Ground plane antenna
A ground plane (GPA) ( English Monopole antenna ), in short also referred to as a ground plane , designated (GP) actually monopole antenna , and quarter-wave radiator , rod antenna or, after its inventor Guglielmo Marconi , Marconi antenna called, is a vertical - antenna , consisting of a vertical pole or a self-radiating mast .
In contrast to a dipole antenna , it is fed asymmetrically . It is usually supplemented by a so-called " counterweight " (see also earthing ), which is supported by the earth's surface, which is a good electrical conductor, an earth network buried in the earth around the antenna or, especially in the VHF range, by several so-called " radials " around the monopoly is formed.
These radials, which are at ground or earth potential, start radially from the central feed point (base point) of the antenna and comprise at least three to several hundred conductors. The length of these conductors is often determined by the wavelength λ, and it is advantageous to choose λ / 4 as the length if only a few radials are used. Self-radiating transmission masts in the medium wave range use z. B. 100 radials made of galvanized steel strips, which are buried about 60 cm deep in the ground around the transmission mast.
Monopole antennas are used as transmitting and receiving antennas for long wave , long wave , medium wave , short wave up to the ultra short wave (VHF) range. Examples of monopole antennas in the VHF range are vehicle antennas for VHF reception, with the metallic body serving as a ground plane instead of the earth.
function
Due to its symmetrical structure, a dipole has a potential plane with zero potential perpendicular to its center. This potential level can be replaced by an extended conductive level like the earth's surface without significantly changing the function of the antenna. The current flows into the conductive plane instead of the missing dipole half. Various construction parameters allow to influence the characteristics of the antenna:
- If the radials of a λ / 4 long ground plane antenna are pivoted downwards by 45 °, a base point resistance of 50 Ω can be achieved and the antenna can then be fed directly with conventional coaxial cable .
- The radiators of medium-wave radio transmitters are often made electrically 190 ° (a little more than λ / 2) long, because this way the strongest ground wave can be achieved. A 5/8 λ long radiator delivers even more gain, but at an elevation angle of approx. 30 °.
- If only a few radials are used as a counterweight, they must be tuned to λ / 4 resonance in order to offer the compensating current a current path with low impedance. 3–4 radials per operating frequency are sufficient for an acceptable antenna efficiency and approximately omnidirectional radiation.
- If, on the other hand, the counterweight of the antenna consists of many radials, the radials do not necessarily have to be tuned to λ / 4 resonance, since the impedances of all radials are effectively connected in parallel. Medium wave broadcasters typically use over 100 buried radials.
A special form of the ground plane is the T-antenna , in which the capacitive effect of the topmost piece of wire (see line theory ) is replaced by a roof capacitance made of horizontal wires. In this way, the overall height can be reduced from 500 m to 150 m, for example. Another type of monopole antenna is the Alexanderson antenna .
Antenna gain
A monopole antenna has a radiation field that is rotationally symmetrical around the vertical monopole, so the antenna gain of a monopole antenna is 3 dBd with an ideal ground plane , based on the ideal dipole antenna, since with the monopole antenna the radiation to the upper half-space with a solid angle of 2π is limited. If the ground plane is insufficient, for example due to the curvature of a metallic vehicle body, the antenna gain is less than 3 dBd.
Impedance
The wave impedance with base feed is 36 Ω. It is therefore only about half the wave impedance of a symmetrically fed half-wave dipole with about 73 Ω and one eighth of a folded dipole (240 Ω).
Like the dipole antenna, monopole antennas can be shortened mechanically by inserting an inductance as an " extension coil" near the base . This worsens the radiation, the wave impedance is guaranteed with a suitable inductance. It should be noted that the adaptation with inductance is narrow-band. Strictly speaking, a monopole antenna can only be shortened for a single frequency. The antenna loses much of its natural broadband. For broadband shortening, which is usually desired for a band, you have to use other methods.
literature
- Ulrich Freyer: Antenna technology for radio practitioners . 1st edition. Franzis-Verlag, Poing 2000, ISBN 3-7723-4693-6 .
- Gerd Klawitter: Antenna advisor receiving antennas for all wave ranges . 6th edition. Verlag für Technik und Handwerk, 2005, ISBN 3-88180-613-X .
