between the reflector and driven element should be
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 tuning has
(ARRL Radio handbook)