Why use noise temperature instead of noise figure?

Noise figure compresses the low end of the scale: a cryogenic LNA with 5K noise temperature has NF = 0.07 dB, and a 10K LNA has NF = 0.14 dB. The difference of 0.07 dB is barely measurable, yet the 5K LNA is twice as good. Noise temperature preserves this distinction: 5K vs 10K is clearly a factor of two. For satellite ground stations and radio telescopes where every Kelvin matters, noise temperature is the standard metric. Noise figure is more convenient for commercial systems where NF values are in the 1 to 10 dB range and the compressed scale is not a problem.

What contributes to antenna noise temperature?

The antenna collects noise from everything in its field of view. Sky noise: at zenith, the atmosphere contributes about 3K at 1 GHz, rising to 10K at 10 GHz, 50K at 22 GHz (water vapor absorption line), and 300K at 60 GHz (oxygen absorption). Cosmic microwave background adds 2.73K in all directions. Ground noise: any part of the antenna pattern that intercepts the ground (sidelobes, spillover) sees 290K. A well-designed satellite dish with 0.5 dB spillover might have antenna temperature of 30 to 50K pointed at the sky, versus 150 to 200K for a terrestrial base station antenna that sees buildings and terrain in its sidelobes.

What is G/T and why is it the key satellite metric?

G/T (gain-to-noise-temperature ratio, in dB/K) is the figure of merit for a receiving earth station. It combines antenna gain (which captures more signal) with system noise temperature (which determines the noise floor). A station with 45 dBi antenna gain and 80K system temperature has G/T = 45 minus 10 log10(80) = 45 minus 19 = 26 dB/K. Improving either the gain (larger dish) or the noise temperature (better LNA, less spillover) improves G/T. Satellite link budgets specify the required G/T at the ground station, and all other parameters (EIRP, path loss, rain margin) flow from it.

System Design

Noise Temperature

T_sys (Kelvin)

A deep-space antenna at NASA's Goldstone complex uses a cryogenically cooled LNA at 15 Kelvin. The 70-meter dish, pointed at the zenith, sees an antenna temperature of about 20K from the sky and cosmic background. Total system temperature: approximately 35K. This translates to a noise figure of 0.5 dB, but saying "0.5 dB NF" hides the engineering. Noise temperature preserves the physics: every Kelvin of system temperature directly maps to noise power via N = kTB. If the LNA warms from 15K to 30K, system temperature rises from 35K to 50K, a 43% increase in noise power and a 1.5 dB sensitivity loss. No NF specification captures this granularity at the low end of the scale.

Category: System Design

Conversion: T = 290(10^NF/10 − 1) K

Key Metric: G/T (dB/K)

NF vs. Noise Temperature: Seeing the Difference

NF to noise temperature:
T_e = T₀(10^NF/10 − 1) where T₀ = 290K

System noise temperature:
T_sys = T_ant + T_LNA + T₂/G_LNA + T₃/(G_LNA·G₂) + ...

G/T figure of merit:
G/T (dB/K) = G_ant(dBi) − 10·log(T_sys)

Noise Temperature Scale

NF (dB)	T_e (K)	Typical Device	Application
0.07	5	Cryogenic InP HEMT (20K)	Radio astronomy
0.3	20	Cryogenic GaAs LNA (77K)	Deep-space comms
0.5	35	Room-temp GaAs pHEMT	Satellite ground station
1.0	75	GaAs MMIC LNA	VSAT, GPS
2.0	170	SiGe LNA	Cellular base station
5.0	627	Consumer WiFi front-end	WLAN access point
10.0	2,610	Passive mixer (no LNA)	Simple receiver

Common Questions

Frequently Asked Questions

Why not just use NF?

NF compresses at the low end: 5K = 0.07 dB, 10K = 0.14 dB. The 0.07 dB difference is barely measurable, but the 5K LNA is twice as good. Noise temperature preserves the distinction. Used for satellite, radio astronomy, and deep-space links where every Kelvin matters.

What heats the antenna?

Everything in view: sky (~3K at 1 GHz, 50K at 22 GHz water line, 300K at 60 GHz O₂), cosmic background (2.73K), ground (290K via sidelobes). A satellite dish at zenith: 30 to 50K. A terrestrial BS antenna: 150 to 200K from buildings/terrain.

What is G/T?

Figure of merit for receive stations: G(dBi) − 10·log(T_sys). A 45 dBi dish with T_sys = 80K: G/T = 45 − 19 = 26 dB/K. Improving either gain or noise temperature raises G/T. Satellite link budgets specify required G/T.

Related Terms

← Noise Floor Notch Filter →

System Analysis

System Temperature Calculator

Enter antenna temperature, LNA noise temperature, and gain. Cascade through the full receiver chain to compute T_sys and G/T for satellite link budget analysis.

Calculate T_sys