The first parabolic antenna was created by Heinrich Hertz in the late s. His early design was created using a brass sphere, zinc sheet metal, and wood. Hertz used one antenna for transmitting signals and another for receiving signals. Hertz used his antenna to help prove the existence of electromagnetic waves. As a result of this and other accomplishments, his name is now the term used for radio and electrical frequency measurements.
These include hertz Hz , kilohertz kHz and megahertz MHz. A single antenna can transmit and receive signals. A parabolic antenna is a unidirectional antenna since it transmits signals in only one direction. The dish part of one is known as the reflector. The reflector creates a large surface area that the antenna uses to receive and transmit signals.
Motorized dishes may experience feedhorn movement when the antenna is moved from one satellite to the next; heavy winds can also temporarily move the feedhorn away from the antenna's focus. Guy wire kits are available which the installer can use to provide additional structural rigidity to the buttonhook support if required for a given installation. Offset-fed Antennas The dish design of choice for most digital DTH systems is called an offset-fed antenna. With the offset-fed design, the feedhorn is no longer positioned at the front and center of the reflector but rather offset to the bottom of the dish.
However, the feed would be centrally located if we extended the parabolic curve of the offset fed dish to the full length of a prime focus parabola. The offset fed antenna design offers several distinct advantages over its prime focus counterparts.
There is no feedhorn blockage, an important consideration when the antenna aperture is less than one meter in diameter. Moreover, the offset angle at which the feedhorn tilts up toward the reflector is such that if the feed looks over the antenna's rim it will see the cold sky rather than the hot earth. Due to these advantages, the offset-fed antenna can achieve higher efficiency levels than prime focus antennas can generally attain.
The low inclination angles required by offset antennas also may be beneficial in certain climate zones. In tropical or semi-tropical environments, rain will not collect inside the reflector. In cold weather climates, snow will slide off of the antenna surface rather than accumulating inside the reflector.
Cassegrain Antennas The cassegrain antenna is most often used for dishes that exceed five meters in diameter. Its use is primarily restricted to uplink earth stations and cable TV head ends. The cassegrain design incorporates a small sub reflector located at the front and center of the dish. The sub reflector deflects the microwaves back toward the center of the reflector, where the feedhorn is actually mounted. Like the prime focus dish, the cassegrain antenna's view of the satellite is partially obscured, in this case by the sub reflector.
However, when the diameter of the dish exceeds 5m, the percentage of blockage is actually quite small. This type of antenna obtains higher efficiencies because the feedhorn looks up at the cold sky and the required illumination taper is reduced.
The precise manufacturing tolerances required to implement this dual reflector approach, however, increases the manufacturing cost and adds complexity to the installation process. Spherical Antennas The spherical antenna design creates multiple focal points located to the front and center of the reflector, one for each available satellite. The curvature of the reflector is such that if extended it outward far enough along both axes it would become a sphere. Spherical antennas are primarily used for commercial SMATV and cable installations where the customer wishes to simultaneously receive multiple satellites with a single dish.
Planar Arrays Some digital DTH systems in Japan and elsewhere have elected to use an alternate antenna design called the planar array. These flat antennas do not rely on the reflective principles used by all parabolic dishes. Therefore no feedhorn is required. Instead a grid of tiny elements is embedded into the antenna's surface. These elements have a size and shape which causes them to resonate with the incoming microwave signals. A spider's web of feed lines is used to interconnect all the resonant elements in such a way that their signal contributions are all combined in phase at a single terminal located at the center of the array which connects directly to the LNB.
Planar arrays are relatively unobtrusive: there is no feedhorn and the LNB is located to the rear of the antenna out of sight. Since these antennas are most always dedicated to the reception of a single satellite or constellation of collocated satellites, they can be mounted in a fixed position on an outer wall or rooftop. One main disadvantage of the planar array is its limited frequency bandwidth which is about MHz.
Parabolic antennas, however, have a broad bandwidth; a single dish, for example, can be used to receive S, C, and Ku-band satellite signals. Another disadvantage of the planar array is the high construction cost: more than four times the cost of manufacturing a feedhorn and parabolic reflector with equivalent signal amplification characteristics. Gain, which is expressed in decibels, or dB, is primarily a function of antenna capture area or aperture: the larger the antenna aperture, the higher the antenna gain.
Gain also is directly related to antenna beam width: the narrow corridor or "boresight" along which the antenna looks up at the sky. The antenna's efficiency rating is the percentage of signal captured by the parabolic reflector that actually is received by the feedhorn. As we have previously seen, the feed-horn's illumination of the outer portion of the dish is attenuated or tapered, which leads us to conclude that antenna gain is not as important a factor as it might first appear to be.
In some areas it is the form of antenna that is used virtually exclusively because of its characteristics. In all these applications very high levels of gain are required to receive the incoming signals that are often at a very low level. For transmitting this type of RF antenna design is able to concentrate the available radiated power into a narrow beamwidth, ensuring all the available power is radiated in the required direction.
In some antennas it may be a simple dipole, in others a horn. Its aim is to illuminate the second element of the antenna, the reflector with an even density of radiation with the minimum spillage or radiation missing the reflector and being radiated elsewhere. Reflector: The reflector is the distinctive part of the parabolic reflector antenna. The parabolic shape is key to the operation of the RF antenna because the paths taken from the feed point at the focus to the reflector and then outwards are in parallel.
However more importantly the paths taken are all the same length and therefore the outgoing waveform will form a plane wave and the energy taken by all paths will all be in phase. This enables the antenna to perform in a particularly effective manner. The parabolic shape of the reflector surface of the antenna enables a very accurate beam to be obtained. In this way, the feed system forms the actual radiating section of the antenna, and the reflecting parabolic surface is purely passive.
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