# Do I ‘need’ a masthead preamp to work satellites on 2m? – G/T vs G/Ta

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.

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.

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 .