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Modem and Dish
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.
When the signal reaches the viewer's house, it is captured by the satellite dish. A satellite dish is just a special kind of antenna designed to focus on a specific broadcast source. The standard dish consists of a parabolic(bowl-shaped) surface and a central feed horn. To transmit a signal, a controller sends it through the horn, and the dish focuses the signal into a relatively narrow beam.The dish on the receiving end can't transmit information; it can only receive it. The receiving dish works in the exact opposite way of the transmitter. When a beam hits the curved dish, the parabola shape reflects the radio signal inward onto a particular point, just like a concave mirror focuses light onto a particular point.
In this case, the point is the dish's feed horn, which passes the signal on to the receiving equipment. In an ideal setup, there aren't any major obstacles between the satellite and the dish, so the dish receives a clear signal.
Types:
Motor-driven dish. A dish that is mounted on a pole and driven by a stepper motor or a servo can be controlled and rotated to face any satellite position in the sky. Motor-driven dishes are popular with enthusiasts. There are three competing standards: DiSEqC, USALS, and 36v positioners. Many receivers support all of these standards.
Multi-satellite. Some designs enable simultaneous reception from multiple different satellite positions without re-positioning the dish. The vertical axis operates as an off-axis concave parabolic concave hyperbolic Cassegrain reflector, while the horizontal axis operates as a concave convex Cassegrain. The spot from the main dish wanders across the secondary, which corrects astigmatism by its varying curvature. The elliptic aperture of the primary is designed to fit the deformed illumination by the horns. Due to double spill-over, this makes more sense for a large dish.
VSAT. A common type of dish is the very small aperture terminal (VSAT). This provides two way satellite internet communications for both consumers and private networks for organizations. Today most VSATs operate in Ku band; C band is restricted to less populated regions of the world. There is a move which started in 2005 towards new Ka bandsatellites operating at higher frequencies, offering greater performance at lower cost. These antennas vary from 74 to 120 cm (29 to 47 in) in most applications though C-band VSATs may be as large as 4 m (13 ft).
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.
When the signal reaches the viewer's house, it is captured by the satellite dish. A satellite dish is just a special kind of antenna designed to focus on a specific broadcast source. The standard dish consists of a parabolic(bowl-shaped) surface and a central feed horn. To transmit a signal, a controller sends it through the horn, and the dish focuses the signal into a relatively narrow beam.The dish on the receiving end can't transmit information; it can only receive it. The receiving dish works in the exact opposite way of the transmitter. When a beam hits the curved dish, the parabola shape reflects the radio signal inward onto a particular point, just like a concave mirror focuses light onto a particular point.
In this case, the point is the dish's feed horn, which passes the signal on to the receiving equipment. In an ideal setup, there aren't any major obstacles between the satellite and the dish, so the dish receives a clear signal.
Types:
Motor-driven dish. A dish that is mounted on a pole and driven by a stepper motor or a servo can be controlled and rotated to face any satellite position in the sky. Motor-driven dishes are popular with enthusiasts. There are three competing standards: DiSEqC, USALS, and 36v positioners. Many receivers support all of these standards.
Multi-satellite. Some designs enable simultaneous reception from multiple different satellite positions without re-positioning the dish. The vertical axis operates as an off-axis concave parabolic concave hyperbolic Cassegrain reflector, while the horizontal axis operates as a concave convex Cassegrain. The spot from the main dish wanders across the secondary, which corrects astigmatism by its varying curvature. The elliptic aperture of the primary is designed to fit the deformed illumination by the horns. Due to double spill-over, this makes more sense for a large dish.
VSAT. A common type of dish is the very small aperture terminal (VSAT). This provides two way satellite internet communications for both consumers and private networks for organizations. Today most VSATs operate in Ku band; C band is restricted to less populated regions of the world. There is a move which started in 2005 towards new Ka bandsatellites operating at higher frequencies, offering greater performance at lower cost. These antennas vary from 74 to 120 cm (29 to 47 in) in most applications though C-band VSATs may be as large as 4 m (13 ft).
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A satellite modem or satmodem is a modem used to establish data transfers using a communications satellite as a relay.
There is a wide range of satellite modems from cheap devices for home Internet access to expensive multifunctional equipment for enterprise use.
A "modem" stands for "modulator-demodulator". A satellite modem's main function is to transform an input bitstream to a radio signal and vice versa. There are some devices that include only a demodulator (and no modulator, thus only allowing data to be downloaded by satellite) that are also referred to as "satellite modems." These devices are used in satellite Internet access (in this case uploaded data is transferred through a conventional PSTN modem or an ADSL modem).
The main functions of a satellite modem are modulation and demodulation. Satellite communication standards also define error correction codes and framing formats.
Popular modulation types being used for satellite communications:
References:
There is a wide range of satellite modems from cheap devices for home Internet access to expensive multifunctional equipment for enterprise use.
A "modem" stands for "modulator-demodulator". A satellite modem's main function is to transform an input bitstream to a radio signal and vice versa. There are some devices that include only a demodulator (and no modulator, thus only allowing data to be downloaded by satellite) that are also referred to as "satellite modems." These devices are used in satellite Internet access (in this case uploaded data is transferred through a conventional PSTN modem or an ADSL modem).
The main functions of a satellite modem are modulation and demodulation. Satellite communication standards also define error correction codes and framing formats.
Popular modulation types being used for satellite communications:
- Binary phase shift keying (BPSK);
- Quadrature phase shift keying (QPSK);
- Orthogonal quadrature phase shift keying (OQPSK);
- 8PSK;
- Quadrature amplitude modulation (QAM), especially 16QAM.
References:
- Nice K. Harris T. How Satellite TV Works [online] Available at: http://electronics.howstuffworks.com/satellite-tv6.htm
- Satelite dish [online] Available at: http://en.wikipedia.org/wiki/Satellite_dish