This equation can be simplified by fixing the known values and the relation between Re and Rb, Refer to the code below:
x = [0:0.5:20];
result = [0:0.5:20];
for i = 1:length(x)
result(i) = cali(x(i));
end
plot(x,result)
function out = cali(Rb)
k = 1;
n= 1;
k = 1;
w = 1;
thefixedta_0 = 1;
Re = 100*Rb;
out = (2*pi*k*n*w*thefixedta_0*Rb)*((besselk(1,n*Rb)*besseli(1,n*Re)-besseli(1,n*Rb)*besselk(1,n*Re))/(besselk(0,n*Rb)*besseli(1,n*Re)+besseli(0,n*Rb)*besselk(1,n*Re)));
end