JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year ANTENNAS AND PROPAGATION Previous Question Paper (August/September - 2021 - R18)

A quarter-wavelength monopole antenna of length l = λ/4 mounted on an infinite, perfectly conducting ground plane can be analyzed using the image theory. The monopole and its image form an equivalent half-wave dipole of length L = λ/2 in free space.

For a dipole of length L = λ/2 oriented along the z-axis, the far-field components are derived from the vector potential. The electric and magnetic field components for the monopole (valid for the region above the ground plane, 0 ≤ θ ≤ π/2) are:

Where:

• I₀ is the peak current at the base of the monopole.
• η is the intrinsic impedance of free space (≈ 120π Ω).
• k is the wavenumber (2π/λ).
• r is the radial distance from the source.
• θ is the elevation angle measured from the z-axis.

Note that the radiation exists only for 0 ≤ θ ≤ π/2. The presence of the ground plane effectively doubles the radiation intensity compared to a dipole in free space for the same current, resulting in a gain increase of 3 dB.

The radiation pattern of a quarter-wave monopole is defined by its behavior in the principal planes:

• Elevation Plane (E-plane, φ = constant): The pattern is a semi-circle (or a figure-eight cut in half). Since the ground plane prevents radiation below the surface, the pattern shows maximum radiation at θ = 90° (along the ground) and zero radiation at θ = 0° (along the axis of the antenna).
• Azimuthal Plane (H-plane, θ = 90°): The pattern is omnidirectional. Because the monopole is symmetric about the z-axis, the field strength is uniform for all values of φ at a fixed elevation angle.

The resulting 3D radiation pattern resembles a toroid (donut shape) that has been sliced in half horizontally, with the flat base resting on the ground plane.