The Sommer design takes advantage of different elements phased to operate on
all of the bands from 20 to 10. The longest element can be adjusted to
operate on either 30 or 40.

One one band all the elements might operate as half wave elements but on
another they would operate as 3/4 wave elements or even ful wave elements.

If properly assembled and adjusted (and that is not easy) the Sommer beam
can be quite effective for the given boom length.

There was an execellent article in Dx magazine about how KM1E slected the
Sommer XP708 over other antennas in late 1995.

Dave K4JRB