Q6Electromagnetics Waves
Question
Q.6. What do you mean by monopole and dipole antenna?
Answer
A monopole antenna is a single straight conductor fed against a ground plane, radiating a hemispherical pattern; a dipole antenna is two collinear conducting arms fed at the centre, radiating a doughnut-shaped pattern, and behaves like two monopoles joined end to end.
Monopole antenna: a single straight radiating conductor (typically a quarter-wavelength, λ/4, long for resonance) mounted vertically over a conducting ground plane (or the Earth itself), fed at its base between the conductor and the ground plane. The ground plane acts as a mirror, and by image theory the monopole together with its image behaves electrically like a full half-wave dipole above the plane, radiating only into the upper hemisphere with double the field strength (four times the power density) of the equivalent half of a dipole for the same input power — common examples include car-radio whip antennas and mobile-tower antennas over a ground/metal base.
Dipole antenna: two straight conducting arms, collinear and each of length λ/4 (for the common resonant half-wave dipole, total length λ/2), fed at the small gap between them at the centre. Being a balanced, self-contained radiator (no ground plane needed), it radiates into free space in all directions except along its own axis, producing the characteristic doughnut-shaped (toroidal) radiation pattern that is omnidirectional in the plane perpendicular to the antenna and has nulls along the antenna's own axis.