Modeling of the G5RV shows that the best length for the Coax section
is 0 feet!
The reason is that the coax section only creates loss, depending on
the SWR at the frequency of operation. Some frequencies have higher
SWR than others.

I provide a sample G%RV antenna file with Coax definition in the demo
version of Nec2go. It is very easy to Sweep the coax length and see
the losses.

Pat W.