spacing between the reflector and driven element should be
Gain vs Spacing
The gain of an an antenna with parasitic elements
varies with the spacing and tuning of the elements, and thus for any
given spacing there is a tuning condition that will give maximum gain
this spacing. The maximum front to back ratio seldom,
if ever occurs at the same condition that gives maximum forward gain.
The impedance of the driven element also varies with the tuning and spacing, and
thus the antenna system must be tuned
to its final condition before the match between the coax and the antenna can be completed. However,
the tuning and matching
may interlock to some extent, and it is
usually necessary to run through the adjustments several times to
insure that the best possible tunning has been obtained.