# Designing high performance VHF/UHF receive systems – Part 6

## Measurement of G/T

G/T can be measured using celestial noise sources provided the antenna can be pointed to them. The noise source that is most appropriate will depend on expected G/T, frequency, time etc.

### Sun

(Duffy 2007) described a method of measuring G/T using the Sun as a noise source, and an expression for calculating G/T for wide antennas.

G/T=10*LOG((10^(Y/10)-1)*2/(SolarFlux*1E-22)/Wavelength^2*4*π*1.3806504E-23)dB/K where:

Y is the measured noise increase in dB; and

SolarFlux is in SFU

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).

## Other celestial noise sources

(ITU-R 2000) details a method of measurement of satellite earth station receive systems, and lists reference noise levels from a number of radio stars

One might have thought that amateur radio operators would share observation of noise rise on celestial objects, both for forming a shared view of the noise flux level and establishing some experience on state of the art G/T achievable with some common configurations on each band.

There is a dearth of information of this type published on the Internet by radio hams, the few boasts of Sun noise rise do not give the solar flux level (or a date to allow determination of solar flux). Some calculators historically gave solar flux based on flawed extrapolation from 10cm flux, some users used the 10cm flux for all bands. Basically, any information published is likely to be amateurish.

## References

• Duffy, O. 2006. Effective use of a Low Noise Amplifier on VHF/UHF. VK1OD.net (offline).
• ———. 2007. Measuring system G/T ratio using Sun noise. VK1OD.net (offline).
• ———. 2007b. Quiet sun radio flux interpolations. VK1OD.net (offline).
• ———. 2007c. noise figure meter. VK1OD.net (offline).
• ITU-R. 2000. Recommendation ITU-R S.733-2 (2000) Determination of the G/T ratio for earth stations operating in the fixed-satellite service .

Designing high performance VHF/UHF receive systems – Part 7