Importance of placement of LNA (Low Noise Amplifier) close to dish and not close to receiver

From: ‘Alex P’ via Society of Amateur Radio Astronomers <sara-list@googlegroups.com>
Sent: 04 January 2025 10:28
To: Society of Amateur Radio Astronomers <sara-list@googlegroups.com>
Subject: [SARA] Re: Adapter attenuation

~ 0.15 dB / adapter 

Please ‘Review’ the Friis Equation

https://www.microwaves101.com/encyclopedias/noise-figure

Linear passive devices have noise figure equal to their loss. 

Expressed in dB, the NF is equal to -S21(dB). 

Something with one dB loss has one dB noise figure.

The relative noise contribution of the five stages shows that after the LNA, not much else is added to the noise factor. After the first two stages (F=1.122 and F=1.116), the noise factor at this point is 93% of the entire chain. Whenever your chain has a lot of gain up front, the denominator of subsequent stages in the Friis equation tends to wipe out the noise factor of the numerator.

If you have 20 dB gain in your LNA or receiver, the noise figure contribution of the subsequent stage will be small (unless the noise figure of the next stage is horrendous!) In our example above, the preselector filter/LNA had 19.5 dB gain and 3.5 dB noise figure. The combined noise figure of the filter, mixer and IF amp which follow is 12 dB, yet it only degrades the noise figure of the entire lineup by 0.3 dB!

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BTW : using LMR400 Cable w/ SMA Connectors is not ideal ..  without good strain relief, you will eventually damage

( mechanically/electrically destroy ) the small SMA  fittings.

That is why LMR240 or RG6 was recommended