The signal loss between opposite polarised linear antennas (vertical to horizontal or vice-versa) is in the order of -23db (quite a lot!)
The signal loss between linearly polarised (either vertical or horizontal) to circularly polarised antennas is in the order of -3db (not a huge amount).
A while ago I made a quick tests with a homemade Skew-Planar (bottom right most antenna in abcd567’s pictures above) cut for 1090MHz and didn’t see a big drop off in received signals.
My theory is that some adsb signals are being ‘skewed/slanted’ in terms of polarisation.
That is, they hit some object on their way from the aircraft to my antenna and the polarisation is no longer true vertical or horizontal, but somewhere in-between allowing the circularly polarised antenna to better receive these signals at less loss than a true vertical.
I didn’t make extensive tests at the time, but will try to do so again when I get some time to spare.
My tests were in a built up area, so may be less relative on a hill top site with no nearby obstructions (causing reflections).
The skew planar is a true omni-directional CP antenna - meaning it will receive equally as well in all directions, unlike most of the other antennas in abcd567’s pics above.
The helical beam is obviously directional (top left pic), the Lindenblad is omni-directional and circularly polarised (top right pic) and the Quadrafilar Helix is circularly polarised but more end fire (more like a beam) - (bottom left pic).
A circularly polarised antenna may work equally as well as a vertical in some locations - that is in densely populated areas with lots of tall objects/buildings in the vicinity that potentially may be distorting the signals polarization at the receiving antenna.
I don’t think it wouldn’t be a total waste of time to look a bit more at CP antennas?
The Skew-Planar is relatively simple to make and presents a pretty good match to 50 ohm cable.