Well THANK YOU MC, - as I had 7 turns, not 8 as your excellent pic shows.
Well Doc, I think you're right, but, - the coil is going to ground not to the radiator, so it's an inductive path to ground from the transmitter in parallel not series with the radiator.
- I had to scratch my brain for a few on this one...
Normally I think this would cause the transmitter to see decreased inductive reactance as it now has two paths to follow.
Remember, this added coil is going to ground and not into the ring / radiator.
In order to rebalance the load reactances the antenna would have to offset that decreased inductive reactance by decreasing the capacitive reactance, or lengthing the antenna...right?
But wouldn't this cause a decline in the performance of the antenna as it would no longer be at an optimum resonant length?
I bet the resonance of the coil is far lower than the resonant frequency of the antenna to the degree that the coil's influence on the tuned length of the radiator should be negligible, otherwise the RF would have two paths, one to ground & one into the matching ring & radiator, and a 3dB loss in performance would be expected.
I think the coil is only there for static discharge and a DC path to ground, not for retuning or additional matching of the radiator.
If one version of the Sigma5/8 is shorter, (prolly the newer version) I'll bet it's to get the tuning closer the center of the newer 40 channel band.
...maybe? 
73
