A reader of Do I ‘need' a masthead preamp to work satellites on 2m? – space noise scenario has written to say he does not like my comments on the hammy adaptation of G/T.
Above is an archived extract of a spreadsheet that was very popular in the ham community, both with antenna designers and sellers and end users (buyers / constructors). It shows a column entitled G/T which is actually the hammy calculation. The meaning possibly derives from (Bertelsmeier 1987), he used G/Ta.
Ta is commonly interpreted by hams to be Temperature – antenna. It is true that antennas have an intrinsic equivalent noise temperature, it relates to their loss and physical temperature and is typically a very small number. But as Bertelsmeier uses it, it is Temperature – ambient (or external), and that is how it is used in this article.
Let's calculate the G/Ta statistic for the three scenarios in Do I ‘need' a masthead preamp to work satellites on 2m? – space noise scenario.
Base scenario
Above is a calculation of the base scenario, G/T=-29.74dB/K.
Also shown in this screenshot is G/Ta=-23.98dB/K.
With masthead LNA
Above is a calculation of the masthead amplifier scenario, G/T=-25.21dB/K.
Also shown in this screenshot is G/Ta=-23.98dB/K.
With LNA at receiver
Above is a calculation of the LNA at the receiver scenario, G/T=-25.754dB/K.
Also shown in this screenshot is G/Ta=-23.98dB/K.
Conclusions
Scenario | G/T (dB/K) | G/Ta (dB/K) |
Base | -29.74 | -23.98 |
With masthead LNA Gain=20dB NF=1dB | -25.21 | -23.98 |
With local LNA Gain=20dB NF=1dB | -25.75 | -23.98 |
Note that G/Ta is the same for all three configurations, it does not contain the important information that differentiates the performance of the three configurations.
Importantly, you cannot derive G/T from G/Ta without knowing either G or Ta (and some other important stuff), the G/Ta figure by itself cannot be ‘unwound'… so if you select an antenna ranked on a G/Ta value (even if mislabeled), the ranking of ‘real' G/T may be different depending on many factors specific to your own scenario, ie the one with the better G/Ta might have the poorer G/T.
References
- Bertelsmeier, R. 1987. Equivalent noise temperatures of 4-Yagi-arrays for 432MHz. DUBUS..
- Duffy, O. 2006. Effective use of a Low Noise Amplifier on VHF/UHF. VK1OD.net.
- ———. 2007. Measuring system G/T ratio using Sun noise. VK1OD.net.
- ———. 2009. Quiet sun radio flux interpolations. https://owenduffy.net/calc/qsrf/index.htm.
- 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 .