Modern Physics 7 Question 8
8. The physical sizes of the transmitter and receiver antenna in a communication system are
(Main 2019, 9 April II)
(a) proportional to carrier frequency
(b) inversely proportional to modulation frequency
(c) independent of both carrier and modulation frequency
(d) inversely proportional to carrier frequency
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Solution:
- Size of antenna is directly proportional to the wavelength of the signal.
Also, the speed at which signal moves =carrier frequency $\times$ wavelength
$$ \begin{aligned} & \Rightarrow \quad f \lambda=c \Rightarrow \lambda \propto \frac{1}{f} \\ & \therefore \text { Size of antenna } \propto \frac{1}{f} . \end{aligned} $$
NOTE Minimum size of the antenna is $\lambda / 4$.
