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Link Budgets

Link Budgets | Noise Temperature | Noise Temperature, Noise Figure and Noise Factor | dB (decibel) Power Gain and Loss | dBW and dBm |
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.

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)

The Noise Factor is the ratio between the input SNR and the output SNR or the amount of noise added to the signal by the LNB.

The Noise Figure is the noise factor, given in dB
NOISE FIGURE  =  10log10( SNR INSNR OUT )    or  =  10log10( NOISE FACTOR) in dB

The Noise Factor  related to the Noise Temperature is

So an LNB with a noise temperature of 15K  would have a NOISE FACTOR =  290+15290 = 1.05172

So quickly on my windows calculator (scientific) I can see that the LNB has a noise figure:
Noise Figure = 10 x log10(1.05172 ) = 0.219 dB 

Noise Temperature:dB
A decibel is the logarithmic ratio between two powers levels.  Being logarithmic, you can simply add and subtract gains and power levels to arrive at the result.

For instance, if you put a power level of -20 dBm into an amplifier with a gain of 60dB, the output will be -20 + 60 = 40 dBm, or about 8 watts.

In practical terms, you don't really need to worry too much about the logarithmic math, but a ratio between to power levels in decibels = 10 x Log10 (Level1/Level2).

Some significant decibel ratios to remember:
+3 dBs is twice the power (log10 2/1) and -3 dBs  is half the power (log10 1/2)
+10 dBs is about 10 times the power and -10 dBs is about 1/10th of the power.
- 30 dBs is 1/10 of a percent or almost nothing.

A level of 47 dBW is twice the level of 44 dBW.  A EIRP of 53 dBW is about ten times the EIRP of 43 dBW.

It is important to use the lowercase "d" and uppercase "B" in dB. The decibel is one tenth of a Bel, named after Alexander Graham Bell that came up with the concept as a unit of audio levels and telephone losses. The whole Bel unit is seldom used today as decibels are a more practical measure.
Decibels can also express absolute values by referencing one of the power values like a Watt or milliWatt.
The benefit of using the logarithmic decibel units is that gains and losses can simply be added and subtracted rather than  multiplied out.

dBW is a decibel referenced to one Watt.
X (dBW) = 10 log10 (X / 1W )
0 dBW = 1 Watt

dBm is a decibel referenced to a milliwatt.
0 dBm = 1 milliwatt

-25dBm = approximately 3 µW