneously without interference,which improves the selectivity of the receiver.Option $A$ is incorrect because modulation does not reduce the time lag between transmission and reception; the speed of electromagnetic waves is constant $(c \approx 3 	imes 10^8 \ m/s)$.

Column $A$	Column $B$
$A$. $D$-layer	$I$. $250-400 \ km$
$B$. $E$-layer	$II$. $170-190 \ km$
$C$. $F_1$-layer	$III$. $95-120 \ km$
$D$. $F_2$-layer	$IV$. $65-75 \ km$

Which of the following options is $NOT$ correct? We need modulation for:

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The frequency of a $FM$ transmitter without signal input is called

The maximum and minimum voltage of an amplitude modulated signal are $60\,V$ and $20\,V$ respectively. The percentage modulation index will be $....\%$

Match Column $A$ (layers in the ionosphere for skywave propagation) with Column $B$ (their height range).
Column $A$ Column $B$
$A$. $D$-layer $I$. $250-400 \ km$
$B$. $E$-layer $II$. $170-190 \ km$
$C$. $F_1$-layer $III$. $95-120 \ km$
$D$. $F_2$-layer $IV$. $65-75 \ km$

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:

The waves which are reflected back to the earth by the ionosphere are:

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