Radial (radio technology)
A radial (plural: radials ), with the meaning originally derived from radius ( German spoke ) for rods arranged in the shape of a spoke , is in radio technology an arrangement of at least one, usually several metallic rods, bands or wires that radiate from the base of a vertical antenna ( radial), similar to the spokes of a carriage wheel .
function
A vertical antenna ideally has a length of a quarter of the wavelength (λ / 4). (It can be significantly shortened mechanically by inserting a coil near the feed point in the antenna base . In this way, manageable rod lengths can also be achieved at lower frequencies such as 7 MHz ( λ = 40 m ), for which a λ / 4-long antenna rod 10 Meters long.)
Since a vertical antenna is fed unbalanced to earth , mostly via a coaxial cable , it also needs an electrical counterweight , whereby the unbalanced monopole (through "reflection") is supplemented to a balanced dipole . Ideal for mirroring is a highly electrically conductive ground plane ( English ground plane ) (red in the left image) as though through the ground (as opposed to sea water ) due to its too low conductivity is often inadequate given good. In particular, dry soil has a significantly higher resistance than a metallic surface or metallic radials.
One way of realizing the required counterweight is to use earth nets made of mostly metal wires or strips laid just below the earth's surface. In a broader sense, these are also referred to as radials. For antennas mounted far above the ground (as in the picture above right), another solution is recommended, namely the arrangement of radials that are separated from the ground (above the ground) and isolated. In the simplest case, the radials (red in the right picture) point horizontally away from the vertical antenna rod.
In general, however, they can also be arranged inclined to the ground (see picture above right). This increases the input impedance of the antenna, thereby improving its adaptation and consequently reducing the standing wave ratio (VSWR).
literature
- Alois Krischke : Rothammels Antennenbuch. DARC- Verlag, 2013, ISBN 978-3-88692-065-5 .
Web links
Individual evidence
- ^ Radius , Duden , Bibliographisches Institut , 2019