A recent online post seeking opinions on the chap’s 70cm weak signal configuration is an interesting subject for study, and one that should be of interest to many weak signal DXers.
This article focusses on just one question in a quite similar configuration, what is the advantage given by the LNA?
Study configuration
The scenario will be evaluated for both terrestrial and satellite paths.
Above is the assumed ambient noise environment, it has great bearing on the results. More on that later.
The system G/T statistic has been used to quantify comprehensively the performance of a receive station over many decades. It has widest application in weak signal / low noise receive systems, eg satellite paths. G/T is the ratio of system antenna gain to equivalent noise temperature, most often given in dB as dB 1/K or simply dB/K.
The article Effective use of a Low Noise Amplifier on VHF/UHF gives some insight into G/T and its application.
Let’s use my G/T spreadsheet to model the scenario.
Assumptions:
- feed line types and lengths as listed in the spreadsheet;
- ambient noise as given in table above;
- LNA option is a MVV-432-VOX;
- system is linear, ie no non-linearity, zero IMD.
Two key metrics are calculated in the spreadsheet, G/T and Signal to Noise Degradation (SND).
- G/T can be used to calculate the S/N ratio given a receive field strength; and
- SND is the degradation of the external S/N ratio by system internal noise (see Signal to noise degradation (SND) concept).
Either metric can be used to calculate the S/N improvement due to adding the LNA.
S/N is a measurable quantity, and an S-meter does not give S/N with any accuracy. Higher S-meter deflection is not synonymous with higher S/N ratio.
Results
Terrestrial
The following assumes an equivalent external noise temperature that is of the order of what might be expected to illustrate the analysis technique, but measured values should be used for a specific scenario.
Above is the baseline terrestrial model with provision for the LNA, but without it.
A perfect receive system would have SND=0dB, so at 6.67dB, this degrades ‘off-air’ S/N substantially.
Above is the terrestrial model with LNA.
A perfect receive system would have SND=0dB, so at 0.73dB, this degrades ‘off-air’ S/N by a quite small amount, and some 6dB improvement over the baseline terrestrial model with no LNA.
Satellite
The following assumes an equivalent external noise temperature that is of the order of what might be expected to illustrate the analysis technique, but measured values should be used for a specific scenario.
Above is the baseline satellite model with provision for the LNA, but without it.
A perfect receive system would have SND=0dB, so at 16.64dB, this degrades ‘off-air’ S/N hugely.
Above is the satellite model with LNA.
A perfect receive system would have SND=0dB, so at 5.14dB, this degrades ‘off-air’ S/N substantially, but 12dB improvement over the baseline satellite model with no LNA.
Summary
Caveats
Linear system
The results above depend on an assumption that the system is linear, in particular this means insignificant IMD.
Note that many ham LNA designs and products lack front end selectivity which predisposes them to worse IMD due to the higher aggregate signal levels reaching the active device.
High gain antenna systems tend to be frequency selective and provide some measure of front end selectivity. It is worth evaluating S/N ratio with a low value precision attenuator inserted before the LNA, if S/N improves there is significant IMD… and addressing that improves internal noise.
External noise
As mentioned, external noise is scenario dependent, antenna pattern dependent, antenna azimuth and elevation dependent, and should be evaluated (see link below).
Other variables
The model has many variables, and more system components could be added if needed. Any of these can be varied to answer questions like:
- what is the impact of less LNA gain;
- is optimal gain just sufficient to offset the coax loss;
- is a NF=0.8dB Gain=10dB LNA better;
- should I choose an LNA with more gain, or lower noise figure;
- what is the benefit of LDF4-50A feed line;
etc.
Whilst discussions on social media about with opinions and hand waving, it is quantitative analysis of your own scenario that gives the most valid answers.
Links / references