5G wavelengths are between 1mm and 10mm depending on actual frequency.
1090 MHz wave length is 10.82831 inches.
You might consider putting a skylite in the attic roof and then you could have the antenna upright on the roof peak. Skylite needs to be big enough so you can boost yourself up out on to the roof with only a step ladder in the attic. Skylite is best located on the north facing roof slope so you can avoid most of the sun.
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I like this idea but it’s not an option I can pursue, sadly 
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Well, upside down went poorly, average range dropped from 155ish to 120ish. So now I’m back to confirming the 2ft adsbx antenna works better than the DPD productions antenna.
Has anyone ever tried running two SDR dongles on a raspberry pi zero 2 W in order to capture both 1090 and 978? (Using some kind of usb hub I suppose)
I don’t have a setup with both 1090 and 978 (no 978 in Europe) but I suspect it will swamp the cpu of the Pi Zero. I’d recommend a Pi2 or Pi3 at least if you want to venture into that road. Usually the 1090 alone will push the CPU to the max and adding a second dongle will only enhance that, causing signals to be dropped and not processed.
So I’d expect that setup to perform less than a single SDR setup.
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Sorry I meant to say “pi zero 2 w”. I’m currently getting around 3420 planes a day and the CPU averages around 11.4% so I should be good on that front.