I’ve got a Sirio 2008 as with 8 radials which are longer than yours. I never tried it with less than 8 so I can’t say. My SWR is the lowest in between 27,215 and 27,605 (1:1) and never goes above 1:3 on either end. If I remember correctly I tuned the antenna (length of the final rod) for 27,205 so must have been 33cm. It is currently attached to my chimney about 30 to 35 feet above ground.