…which also explains:
Testing
When building my first collinear using this design, I made the loops in a similar way to those seen on many commercial antennas which have multi-loop coils, with the wire above and below the coil being centrally located with regards to the loop.
how NOT to do the loops / first attempt at the loops
However, after doing some further research, I realised this was incorrect, and made another collinear using the loops as per the approach described above.
Some quick comparison testing indicates the collinear with the offset loops performs much better than the collinear with the centered loops.