[TowerTalk] 40 m 2 el yagi bandwidth very narrow

[Kari Dx Supply]

Hi! 
I just recently built a 2 el 40 m shorty with linear loading. I have reused Hy- gain parts. Boom is 23 feet long and 0.17 wl spacing. Element length like most shortys. After some adjustment with a hair pin coil I got it well matched at 7040 (1.12:1). The feed point impedance was like theory  predicts - z= 17 ohms - j23.
But the 2:1 SWR bandwidth is very narrow only about 80-90 kHz. I have not adjusted reflector more then made it  abt 4% longer. The reflector element is electrically in contact with the boom ( driver is isolated). According to theory the SWR bandwidth should be greater with this spacing or? Looking at reflector on similar threads also supports much broader bandwidth.
How can I achieve a greater bandwidth? Is it that the relector is a little bit to short and influences the driver to much? Again doing the practical way by using a signal source some distance away and adjusting reflector for actually the best F/B and hoping this improves the SWR is a feasible way. What says theory in this respect? Thanking for valubale tips. I have tried to read  the Yagi design book and see what is says but I can not get hold of a copy.
73s Kari SM0HRP