Theory and Design of Ground Plane Antenna25/11/2022
Ground-mounted antennas are frequently quite simple and practical to operate. However, using a ground plane is another option when mounting an antenna on the ground is not an option due to the terrain or when it is required to raise the antenna.
The antenna ground plane functions as a mimicked ground, as its name suggests. The ground functions as a plane to reflect the radio waves for monopole antennas like a quarter wavelength vertical, allowing the Earth to perceive an image of the upper half of the antenna. By substituting a conducting plane for the actual earth, this function may be mimicked. The conducting surface must reach at least a quarter wavelength from the antenna base in order to serve as an antenna ground plane.
A complete circular conducting plate is not actually required for a ground plane. Cost and wind resistance management would be challenging in this situation. Instead, a ground plane made up of many quarter-wavelength radials is typically adequate. Four conducting radials are frequently employed, and they frequently succeed in simulating the whole circular ground plane.
A minimum of four radials is not always required; frequently, even two will do. Additionally, they may be cut down little without negatively affecting performance, particularly if they are loaded to maintain their electrical length.
In comparison to a dipole, which has a feed impedance of 73Ω, a quarter wavelength vertically fed against a ground plane has a feed impedance of 37Ω. If the antenna needs to be fed with a conventional 50Ω coaxial feeder, then this can be a problem. To get around this, the feed impedance can be raised by bending the ground plane conductors downward from the horizontal. When the angle between the ground plane rods and the horizontal is 42 degrees, a 50Ω match will be produced.
In the limit situation, the radials all run down the same axis as the primary radiating element to create a vertical dipole, for which the radiation resistance/feed impedance is 73Ω. If the radials are dropped much farther, the impedance climbs even further.
The operating frequency of the antenna and, thus, the length of the wires, determine the typical methods for making ground plane conductors.
At HF, the conductors typically consist of regular wires linked to the antenna’s ground point. These are transported to accessible locations so they may be moored. Usually, the conductor’s end has to have an insulator.
At VHF and higher frequencies, the conductors are often considerably shorter and constructed of rods, which is typically the same material as the vertical element itself.