The effect of changing the angles or length of v antenna on gain

if i change the angle of v antenna...i see that the tv channels become better ...why .... is it when u open more angle u cover more range of there are some rules acoording to theoritical part i hope to see furmules the effect of changing the angles or lenght of v antenna on gain and imedance effecancy


one more think what is F/B ratio and how can i do a relation betwwen gain and f/b
thx