This is designed to be installed at ground level, preferably with at least 3 radials.
The length should not be changed by too much, first the coil should be adjusted (just by stretching it or squeezing the turns closer should be enough). This is quite dependent on the grounding or addition of radials. Then the length may be adjusted for fine tuning.
Radials will improve performance.
Note the top end of the 300Ω line is shorted, and the bottom is open.
It helps to wind the coil in three pieces (ie 8t & 8t & 3t on a 25mm former 50mm long) this makes it easier to adjust, and if a turn needs to be taken off or added, it can be done easily on the small 3t coil.
I built a 10m version for testing, my final SWR after a short time tweaking was 1.03:1