Antenna gain
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Antenna gain is the measurement of an antennas ability to amplify the incoming microwave signals in a particular direction, compared with the sensitivity of an isotropic antenna in any direction, or a dipole antenna in the equatorial direction. The two measurements are measured in decibels and denoted by dBi and dBd respectively.
For dish-type antennas, gain is proportional to the aperture (reflective area) and surface accuracy of the antenna, as well as the frequency being transmitted/received. In general, a larger aperture antenna has a higher gain. Also, the higher the frequency, the higher the gain. Surface inaccuracies are more of a factor at higher frequencies.
Gain, 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. 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.
The ultimate figure of merit for all receiving antennas is the G/T (pronounced "G over T"); that is, the gain of the antenna (in dB) minus the noise temperature of the receiving system (in dB). A typical C-band system will have a G/T of around 20 dB/K, while most Ku-band digital direct to home systems have a G/T of 12.7 dB/K. The more powerful the satellite signal, the lower the G/T value that will be needed at the receiving system down on the ground.
The noise value (T) primarily comes from two sources. The antenna noise is a function of the amount of noise that the feedhorn sees as it looks over the antenna rim towards the hot earth (which has a noise temperature of 290 K). Antenna noise generally ranges between 30 and 50 K.
The noise contribution of the LNB's internal circuitry is the other major source of concern. C-band LNB performance now ranges as low as 20 K. If we add an antenna/feed noise of 40 K to LNB noise of 35 K = 75 K. Ten times the Logarithm of 75 K equals a (T) of 18.8 dB. A typical 1.8m diameter C-band antenna will produce a gain of 38 dB. Therefore the G/T of the system described above would be (G) 38 dB minus (T) 18.8 equals 19.2 dB/K.