Spurred on by you mentioning that your Airspy site 97430 does better than your reference site 72716, I made one more stepwise refinement today to my own Airspy site hphillip#stats-123922 around 15:33
To my surprise it has made more difference than the firmware update
The simple change was to put a dark blue FA filter immediately before the Airspy.
Already comparing my reference ProStick+ site hphillip#stats-117925 with the airspy plus filter, the 1090graphs are already suggesting enhanced aircraft numbers.
First pic is the reference site:
Pic below is the Airspy with dark blue filter:
On the face of it my number of Aircraft have already gone up by 10.
Really I’ll need a few days to definitively know if the change was a significant improvement or just a fluke!
In a previous lifetime I used to play with UHF TV DXing and had a rather nice (though rather ugly for the neighbours) stacked and bayed bowtie array on a lovely Yaesu rotator. The reason I mention that was the system had a 4-into-1 combiner, which on the face of it was never going to be the same gain as a single ultra fringe yagi.
However what I found in practice (and really I am talking about days before DTV) the height differences of the bowties pulled in the signals really well and was not as sharp as a yagi.
I contemplated a similar approach for 1090Mhz i.e., have multiple directional antennas and use a 3-way or 4-way cavity combiner.
My present omni works well but I realise I have a couple of directions SSE and NE where something with more directional gain may be useful.
Problem is cost! But if you ever go down a multi-antenna approach I would be very interested to hear how you get on verses an omni.
I jokingly concluded to myself that the best of both worlds might be a fast rotating high gain directional antenna (preferably silent and no visual impact to the neighbours, perhaps a red light on the top).
I can always dream