Designing high performance VHF/UHF receive systems – Part 1

G/T

A metric that may be used to express the performance of an entire receive system is the ratio of antenna gain to total equivalent noise temperature, usually expressed in deciBels as dB/K. G/T is widely used in design and specification of satellite communications systems.

G/T=AntennaGain/TotalNoiseTemperature 1/K

Example: if AntennaGain=50 and TotalNoiseTemperature=120K, then G/T=50/120=0.416 1/K or -3.8 dB/K.

The utility of G/T is that receive S/N changes dB for dB with G/T, in fact you can calculate S/N knowing G/T, wavelength, bandwidth and the field strength of the signal (Duffy 2007).

S/N=S*λ2/(4*π)*G/T/(kb*B) where:
S is power flux density;
λ is wavelength;
kb is Boltzmann’s constant; and
B is receiver effective noise bandwidth

Calculating G/T for configuration alternatives gives a single performance metric that allows comparison of the performance of the configurations. Different levels of performance might be weighed against cost in a cost benefit analysis, eg whether funds applied to antenna improvements will be more cost-effective than applying funds to feed line improvement, whether an LNA with NF=0.8dB and Gain=30dB is more cost-effective than one with NF=0.5dB and Gain=18dB.

Such a quantitative approach cuts through the old wives tales about the relative merit of some approaches, and will quantify the relative performance in the system context. Some options will have different effects depending on other system details, and Rules of Thumb (ROT) that ignore this sensitivity to the rest of the system becomes simply rot.

Inclusion of gain in the metric allows quantitative comparison of configuration changes that include a change in antenna gain.

A key limitation of simple system models is that they assume the system is linear, in particular that there is no significant IMD noise created by system components. Systems with significant IMD are basically unpredictable, and whilst the performance of a particular system can be measured, that does not provide knowledge that is extensible to another system because the environment, and response to the environment are probably different. The bottom line is that components with significant IMD noise don’t belong in a high performance system, fix them or get rid of them.

Note that the popular ham antenna tables published by VE7BQH which show a calculated quantity labelled G/T value for antenna systems appears to ignore receiver equivalent noise (ie it is for a noiseless receiver), and as such, the usage is not consistent with the industry meaning of the term (eg ITU-R. 2000).  Don’t confuse this discussion of G/T with those tables.

G/T is calculated with respect to some nominated reference plane. The most common practice is to use the antenna waveguide flange or connector as the reference plane, and to consistently calculate system component contributions relative to that reference plane. In the case of an array of four Yagi antennas, the reference plane would be the connector on the power divider/combiner that provides single connector access to the antenna system.

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).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 2