This is the 20 meters azimuth plot of a two element quad antenna and shows a front to back ratio of only 16.2 db's. The measurements used in the plot are those measured after construction and without tuning. To improve the front to back ration requires tuning of the reflector element, usually accomplished my inserting a small tail section. Best F/B is obtained when both loops approach the same electrical size. In order to archive this, it would be necessary to construct the reflector loop initially smaller than the driven loop in order to insert the tuning tail, which would then add physical wire length to the loop.
There are several draw backs to this idea in that countless calculations shows a reduced antenna gain as the loops now have the same physical size, the feed point impedance also becomes unstable and requires re matching to the feed line, F/B tuning except under ideal test condition can often be inconclusive.
It is a difficult, and time consuming affair then changing the entire loop dimensions when the antenna is installed. It is very tempting to try tuning the reflector as theoretical result under ideal conditions show a F/B ratio in excess of 35 Db's. Personally opting for a stable and reproducibly antenna design without tuning stubs reduces time and effort and in the long run has very little practical benefit.