It is very difficult to determine the exact radiation pattern of your
Inverted V antenna in advance simply because there are too many factors that
can influence the pattern, e.g. ground effects and the effects of other
surrounding objects. When we think of ground effects alone, as an example,
just the composition of the earth below and around your antenna can
influence the radiation pattern. 

Most of the radiation patterns discussed regarding any particular antenna
are discussed in reference to such an antenna in free space. It is like the
economist telling us, in theory, what will happen to the influence of a
given economic factor when all other factors are held constant. The truth is
that in the real world all other factors are never held constant. Thus, when
you take any given antenna out of the land of theory and put it in the land
of the real world, there are many factors that influence the radiation
pattern. That is why we often put up an antenna thinking it will give us
especially excellent results in a particular direction, only to find out
that it ends up working more effectively in some other direction. 

Ron, K8HSY