JNTU B.Tech III ANTENNAS AND WAVE PROPAGATION Previous Question Paper (November/December - 2020 - R16)

Directivity of an antenna is defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. It is a measure of how much an antenna concentrates energy in a particular direction compared to an isotropic radiator.

Gain is a measure that takes into account the efficiency of the antenna as well as its directional properties. It is defined as the ratio of the intensity in a given direction to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. It is related to directivity by the efficiency factor, where Gain = Efficiency × Directivity.

Resolution (or beamwidth) refers to the ability of an antenna to distinguish between two closely spaced targets. It is typically defined by the half-power beamwidth (HPBW), which is the angular width of the main lobe where the radiation intensity is at least half of its maximum value. A narrower beamwidth provides higher angular resolution.

A quarter-wave monopole antenna placed over a perfectly conducting ground plane acts as a half-wave dipole due to image theory. The image of the monopole creates a virtual current distribution identical to the lower half of a half-wave dipole.

The current distribution on a monopole of length L = λ/4 is given by I(z) = I₀ sin(k(L-z)). The radiated power P_rad is calculated by integrating the Poynting vector over a hemisphere:

The radiation resistance R_rad is defined by the relation P_rad = (1/2) I₀² R_rad. For a half-wave dipole in free space, the radiation resistance is approximately 73.12 Ω. Since the monopole radiates only into the upper hemisphere, the radiated power is exactly half that of the corresponding half-wave dipole.

Thus, the radiation resistance of a quarter-wave monopole is 36.56 Ω, which is half the resistance of a half-wave dipole.