Antenna gain
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In antenna design, gain is the logarithm of the ratio of the intensity of an antenna's radiation pattern in the direction of strongest radiation to that of a reference antenna. If the reference antenna is an isotropic antenna, the gain is often expressed in units of dBi (decibels over isotropic). For example, a dipole antenna has a gain of 2.14 dBi [1]. Sometimes, the dipole antenna is used as the reference (since a perfect isotropic reference is impossible to produce), in which case the gain of the antenna in question is measured in dBd (decibels over dipole).
The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has a positive gain in some directions, it must have a negative gain in other directions as energy is conserved by the antenna. The gain that can be achieved by an Antenna is therefore trade-off between the range of directions that must be covered by an Antenna and the gain of the antenna. For example, a dish antenna on a spacecraft has a very large gain, but only over a very small range of directions - it must be accurately pointed at earth - but a radio transmitter has a very small gain as it is required to radiate in all directions.
For dish-type antennas, gain is proportional to the aperture (reflective area) and surface accuracy of the dish, as well as the frequency being transmitted/received. In general, a larger aperture provides a higher gain. Also, the higher the frequency, the higher the gain, but surface inaccuracies lead to a larger degradation of gain at higher frequencies.
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.
