In reply to G3CWI:
Coincidentally, I have spent a few hours yesterday and today experimenting with a slim jim - made with 4mm stranded copper wire, taped onto a length of plastic trunking. The final dimensions are close to published designs, and I am reasonably confident that it is resonant. The SWR is fairly flat across 2m, lowest around the middle of the band.
Now to compare it with my basic ground plane, made by soldering a quarter wave length of cadmium copper wire to the centre of an SO239 socket, and 3 radials to the outer. SWR adjusted by altering the angle of the radials. I’ve used this on all my activations so far.
With the base of the antenna about 2m above ground in each case, the ground plane gets me into the Stoke repeater (from Shrewsbury) with 2 bars of signal strength showing on the rig. The slim jim does not hear the repeater.
Raising them each in turn on a 5m fishing pole - the ground plane now hears the repeater with 5 bars on the display, and so does the slim jim, also showing 5 bars. For comparison, my commercial colinear five eighths over five eighths, at the same height, displays 8 bars.
In a garden environment with trees and houses around, the slim jim seems to respond to a change in height in a more complex way than the ground plane. I have no means of getting them any higher, but I may tidy up the slim jim and test them in more open surroundings.
In the absence of any 450 ohm feeder, my plan was to sandwich a wire slim jim in between two layers of duck tape, but I haven’t tried that bit yet…