Ground plane

A ground plane antenna (GPA ) is shortened, often simply ground plane (GP), is a technical realization of a Marconi antenna. It is characterized in that it is an asymmetrical design and the second half of the antenna is replaced by a terrestrial network or by at least three λ / 4 long radials. Medium-wave radio transmitter using, for example, 100 radials of galvanized steel strips which are buried about 60 cm in the ground.

Function

A dipole, due to its symmetrical structure, perpendicular to its center a potential plane with zero potential. This potential level can be replaced by a broad conductive plane, without changing the function of the antenna significantly. Here, the current that would flow in the absence of real dipole half flows in the conductive layer. Various design parameters permit to influence the properties of the antenna:

• If the radials of a λ / 4 long groundplane antenna rotated by 45 ° downwards, so can a base impedance of 50 Ω achieve. The antenna can then be fed directly with standard coaxial cable.
• The spotlight of medium wave radio transmitters are often electrically 190 ° (slightly more than λ / 2) made ​​long, because as the strongest ground wave can be achieved. Although a 5/8 λ long radiator delivers even more profit, but under an elevation angle of about 30 °.
• While they must be matched exactly to λ/4-Resonanz Above ground radials tense, but are enough 3-4 radials per working frequency for an acceptable antenna efficiency and approximate omnidirectional coverage.
• A few cm in the soil buried radials must not necessarily be matched to resonance, but much more must be used it. Medium-wave broadcasting stations usually use about 100 buried radials.

Examples

The conductive layer is realized in mobile antennas using the body panel (e.g., on a roof ), or another " counterweight ". The counterweight must absorb the flows that otherwise takes the other Dipolast. There are ground planes with three ( triple-leg ) to several 100 conductors. These are called because they come from the feed point of the antenna radial Radial.

They are therefore a much used form of mobile antenna (car ) to the medium wave radio stations. In the UHF range (eg in the 70 cm band ) represents the body panel, a plurality of wavelengths is large area and thus forms an almost ideal counterweight.

A special form of the ground plane is the T- antenna in which the capacitive effect of the top piece of wire (see line theory ) is replaced by a top capacity of horizontal wires. In this way, the height of for example 500 m to 150 m can be reduced.