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 Ω and 40 Ω.
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 Ω 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.