Question: I've looked through some Semtech reference board schematics and found one strange thing: they put SAW filters both before and after the LNA. For example here is part from a Semtech reference board electrical schematic ([1](https://www.semtech.com/products/wireless-rf/lora-core/sx1302cfdxxxgw1) and [2](https://www.semtech.com/products/wireless-rf/lora-core/sx1302cfd915gw1h).)

[![enter image description here](https://i.stack.imgur.com/RIoz8.png)](https://i.stack.imgur.com/RIoz8.png)

The same thing from the [SX1301](https://www.semtech.com/products/wireless-rf/lora-core/sx1301) datasheet:

[![enter image description here](https://i.stack.imgur.com/sVnXy.png)](https://i.stack.imgur.com/sVnXy.png)

Here is photo of RAK831 which uses the same scheme:

[![enter image description here](https://i.stack.imgur.com/uSe8o.jpg)](https://i.stack.imgur.com/uSe8o.jpg)

Its SNAILTEK copy:
[![enter image description here](https://i.stack.imgur.com/y7vDj.png)](https://i.stack.imgur.com/y7vDj.png)

The same thing in an IC880A gateway:

[![enter image description here](https://i.stack.imgur.com/ipiYW.jpg)](https://i.stack.imgur.com/ipiYW.jpg)

They also have filters before the LNA, which reduces SNR. What can be a reason for that?

Answer: 
You are correct in that a SAW filter (any filter for that matter, or any device in the more general case) before the first LNA will have an adverse affect on the noise figure of your system. So in that regard you want the filter after the LNA.

But with the filter after the LNA, there's the possibility that the LNA will go into compression if a large out of band (band meaning your desired frequency range of interest) signal comes in that overloads the LNA. If this is the situation, then you want a filter before the LNA to attenuate the interferer.
The performance of the SAW filters just out of band is good, with -60 dB notches ([datasheet](https://www.mouser.co.uk/datasheet/2/842/tdkc_s_a0001072893_1-2295533.pdf)) but the performance falls off well out of band (-35 dB @ ~400 MHz, -28 dB @ ~2 GHz). A second SAW filter improves the wide-band filtering. Putting it after the LNA keeps the noise figure low.

[![Graph of SAW filter amplitude response](https://i.stack.imgur.com/EkNyG.png)](https://i.stack.imgur.com/EkNyG.png)
Plot from datasheet, page 7