Here you can see the positions of the aerials - The red circle is roughly where the internal aerial is located in the loft. As you can see, it’s right by some internal brickwork and is therefore directly shielded slightly west of south.
I’m feeding them both into VRS and here’s a range plot from the two sources. Red is the internal aerial, grey is the external.
Despite the fact that the external aerial has (allegedly) much higher gain and with a clearer view all around, the maximum range of the internal aerial isn’t too different in some directions although the internal aerial is well down in directions it’s directly blocked. The Moonraker collinear is in the clear with nothing really in the way to the horizon.
I spot almost exactly the same number of aircraft/day between the two systems although the reports received on the external aerial is about 10% higher than that on the internal.
I’ve tried this with the Pro Stick Plus in the loft and the RTL-SDR/Uputronics pre-amp on the external aerial with no real noticeable difference.
Next step is to get a Pi and receiver mounted at the base of the collinear, I’m not sure which one to stick up there, whether to go with the Pro Stick Plus or the RTL-SDR/Uputronics combo. The former will probably be easier to do but my ultimate goal is simple - Get rid of the coax as much as possible.
However, this just shows that a perfectly good system can be set up with a simple internal aerial.