Monopole antenna

Introduction

A monopole antenna is a class of radio antenna that consists of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane, which is often the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.

A straight rod-shaped conductor mounted perpendicularly over a conductive surface.

Design and Operation

The general term monopole is used to describe an antenna which radiates a directional (asymmetric) pattern that is a function of angle. The most common form is the quarter-wave monopole, in which the antenna is approximately 1/4 of the wavelength of the radio waves. The radiation pattern in the vertical plane is similar to that of a dipole antenna, but it is compressed, with zero radiation at the highest point. Higher performance monopole antennas are also possible, and there designs can include additional conical skirts or cylindrical sleeves to adjust the antenna's impedance and radiation pattern.

Quarter-Wave Monopole

The most widely used design of a monopole antenna is the quarter-wave monopole, which is roughly a quarter of the wavelength of the radio waves it is used with. The radiating element of a quarter-wave monopole is a rod or wire, usually mounted vertically, with one end grounded or connected to the radio's chassis. The radio waves are emitted from the top end of the rod, and the electric field lines are approximately circular around this axis. The antenna is usually fed at its base. A quarter-wave monopole, like any other similar antenna, has a single resonant frequency, and is effective over a relatively narrow bandwidth.

Half-Wave Monopole

A half-wave monopole is a variation of the basic monopole antenna in which the antenna is half the wavelength of the radio waves. The radiation pattern of a half-wave monopole is similar to that of the quarter-wave monopole, but it is slightly more directive (has a higher gain). However, the half-wave monopole has a higher impedance, making it less efficient than the quarter-wave monopole. The half-wave monopole can be fed at the base as a quarter-wave monopole, but it can also be fed at its center, in which case it is called a half-wave dipole.

Ground Plane

The ground plane of a monopole antenna can be either a physical surface, such as the Earth, or a network of conductive elements that simulate a surface. In either case, the ground plane provides a return path for the radio waves and greatly affects the radiation pattern of the antenna. The size and shape of the ground plane, as well as its proximity to the antenna, can have a significant impact on the antenna's performance. For example, a larger ground plane generally provides a more stable and efficient antenna, while a smaller ground plane can lead to a less efficient antenna with a more complex radiation pattern.

Applications

Monopole antennas are used in a wide variety of applications. They are commonly used in mobile and portable wireless devices because of their simplicity and omnidirectional radiation pattern. They are also used in base station antennas for wireless communications, in which they are often arranged in arrays to increase gain. Other applications include broadcasting, where they are used as transmitting antennas for AM and FM radio, and in aviation, where they are used for communication and navigation systems.