Question

To transmit a signal of $3KHz$ frequency, the minimum length of antenna is ____ km.

• A. $25$
• B. $20$
• C. $50$
• D. $75$

Answer 1

The correct option is A $25$

Key Concept Marconi antenna is a straight conductor of length equal to a quarter of the wavelength of radio signals to be transmitted or received i.e. $l=\lambda /4$.
It depends on frequency of carrier wave and directive of the beam etc.
Given, $v=3\times {10}^{3}Hz$
Now, speed of light, $c=v\lambda$
$\Rightarrow 3\times {10}^8=3\times {10}^{3}Hz\cdot \lambda$
$\Rightarrow {10}^5=\lambda$
For minimum length of an antenna is
$l=\frac{\lambda }{4}=\frac{{10}^5}{4}=25000m=25km$.