A 900 Yagi would usually be designed for the 902 to 928MHz range. One aspect of a Yagi is the directors will have a big impact on gain when you go higher than the design frequency and they eventually become a reflector. 1090MHz is probably getting into the range where a 900 Yagi is starting to have some problems.
If you connect 4 Yagi's with a power divider to try and cover 369deg omni coverage, you will loose 6dB of gain right away from the power divider loss and only part of that is made up by overlapping coverage of the Yagi's. In this case something like 3 element Yagi's would have a broad enough pattern to overlap well but larger and higher gain Yagi's will have narrower patterns leaving holes in the omni coverage.
I think you would be much better off with a large high gain omni designed for 1090MHz. The more gain it has, the more it will pick up towards the horizon where you need it for extreme distance. When aircraft are at high altitude and within 200mi they are usually line of sight to most any antenna on your roof and when overhead at say 37,000ft where the antenna pattern is really bad they are only about 7mi away you don't need much to pick that up.
prcguy
on i will buy some many 900mhz yagi and put it on my roof to get the best results.
