I made the antenna of an insulated copper wire with a diameter if 1.8mm (3.5mm with insulation).
The centerfrequency of the 868 MHz ISM band (863 MHz - 870 MHz) is 866,5 MHz.
The circumference of one loop is: 27295 / 866,5 MHz => 31.5cm
The lengh of 1 radial for best swr is: 11264 / 866,5 => 13cm
The distance between the lower edge of the loop and the Radials are 3412 / 866,5 MHz => 4 cm 
In my case, these values gave the best results. Using a different wire will likely result in different values.
