Quad antenna
A quad antenna (also quad or cubical quad ) refers to full-wave loops (wire length = wavelength ) in a preferably square shape. It can consist of just one or more elements as well as cascaded elements (radiators). In addition, one or more parasitic elements (directors and reflectors) can increase the directivity and the associated antenna gain .
construction
The elements can also form a diamond or a circle. The following applies: the larger the area framed by the full-wave loop, the higher the directional effect and the antenna gain. The maximum is achieved with the circular shape, but for structural reasons, square shapes are mostly used. The diamond shape is also possible here (square on top). The latter has the advantage that only one boom is required. In the hobby sector, early quad antennas were often made of bamboo material, which, if left untreated, allowed several years of operation. Modern quad antennas consist of GRP pipes, Ramin, epoxy pipes, and special quad center pieces made of aluminum injection molding.
properties
If the Quad is fed from below, the polarization is horizontal, if fed from the side, it is vertical, which makes it a flat radiator. Special shapes such as the X-Quad also enable circular polarization. With horizontal polarization, the quad has a stronger vertical focus than a single dipole and has a figure-of-eight characteristic horizontally. When using directors and reflectors (with short wavelengths often also a metal or grid surface) as well as with several radiators (two or more quad antennas fed one behind the other), the directional diagram changes to a lobe. Due to the additional (active or parasitic) elements, the quad antenna achieves a relatively high antenna gain and a stronger directivity in the tuned frequency range.
While the loop fed must have a circumference that corresponds to one wavelength, the circumference of the directors is slightly shorter and the circumference of the reflector is slightly longer. The fed element, reflectors and directors are not galvanically coupled, but can be mounted on a common support rod (boom). The exact lengths are influenced by the ambient conditions, so coordination (e.g. using so-called stubs) is usually necessary at the installation site.
The quad antenna is a particularly quiet antenna because of its electrically closed structure. It receives less interference than open antenna forms, for example the typical crackling noise in statically charged rain or snowfall is at least significantly less. Even when transmitting, it significantly reduces the interference from electronic devices in the neighborhood, which are often susceptible to this due to technical and structural inadequacies. The disadvantage is that very bulky structures are required at low frequencies.
history
This type of antenna was developed in 1942 high in the Andes jungle near Quito in Ecuador for the radio station Radio HCJB , which has been in operation since 1938 , by Clarence C. Moore (W9LZX) one of the station engineers and radio amateurs. The reason for this was that at an altitude of 3000 m the air ionized so strongly that the element ends of the 4-element directional radiator initially used melted away due to corona discharge ( Elms fire ). To avoid this, he looked for a closed antenna construction. This is how the first cubical quad was finally created.
Clarence C. Moore made the first publication in 1947 when he returned to the USA, applied for a patent for the quad antenna and received it in 1951.
application
Quad antennas have found and are widely used in professional radio services, in the military and in foreign services. The typical frequency spectrum ranges from shortwave to the gigahertz range.
Quad antennas have also been valued for decades in the amateur radio service and have proven themselves particularly as DX antennas (wide connection). Because radio amateurs have extensive experience and literature on this type of antenna, they are usually built by radio amateurs themselves and are still being developed today. The building materials for this come from the areas of hardware stores, hang gliding, amateur radio and natural products.
variants
There is a wide variety of types of quad antennas, including: Double Quad (also Bi-Quad), X-Quad, Cubical Quad, Bird Cage, Swiss Quad, German Quad, UA3IAR-Quad, Multiband Cubical Quad, Three-band Quad, Five-band Quad, Mini-Quad, G3YDX-Mini-Quad, Spider-Quad , DJ4VM-Quad and others.
Bi-quad antenna
A special form of the quad antenna is the bi-quad. She is z. B. expanded with a metal plate as a reflector for directional antenna. The Bi-Quad with metal reflector has a base point resistance of approx. 60 Ω, which enables the connection to coaxial cables, whereby the shield of the cable is connected to the reflector. As a result, the asymmetrical coaxial cable is also adapted to the symmetrical radiator and there are no standing waves . An “8” lying horizontally has a vertical polarization.
This type of antenna has been used in amateur radio for a long time, and recently bi-quads have also been used in WLAN technology and the wardriving scene. The Russian VHF radar "Vostok-E" uses this type of radiator in its directional antenna.
literature
- Karl Rothammel , Alois Krischke: Rothammels Antennenbuch . 11th edition, Franckh-Kosmos Verlag, Stuttgart 1995, ISBN 3-440-07018-2 .
- K. Weiner DJ9HO: The Cubical Quad and its special forms . DARC-Verlag, Baunatal 1999.