[amsat-bb] [Video] Preamp test in front of radio / satellite ground station build update

[Zach Metzinger]

On 2020-01-28 14:56, John Brier via AMSAT-BB wrote:
> Here is a little update on my satellite ground station build project
> and a demonstration of a test I did with my Advanced Receiver Research
> 2 meter preamplifier.

Hello John,

As others have said, FM is not ideal for this testing.

Ideally, you'd want to run SSB and feed that into your rig. Then, take 
the audio output and put it into Spectrum Lab or similar.

For a weak CW signal (less than 1 S unit), you should see a peak at the 
signal just above the noise. This ensures that AGC is inactive, if it 
can't be disabled.

Now, switch in the LNA. The CW tone should jump by the gain of the 
preamp. Ideally, the noise floor will not change at all. If it does, 
then the preamp is not a low-noise type or something else (overload) is 
going on.

Even with low-loss coax, putting the LNA at the antenna is the right way 
to do it. Use the following calculator to experiment with this lineup:

https://www.pasternack.com/t-calculator-noise-figure.aspx

(Noise-dB / Gain-dB)

Case 1: LNA (1/20) -> Coax (2.5/-2.5) -> Rig (6/90) == 1.2dB
Case 2: Coax (2.5/-2.5) -> LNA (1/20) -> Rig (6/90) == 3.6dB

You're giving up 2.4dB of NFdB for the convenience of putting it 
indoors. The numbers above assume LMR-400 at 100' loss.

https://www.rtl-sdr.com/tutorial-on-properly-positioning-a-preamp-lna-in-a-radio-system/

Local noise sources, unless they are incredible broadband, won't impact 
the usefulness of a quality LNA. The thing to look for here is 
linearity, which is indicated by the 3rd order intercept (IP3) of the 
LNA. Most modern designs have eye-popping linearity and noise figures as 
compared to the lowly, ancient U310-based amplifier.

https://www.maximintegrated.com/en/design/technical-documents/tutorials/5/5429.html


--- Zach
N0ZGO