Gain-to-Noise Temperature Ratio

Gain-to-noise temperature ratio, usually written G/T, is a receiver antenna-system figure of merit. It compares receive antenna gain with the system noise temperature that the receiver sees at the same reference plane. A high G/T means the station collects wanted signal energy efficiently while adding relatively little thermal noise.

The metric is common in satellite, telemetry, microwave, radar and weak-signal RF budgets because it lets engineers compare receiving systems independently from a particular transmitter or path. It is not only an antenna number and not only a receiver noise number; it is the combination that matters at the receive boundary.

Basic Definition

In linear form:

where G is receive antenna gain as a linear ratio and T_sys is system noise temperature in kelvin. In decibel form:

The dBi reference is important. If gain is reduced by pointing loss, polarization mismatch, radome loss or feed loss before the reference plane, the reduced gain must be used.

Link-Budget Use

For a one-way RF link, G/T enters the carrier-to-noise density expression:

with:

This form keeps transmit capability, propagation loss and receiver quality separate. It is especially useful when several receive stations, antenna sizes, pointing modes or weather states must be compared against the same spacecraft, microwave hop or telemetry transmitter.

Worked Example

A receive station has clear-sky antenna gain:

The system noise temperature at the antenna reference plane is:

Then:

For an incoming link with:

and total path loss:

the available carrier-to-noise density is:

If the bit rate is:

then:

Against a required Eb/N0 of 10.5 dB plus 1.5 dB implementation allowance, the margin is:

Degraded Conditions

Rain, wet radomes, feed losses and pointing errors can degrade G/T by changing gain, noise temperature or both. If pointing loss is:

and rain plus background noise raise system temperature to:

then:

The receiver figure of merit has dropped by:

This degradation passes directly into C/N0, so the link margin falls by the same amount unless adaptive coding, lower bit rate, higher EIRP or a larger antenna compensates.

Boundary With Related Metrics

G/T is downstream of antenna gain and system noise temperature. It does not describe transmit power, spectral mask compliance, receiver linearity, adjacent-channel rejection or modem implementation loss. Those must remain visible elsewhere in the link budget.

System noise temperature explains the temperature denominator. Carrier-to-noise density uses G/T to predict C/N0. Receiver sensitivity translates required SNR, bandwidth and implementation loss into minimum signal level. Link margin compares the available result with a requirement. Keeping these terms separate prevents a clean-looking budget from hiding a bad antenna pointing assumption or an optimistic clear-sky temperature.

Validation Evidence

A defensible G/T statement includes frequency, polarization, antenna gain pattern, pointing state, feeder and radome losses, reference plane, measured or specified T_sys, sky condition, weather state, calibration method, uncertainty and whether the value is peak, scan-average or operational minimum.

Operational trending should preserve the same reference plane and measurement bandwidth. Otherwise a reported G/T improvement may only reflect a changed calibration setup, a different receiver configuration or a quieter sky condition during the test.

Common mistakes include using transmit antenna gain instead of receive gain, mixing dBi and dBd, applying feeder loss to noise temperature but not gain, using clear-sky G/T during rain fade, reporting a vendor value without the reference plane and comparing terminals at different frequencies without accounting for antenna aperture, beamwidth and background temperature.