After a couple days with an indoor DIY spider and a short run of RG8X, I've decided to upgrade to a FlightAware 5.5dB antenna outside on top of my roof and about 40' of LMR-400. I found some good condition LMR-400 that I had laying around, measured it, and it was exactly long enough to run down from the roof up through the floor of my radio room. I also had a couple N-connectors on hand that I fitted and soldered. I'm curious about what kind of performance increase/decrease I may expect? I understand I'm upgrading to a higher gain antenna that's located outdoors, but I'm also adding about 35' of coax run which I'm sure will rob some precious dB. With the DIY spider handing from my ceiling, I was able to pick up approximately 50 flights that were up to about 160 miles away that were above 15,000. Flights closer to me I was able to track down to about 3,000. Flights within about 30 miles I could track at 1,500. If I plugged in a portable USB battery pack to the Raspberry Pi and took it all outside, with the DIY spider hanging from a low tree branch (about 6') I was able to pick up over 100 flights! I'm pretty happy with my 160 mile distance range, but I'm not happy with the altitude at which I can track down to. I'd like to track flights lower in altitude, if that makes sense. I'm thinking that having the antenna up higher should help with that, but again, I'm worried about coax loss. A 40' run of LMR-400 at 1090 is going to be about 2.25 dB loss if my math is correct. The 6' length of RG8X at 1090 was about 1 dB loss if my math is correct. So I obviously am going to have more loss, but a higher gain antenna and an outdoor placement to offset that. I'm positive in thinking that these aspects should net me a fair performance increase?
It depends on which dongle you use and also can depend on interference picked up by the antenna. But yes normally this is a much better setup, the 2 dB loss are perfectly acceptable. Antenna placement is really key for ADS-B as it is line of sight mostly. A good option is the green radarbox dongle or the blue FA prostick plus, they both have an LNA and filter built in (different order in the signal path, making the radarbox better for heavy interference). A really good setup would be this triple filtered LNA: https://www.rtl-sdr.com/new-product-rtl-sdr-blog-1090-mhz-ads-b-lna/ It provides phenomenal results and it's not an issue installing it after the 2 dB loss of the LMR400, no need to install it directly at the antenna (which would be absolute optimum). Then a good dongle, either the rtl-sdr blog v3 dongle (can supply the LNA with power) or if you don't want to fiddle around with software, use this always on biast dongle: https://www.amazon.com/NooElec-NESDR-SMArTee-SDR-R820T2-Based/dp/B079C3FHPG Anyhow you can also use a standalone bias-t obviously, i'd still recommend the rtl-sdr blog v3 dongle or even better an Airspy Mini (quite a bit more expensive). rtl-sdr LNA plus Airspy Mini is a very potent setup for ADS-B reception, probably on par with the RadarCape (quite expensive hardware receiver, somewhat industry standard some would say) The Airspy Mini shows best results on the pi4, with the pi3 it's somewhat limited by the CPU but still quite good. At that point, if you still feel like spending lots of money, the DPD colinear aerial is probably the best antenna around But the setup with the airspy and rtl-sdr LNA should already mean your limit is the radar horizon. So the DPD antenna in that case will not necessarily increase reception at all.
The filter comes after the LNA on the blue ProStick Plus, some people need the extra FA filter in front of it for good reception. Anyhow the rtl-sdr LNA combo with their receiver is a slight improvement on that, but not by much in my experience. The airspy with an rtl-sdr LNA is clearly superior, but mostly in message rate, not so much in range.
Thanks! I think I'm going to try the outdoor antenna first, and if I end up needing another filter I'll grab one from Amazon. I'm out in the country though, very rural area. Not much in terms of RF interference here, other than a couple power lines and what I have in the house here (Wifi router, cell phones, ham equipment). The closest cell tower is maybe a mile and a half or two miles. The closest FM radio station is about 7 miles.
