LNBs are described either in terms of noise temperature (degrees K) or noise figure (dB), two interchangeable yardsticks. LNB noise figure is a measure of the degree by which an LNB degrades the signal-to- noise ratio of the satellite signal as it passes through the device. Since an LNB is the "front end" of a satellite reception system the noise it adds to the incoming signal sets the noise floor and plays a large part in determining picture quality. LNBs operating in the Ku-band range are now available at reasonable prices with noise figures in the 1.8 to 0.6 dB range, equal to temperatures ranging from 149 to 43°K. C-band LNBs are available in noise temperatures as low as 20°K.e improving noise performance of LNBs has brought benefits to many in difficult reception areas. For example, in regions where temperatures regularly soar to above 100°F during the day, LNBs perform well enough to overcome associated performance degradation. In weak footprint regions of the world, ultra-low noise LNBs combined with large dishes can greatly enhance reception of signals.

Link Budgets

Link budgets add up all the gains and subtract all the losses in a satellite system to predict the value of the receive signal for a given transmit level at the distant end. This takes into account the transmit power, the gain of the transmit antenna, the space losses, the receive gain and noise of the antenna on the satellite, and then the same again from the satellite back to the receiving ground station.

This is only necessary when calculating what size of antenna and transmitter is required for a given satellite link and receiving antenna and LNB.

There are many link budget calculators and spreadsheets available on the internet and it is quite easy to fill in the details and get a result. The difficult part however, is getting the right information to put into the spreadsheet.

Noise Temperature

The small amount of noise produced by a C-Band LNA or LNB is called Noise Temperature even though for our practical purposes this has little to do with temperature. C- Band uses Noise temperature but Ku and Ka-band LNBs are usually rated by Noise Figure in dBs, rather than Noise temperature.

All matter above -273°C or Absolute Zero Kelvin radiates some level of noise. The noise added by the LNA or LNB, that reduces the quality of the signal and decreases the signal to noise ratio is expressed in Kelvin. C-band LNAs and LNBs typically have a noise temperatures of about 15K to 30K (Kelvin)or a Noise Factor of about 0.2dB to 0.4dB.

Why this is called a "temperature" can be confusing. The amount of noise is expressed as the equivalent of the noise that a resistor would generate at an ambient temperature of 290 Kelvin (17°C). A theoretical resistor at 0 Kelvin would generate no noise.

Ku Band LNB noise is normally expressed in dBs not Kelvin, and are considerably higher. A typical Ku-Band LNB has a noise figure of 0.8dB (or 59K).

Noise temperature = Boltzman Constant x Absolute temperature of 290 Kelvin x bandwidth
N=kTB= 1.38x10^-23 x 290(K) x Bandwidth (Hz)