A / B Switch
A switch in a satellite receive earth station, which has two inputs and a single output. Each switch input is connected to the output of an LNB. The single switch output is usually connected to a satellite receiver (set-top box). The switch permits selection of one of the two LNB inputs (A or B) for routing it to the common output (to a receiver for example), whilst providing adequate isolation between the signals produced by each LNB.
An electronic device used to increase the strength (power) of a signal fed into it.
A device for transmitting or receiving radio waves. Also known as an aerial. In satellite communication systems the antenna usually consists of a parabolic reflector and a feedhorn. In a receiving system the reflector focuses radio waves onto the feedhorn for detection and conversion into electrical signals. In transmitting systems the reflector concentrates the radio waves emitted by the feedhorn into a narrow beam aimed towards the satellite.
The total reflective area of a parabolic antenna (dish) over which radio waves are captured or radiated. The effective aperture is smaller than the physical aperture and is related to it by the Antenna Efficiency.
The ratio of the signal strength transmitted towards or received from a particular direction in space by a real antenna to the signal strength that would be obtained with a theoretically perfect antenna of the same physical size. This ratio is usually expressed as a percentage.
The radiation of electromagnetic energy from the feedhorn to the surface of the parabolic reflector of a transmit antenna, or the focusing of electromagnetic energy captured by the reflector of a receiving antenna towards the feedhorn. With perfect illumination no signal energy is lost to the surrounding terrain. In practice there is always some loss.
Aspect Ratio (Television)
The ratio of the width of a television picture or television screen to its height. The ratio for conventional television systems is 4:3. In advanced television systems (e.g. widescreen) the ratio is usually 14:9 or 16:9, which better approximate the aspect ratio used in cinema.
A passive device that weakens a signal that passes through it. The amount by which the signal power is reduced is usually expressed in decibels.