Monopole Antenna - The operating principle of rod antennas (or monopoles) is based on the fact that the current distribution on an antenna structure that is only a quarter wavelength long is identical to that on a half-wave dipole if the antenna element "missing" from the dipole is replaced by a highly conducting surface. As a result of this reflection principle , vertical quarter-wave antennas on conducting ground have basically the same radiation pattern as half-wave dipole antennas. There is of course no radiation into the shadowed half of the space. The input impedance is half that of a dipole, exhibiting values between approx. 30 Ξ© and 40 Ξ©.
The conducting surface on which the monopole is erected plays an important part in enabling the reflection principle to take effect. Even on reasonably conducting ground (such as a field of wet grass) and particularly on poorly conducting ground (dry sand) it is usual and helpful to put out a ground net of wires (commonly also called radials).
Figure below shows the influence of ground conductivity on the vertical pattern of a monopole antenna:
A special form of a monopole antenna is the so called groundplane antenna as figure below. It is characterized by several wires or rods (known as radials) which are arranged in a radial configuration from the feed point under a certain angle. Typically an angle of approx. 135° to the quarter wave monopole is used in order to increase the feed point resistance to a value of approx. 50 Ξ© which can easily be matched to commonly used coaxial cables.
Groundplane antennas are used as vertically polarized omnidirectional antennas even in the VHF/UHF frequency range.
