G/T is defined as the ratio of antenna gain to total equivalent noise temperature.
For clarity, lets define those terms.
Gain
Gain of an antenna is defined (IEEE 1983) as the ratio of the radiation intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. (Isotropically simply means equally in all directions.)
Gain is often expressed in deciBels as 10*log(gain), and units are dB (or dBi to distinguish it from other references).
If you are a devotee of gain with reference to a half wave dipole in free space (dBd) for whatever reason, add 2.14 to dBd to get gain in dB (dBi). (Thinking in dBd is a handicap, don’t forget to adjust any dBb figures to gain as defined above.)
Total equivalent noise temperature
In addition to external noise, each component of a receiver generates internal noise.
It is convenient to think of a practical amplifier as being noiseless with an input noise source that produces output noise equivalent to that of the practical amplifier.
One of the many methods of expressing that noise source is as a noiseless amplifier with matched input source (Zs=Zin*) where Rs is raised to some temperature to create the same output noise power as the practical amplifier. This temperature is absolute temperature in Kelvins and referred to as the equivalent noise temperature, or Te.
Noise in cascaded amplifiers (and attenuators) can be referred to the input of the previous stage, and hence an equivalent noise temperature can be found for cascaded stages right up to entire systems.
The “total equivalent noise temperature” used for G/T includes not just internal noise, but also the external noise.
It is convenient in some cases to divide external noise into two components, noise received by the antenna ‘on axis’ (skynoise), and noise from sidelobes (spillover or sidelobe noise). In some situations (eg antenna pointed skywards) the background noise field strength might be quite low and antenna gain high, and sidelobe noise may be quite high field strength and low antenna gain, so for example the combined noise might be calculated as 100% of the on-axis noise and 20% of the off-axis noise (that portion calculated from antenna pattern).
References
- Duffy, O. 2006. Effective use of a Low Noise Amplifier on VHF/UHF. VK1OD.net (offline).
- ———. 2006. Receiver sensitivity metric converter. VK1OD.net (offline).
- ———. 2007. Measuring system G/T ratio using Sun noise. VK1OD.net (offline).
- IEEE 1983. IEEE Standard Definitions of Terms for Antennas (IEEE Std 145-1983).
- 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 3
More installments to come…