I have the following setup
FA antenna above roofline
Uputronics LNA with SAW filter
50 ft RG11
Airspy Mini, directly plugged into x86 PC in the basement.
This has worked reasonably well… I’m usually number two or three for positions in my local area, and not far behind those who score better.
Over the last little while I’ve felt that my range and positions has been decreasing, so I set out to obtain a spectrum plot. This ended up being much more complicated than you might expect, as I need bias tee for the LNA, and so I not only needed software that could capture spectrum data on the Airspy, but it also had to support enabling bias tee. It turns out there isn’t any, so in the end I added an external power inserter to the chain and used QSpectrumAnalyzer. The plots you see here are from that setup.
Here’s an 800 - 1200 MHz plot:
There is obviously a strong cellular signal making it through the masthead filter, but there are also those very strong peaks repeating every ~20MHz.
Out of interest I ran the same plot with the antenna disconnected from the Airspy, with the antenna connection shorted. (With bias tee disabled of course! 
):
You can see that even in this situation those repeating peaks are still there, suggesting to me that they are very local to the Airspy and the PC. Not sure what I can do about that. You can also see that even in this setup there is a significant amount of cellular traffic being picked up, which to me seems unexpected.
With the antenna system reconnected, and zooming in to 1090MHz:
I was kind of expecting to see more SNR at least for local aircraft.
Anyway, I’m at the limits of my knowledge and experience here. Looking at the first plot above, I’m wondering if maybe the strong cellular signal is overloading the LNA, and maybe I should look at a cavity filter? That’s quite an expensive thing too take a punt on, so I’d appreciate any advice or opinions
