I figured out that my RTL-SDR would receive ADS-B (and a lot of other stuff) this evening, so I figured I'd take a snapshot of what the spectrum looks like here. Keep in mind, this is the RTL-SDR dongle, and not my FlightAware Pro Stick Plus with the on board filter that it supposedly has. Does it look like I could benefit from some additional filtering? Or could these spurs simply be artifacts of appliances and electronic equipment in my home? I was using a 1090 "rubber duck" antenna that came from Amazon for this test, and the RTL-SDR was nearby my PC and lots of other electronic equipment. https://i.imgur.com/22mc2oY.jpg
A normal rtl-sdr lacks sensitivity for best ADS-B performance. And if you use an LNA, a filter is required as the rtl-sdr can't handle strong out of band signals. Noise from LTE or GSM can be quite a problem, despite being at 700 to 900 MHz.
A lot of those spikes are probably ghosts from strong signals outside of where you are looking. Since a SDR doesn't have the circuitry a normal radio has for tuning you get all kinds of "echos" from strong stations all over the spectrum, I've seen pager signals pop up far away from where they were supposed to be, those seem to be the strongest signals I see. I have one of the Flightaware 1090 filters and it does help a lot so you can max out the gain on the SDR, but a regular SDR still doesn't pull in as many aircraft as the Flightaware pro plus. Without the filter how many aircraft you get depends mostly on how much radio noise you have in your area. The pro plus does have a good built in filter, plug a long wire into it and try to use your SDR software get, municipal, pager, or FM signals and you won't see much if anything. FYI: If you want to see strong signals with tons of ghosts plug a Ham It Up into the SDR to try and listen to shortwave. Amazon US doesn't seem to sell the 1090 filter anymore, only the dual one, but there are other 1090 only filters for less than $15.
https://www.amazon.com/Stratux-1090-MHz-Splitter-Filter/dp/B07NDNBW6G/ Works good, I had one bad one. AMP post filter. https://www.amazon.com/ADS-B-Dual-1090-Band-Pass-Filter/dp/B010GBQXK8 CHINA CHEAPS: https://www.banggood.com/search/1090-lna.html?from=nav
