Power radiated by an antenna is directly proportional to (\( l \) – length of antenna, \( \lambda \) – wavelength of electromagnetic wave)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( \left(\displaystyle\frac{l}{\lambda}\right)^{2} \)
For a linear antenna of length \( l \), the power radiated at a given current depends on the size of the antenna compared with the wavelength it emits, and it varies as the square of that ratio.
\( \displaystyle P \propto \left(\frac{l}{\lambda}\right)^{2} \)
This is why a signal of short wavelength (high frequency) is radiated efficiently by an antenna of practical length, and why an antenna is usually made comparable in size to the wavelength (for example \( l = \lambda/4 \)).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Effective Power Radiated by an Antenna – “A theoretical study of radiation from a linear antenna (length l) shows that the power radiated is proportional to (l/λ)2.”