function for solving for a variable in an equation given a range of values for another variable

I want to solve eqn for r given a range of values for b

b = 0:0.01:pi;
eqn=(4/(6-8*r*(cos(b)-1)))+((sqrt(4+16*r*(cos(b)-1)))/(6-8*r*(cos(b)-1)))) <= 1;

I've seen vpasolve and just solve, but I do not see how to apply those functions to my problem. Any help would be appreciated!