Thought I’d get in touch with you to let you know how this went. It was always the radiator length that had me stumped. Once I was set on the right track of researching velocity factor (thought it only applied to coax!) and end effect or K-Factor, it all began to become clear.
Anyway, I went ahead and cut the six radials and radiator to 4.88m (16ft) and put it together. I ended up trimming the radiator down to 4.83m (15ft9in) for an SWR of 1.2:1 on 20m
My initial calculation for the 40m loading coil of 12 turns of wire on a 45mm (1 3/4in) former was out and was reduced to 10 turns. This gave a SWR of 1.3:1 on 40m initially, which was improved by slight pruning of the radiator by another 2cm (3/4in). SWR is now at or below 1.2:1 over both 40m and 20m bands.
As I am using a 7m (23ft) squid pole, the feed point is about 2.5m (8ft) above the ground. In this case, the radials CAN be stretched out with the ends pinned to the ground resulting in the radials dropping at about a 40 degree angle from the horizontal. Attempting to use them as tight guy wires, the squid pole will collapse telescopically back into itself in anything other than a the slightest of breezes.
So, the antenna also works very well. Over the holiday break, I took it away with me to VK3 land (South Eastern Australia) and used it portable. Good contacts into Japan, Italy and the Canary Islands were made on 20M in the late afternoons. The Antenna also performed very well on 40M netting multiple local contacts out to 1500km (1000miles) or so in the evening hours.