Variance in characteristic equation


One of my characteristic equations looks like this:


Expected value of R is calculated by Dynare, but what is the case with variance? How Can I make Dynare to calculate variance from the above equation?

Please help

By using at least a second order approximation and entering the formula for the variance explicitly. You also need to make use of an auxiliary variable, see
[Expected value of a power)
However, you will get a problem with this equation in steady state.