Measuring system G/T ratio using Sun noise

Rex Moncur (VK7MO) used measurements of Sun noise using NFM and an SSB receiver to determine system Gain Temperature ratio (G/T)

G/T ratio

This analysis focuses on a single figure of merit for the entire receive system, and that is the G/T ratio (the ratio of antenna gain to system equivalent noise temperature. G, Antenna gain is with reference to the antenna connector. T, system noise temperature, is the total system noise, including sky temperature (or ambient noise) referred to the antenna connector.

The G/T ratio is a meaningful indicator of system performance, unlike quoting receiver noise temperature in isolation of sky temperature and antenna gain. The ability to receive weak signals is directly related to G/T, the higher G/T, the better. Signal/Noise ratio is proportional to G/T (Signal/Noise=S*λ2/(4*π)*G/T/(kb*B) where S is power flux density, kb is Boltzmann's constant, and B is receiver effective noise bandwidth).

So, if changing the receiver Noise Figure from 5dB to 1db improves the G/T ratio by just 0.1dB/K, the real benefit of the change is just 0.1dB.

Expected G/T

Fig 1:

Fig 1 is a model of the expected G/T of the receive system.


Measurements were made on 23/06/2007 using NFM of receiver noise output with the antenna pointing at cold sky and then directly at the Sun. The ratio of the 'hot' to 'cold' measurement was 6.28dB. It is important that the noise measurements are made to high resolution, and NFM is a tool for making high resolution noise measurements in audio bandwidth.

On the day, the quiet Sun radio flux observed at Learmonth observatory was interpolated to 2304MHz, and the value was 61SFU.

The expression above can be used to calculate the G/T ratio indicated by a noise rise of 6.28dB due to 61SFU Sun noise.

where Y is the measured noise increase in dB and SolarFlux is in SFU

In this case, the indicated G/T is 10.4dB/K which reconciles well with the G/T model, giving some confidence in the most uncertain element, the estimate of spillover noise.

Note that for antennas where the beamwidth is not large compared to the angle subtended by the radio Sun, less Sun noise is captured and the SolarFlux should be divided by a beamwidth correction factor (BWCF=1+0.38*(Ws/Wa)^2 where Ws is the angle subtended by the radio Sun and Wa is the antenna half power beamwidth).



Version Date Description
1.01 20/10/2007 Initial.

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.

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