Re: Some calculations about GAIN, LNA, ATTENUATORS, COAX...


On Fri, Nov 17, 2017 at 07:16 am, Ruben Navarro Huedo (EA5BZ) wrote:
Hello friends:
My knowledge of RF concepts are medium to low.
I am learning a lot reading this group.
Here are some thinkings about my setup at this moment:

Using an Airspy R2.
Main use are frequencies from 130 to 150 Mhz (Mainly 137 SAT Frequencies).
I am using:
Antenna --> Around 6 meters CO22 Coax Cable--> LNA with HPF and BPF included for 137.5Mhz ( ) --> 30 meters Ecoflex 10 Coax cable --> Airspy R2

Ecoflex 10 has an attenuation of around 5db in 100m (144mhz). Then i will have 1.5db of attenuation in 30 meters.
Lets estimate a loss in connectors and converters of around 1 db more ¿? ----> Then i have around 2.5db of total attenuation.

With this calculation i think my LNA has too much gain. (M
anufacturer talks about 23db gain).
Is this correct?
I will have best results with a 15db attenuator after the LNA.

I am having the best results using GAIN 10 and Linearity.
What do you think about all this?

Thank's a lot.
It depends on your antenna and your RF environment. Note that this particular LNA has a HPF at the input and a BPF at the output but it could still overload if the antenna has too much gain and the FM BC signals too strong.
Try comparing the SNR of a stable signal with and without the LNA. If it helps, then you can keep it.

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