Antenna Noise Temperature Calculator computes total system noise temperature, noise power density, and G/T figure of merit for a satellite ground station or RF receiving system.
Antenna Noise Temperature Calculator for System Noise and G/T Sensitivity
This tool combines antenna noise temperature, feedline loss, and receiver noise figure into total system noise temperature, integrated noise power, and the G/T figure of merit. Satellite ground station and RF receiver engineers use it to predict receiver sensitivity and feed those numbers into a link budget.
How to Use
Enter antenna noise temperature, antenna gain, feedline loss, feedline physical temperature, receiver noise figure, and channel bandwidth. The calculator returns system noise temperature at two reference points, noise power density, integrated noise power, and G/T.
Formula
The cascaded noise-temperature approach and G/T definition follow the system noise concepts documented in the NASA/JPL Deep Space Network (DSN) Telecommunications Link Design Handbook. Noise figure converts to noise temperature against the standard reference temperature $T_0=290$K:
$$F = 10^{NF/10}, \quad T_{rx} = T_0(F-1)$$
The feedline’s own physical temperature $T_p$ contributes noise based on its loss, referenced at the feedline’s output:
$$L_{lin} = 10^{L/10}, \quad T_{line} = T_p\left(1-\frac{1}{L_{lin}}\right)$$
Antenna noise is attenuated by the same feedline before it reaches the receiver:
$$T_{a,rx} = \frac{T_a}{L_{lin}}$$
System noise temperature at the receiver input, and restated back at the antenna terminal:
$$T_{sys} = T_{a,rx} + T_{line} + T_{rx}, \quad T_{sys,ant} = T_{sys} \times L_{lin}$$
Noise density and total noise power over the channel bandwidth $B$, using Boltzmann’s constant $k=1.380649\times10^{-23}$ J/K:
$$N_0 = kT_{sys}, \quad P_n = N_0 + 10\log_{10}(B)$$
Figure of merit compares effective receive gain against system noise temperature, both at the same reference point:
$$G_{rx} = G_{ant} – L, \quad G/T = G_{rx} – 10\log_{10}(T_{sys})$$
The most common input mistake is mixing reference points — pairing antenna-terminal gain with receiver-referenced noise temperature (or the reverse) without adjusting for feedline loss. G and T must be measured at the same physical point in the chain, or the resulting G/T is wrong even though every individual input was correct.
G/T is invariant to which reference point you use, as long as you’re consistent, because moving from the antenna terminal to the receiver input scales both the gain and the noise temperature by the same feedline loss factor — the ratio in dB doesn’t change.
Computing $G_{ant} – 10\log_{10}(T_{sys,ant})$ instead of $G_{rx} – 10\log_{10}(T_{sys})$ gives the identical 3.58 dB/K result here. That agreement is also a built-in sanity check: if the antenna-referenced and receiver-referenced G/T don’t match on a hand calculation, a reference point was mixed up somewhere in the chain.
FAQ
Why does the calculator show two system noise temperatures?
$T_{sys}$ is referenced at the receiver input; $T_{sys,ant}$ is the same total noise restated at the antenna terminal. They differ by the feedline loss factor and both are valid, provided gain is referenced at the matching point.
Why use noise temperature instead of just noise figure?
Noise figure assumes a 290K source and doesn’t combine linearly across cascaded stages. Noise temperature adds directly stage by stage, which is why it’s preferred for satellite and other high-sensitivity receive systems.
Does feedline physical temperature really matter?
Yes. A lossy feedline radiates noise proportional to its own physical temperature. A cable baking in the sun contributes more noise than the same cable at a cooler ambient temperature, even with identical loss in dB.
Why is thermal floor listed separately from noise density?
Thermal floor ($kT_0$ at 290K) is a fixed physical constant used for comparison. Noise density ($kT_{sys}$) is this specific system’s actual noise floor, which is normally higher than $kT_0$ once receiver and feedline noise are added.
Does a higher G/T always mean a better system?
For receive sensitivity, yes — higher G/T means more gain per unit of system noise. It says nothing about transmit power, bandwidth, or modulation, which also determine overall link performance.