An antenna is a key component of a wireless link which efficiently couples electromagnetic energy from the transmitter to free space and from free space to the receiver. An antenna is generally a bidirectional device, i.e, the power through the antenna can flow in both the directions, hence it works as a transmitting as well as a receiving antenna. An antenna acts as an interface between the radiated electromagnetic waves and the guided waves. It can be thought of as a mode transformer which transforms a guided wave field distribution into a radiated-wave field distribution.
The radiation efficiency of an antenna with input power 100 W and power dissipation 1 W is :
99%
Radiation efficiency is the fraction of the input power that actually leaves the antenna as radiation rather than being lost as heat:
\(\eta=\dfrac{P_{rad}}{P_{in}}=\dfrac{P_{in}-P_{loss}}{P_{in}}\)
Step 1 — find the radiated power. Of the 100 W supplied, 1 W is dissipated, so
\(P_{rad}=100-1=99\ \text{W}\)
Step 2 — form the ratio.
\(\eta=\dfrac{99}{100}=0.99=\mathbf{99\%}\)
which is option 3.
Where the distractors come from. Option 2, 0.99 %, is the answer obtained by taking the loss fraction 1/100 and mislabelling it — it is in fact the loss, not the efficiency. Option 1 slips a decimal place. Option 4 would require zero dissipation, which no real conductor can achieve.
The resistance form, which is how efficiency is actually designed for. The input resistance of an antenna splits into a radiating part and a lossy part:
\(\eta=\dfrac{R_{rad}}{R_{rad}+R_{loss}}\)
Here \(R_{loss}/R_{rad}=1/99\). Radiation resistance is not a physical resistor — it is the equivalent resistance that would absorb the power the antenna radiates — whereas Rloss is genuine ohmic resistance in the conductors, plus dielectric and ground losses.
Why efficiency collapses for electrically small antennas. Radiation resistance falls as the square of length in wavelengths:
\(R_{rad}=80\pi^{2}\left(\dfrac{dl}{\lambda}\right)^{2}\)
so a very short antenna may have \(R_{rad}\) of a fraction of an ohm while its conductor and ground losses stay at several ohms — efficiencies of a few per cent are common on portable low-frequency antennas. A half-wave dipole, with 73 Ω of radiation resistance against a fraction of an ohm of loss, easily achieves the 99 % of this question.
How efficiency links gain to directivity.
\(G=\eta D\)
Directivity depends only on the shape of the pattern; gain is what the antenna actually delivers. At 99 % efficiency the two differ by less than 0.05 dB, which is why for a good antenna the terms are often used interchangeably — but for a short or lossy one the distinction matters a great deal.
Hence, the radiation efficiency is 99 %.
Similar Questions
Match the following lists :
List - I List - II a. Beam efficiency i. \(4\pi/\Omega_A\) b. Directivity ii. \(kD\) c. Gain iii. \(\dfrac{\Omega_M}{\Omega_A}\) d. Aperture Efficiency iv. \(A_e/A_P\) Correct Codes are :
The standard reference antenna for the directive gain is :
A dipole antenna has a radiation resistance of 67 Ω and has a loss resistance of 5 Ω measured at the feed point. The efficiency of dipole antenna is :
The directivity of an antenna is 30 and it operates at a frequency of 100 MHz. The value of maximum effective aperture is given by
When the Q of an antenna increases, the bandwidth
Important Questions from Antenna Characteristics - Teaching
An amplifier has power gain of 800. Its decibel power gain is:
Which of the following is a measure of the antenna’s radiated power in a given direction?
The directivity of an antenna array can be increased by adding more antenna elements, as a larger number of elements:
Match the following lists :
List - I List - II a. Beam efficiency i. \(4\pi/\Omega_A\) b. Directivity ii. \(kD\) c. Gain iii. \(\dfrac{\Omega_M}{\Omega_A}\) d. Aperture Efficiency iv. \(A_e/A_P\) Correct Codes are :
The standard reference antenna for the directive gain is :
