You have come up with some valuable info being the Velocity Factor of copper
wire with different insulator coverings. Do you have a source for this?
Also, do you have a source for calculating the Vf when inside white PFC conduit?
I’ve made co-linears in the past and gone crazy adding extra sections in the hope of
getting more gain, and was shocked at the extremely narrow band width, however, at 1090
you dont need bandwidth, just room for errors of measurement or cutting.
From what I’ve done in the past, I’ll guess that 8 sections of a Franklin should be ok, except for, the narrowing of the bandwidth which means get the calculations and measurements correct the first time.
Now as for not being able to receive directly above, I do not buy it, when the signal course is closer, it will be far stronger, and the antenna will also have lobes of gain intermittently or jaged lines intersecting the flight path. I’d predict that there is unlikely to be any loss directly above due to the antenna but there could be from the aircraft if directly above you but only for a very short period of time.
I’m also seeing a directional pattern on a tin can antenna in the direction the slope of the can is leaning. I must try a cone of metal pointing down at 45 degrees and see if that performs differently.