Question

To radiate EM signal of wavelength \( \lambda \) with high efficiency, the antennas should have a minimum size equal to:

• A. 2\(\lambda\)
• B. \(\frac{\lambda}{4}\)
• C. \(\frac{\lambda}{2}\)
• D. \(\lambda\)

Answer 1

The Correct Option is B

To efficiently radiate an electromagnetic (EM) signal with a given wavelength \( \lambda \), the antenna's size plays a critical role. This is particularly important due to the fact that the antenna serves as a resonator for the EM waves.

The efficiency of radiation is significantly enhanced when the size of the antenna is properly matched with the wavelength of the signal. Let’s explore why \(\frac{\lambda}{4}\) is the correct choice for the minimum size of an efficient antenna:

1. Understanding Antenna Resonance:
   • Antennas can be thought of as resonant circuits. For antennas to work efficiently, they must be resonant at the frequency of the transmitted or received signal.
   • The resonant length of a dipole antenna, one of the most common types, is typically a half-wavelength of the signal (\(\frac{\lambda}{2}\)). However, for monopole antennas, which are essentially a quarter-wave radiator, the suitable resonant length is a quarter-wavelength (\(\frac{\lambda}{4}\)).
2. Quarter-Wave Dipole Antennas:
   • A quarter-wave antenna or monopole is designed to have a length of \(\frac{\lambda}{4}\). At this length, it naturally resonates and can efficiently emit or receive EM signals at the frequency corresponding to the wavelength \(\lambda\).
3. Choice Justification:
   • The other options - 2\(\lambda\), \(\lambda\), and \(\frac{\lambda}{2}\) - represent lengths larger than necessary for basic or minimal efficient radiation for a simple antenna design, making them more suitable for other specific cases or types of antennas.
   • Thus, for typical radiative efficiency and standard design applications like vertical antennas, choice \(\frac{\lambda}{4}\) stands as the appropriate minimum size for high efficiency.

Hence, the correct answer is that antennas should have a minimum size equal to \(\frac{\lambda}{4}\) for efficient radiation of an EM signal.