My 15 MHz bandpass filter finally arrived, and I have had installed viedehopf’s graphs1090 just in time for a comparison. (“Receiver 2” is currently fitted with the FlightAware brown rod.) Daytime noise dropped 6 dB, and nighttime noise dropped 10 dB! This improves usable dynamic range by 3 dB to 5 dB.
Reduced noise on the high-gain antenna, combined with mounting the rod vertical on empty hamster cage (from horizontal) means that this experimental feeder now surpasses my PiAware feeder with short antenna in both aircrafts and positions reported, even though the rod is on the “wrong” side of my building (facing away from most traffic with two more dry walls) and 1.5m lower.
High antenna gain even overcame low elevation and absorption to attain better range.
Of course, low antenna gain isn’t the only factor against Receiver 1. As I barely diagnosed a day ago, that setup is suffering from COVID interference. Not that carbon-based SARS-CoV-2 somehow infects silicon-based receiver, but a bad HDMI adapter from workplace that invaded my home due to COVID shelter-in-place spills extreme RF leakage to saturate the preamp just as it did - and more severely so, when the brown rod was attached to it.
As I have experimentally determined in the other thread, sharp weekday elevation in noise level is attributable to this bad video adapter. (Again, a shoutout to @wiedehopf for the wonderful graphs1090!) I still need to further diagnose the smaller noise elevation during weekend days.